Projection Geometry Part I

February 13, 2014 in AV Design Tips, The Basics by Sam Davisson

geometry_1237605Time to get back to the "Basics". I took some time away for the holidays and have been suffering a bit of blogger envy. There really are some very interesting changes happening in the industry to blog about but I think it’s important to get back to the basics now and then. No matter how much the industry changes, the basics are always going to be important to understanding how things work.

The projection screen is one of the key elements in a successful presentation space. If the audience is squinting in the back or the folks in he outer row seats are forced to contort their bodies or twist their necks to see the presentation, the space simply isn’t working. If the presentation and the available screen space isn’t properly matched it can be equally as horrendous. Distracting black lines or blank space around the carefully prepared presentation or by the projected image spilling off of the screen and onto the surrounding walls simply ruin the presentation. Therefore specifying the right projection screen is extremely importance to creating a functional presentation space.

Projection Screen Size Matters

The first step to creating a space that is an effective presentation room, where each member of the audience can easily view the presentation, is to select a screen that is the right size. An appropriate screen size is determined by taking both the seating arrangement and dimensions of the room into consideration.

A couple of decades ago, the standard practice was to establish the image width of the projection screen first. The image width of the screen was determined by measuring the distance from the screen to the most distant viewer and then dividing by six. Previously, projection screens just needed to be big enough for everyone to see, now they must be big enough for everyone to read and evaluate data, especially that guy who likes sitting in the very back row on the outter edge of the room.

The rule of thumb today dictates that designers begin by determining the image height dimension of the screen. The rule for conference rooms and classrooms, or any room where viewing text, is that the screens image height should be equal to one-sixth of the distance between the screen and the most distant viewer. Once you establish the image height it must be checked against the dimensions of the room.

Another basic rule for most presentation area’s is that the bottom of the image should be a minimum of 4′ (48") above the audience floor, allowing audience members seated in the back of the room to see the entire screen. It is also recommended that there be 12" of top black masking between the ceiling and the top of the image or 18" if black masking is not to be used. When specifying a screen for a project, verify that the height of the screen plus top black masking plus the empty space below the bottom of the image area do not exceed the room’s ceiling height.

Basically what that means is that in a typical room with a 10′ ceiling the maximum image height will be 60"

Once the image height is established, the width of the screen will be determined by the format of the information that will be projected and its associated aspect ratio. The aspect ratio is a ratio of width to height in which the material was recorded and will be projected.

The Aspect Ratio Determines Width

Aspect ratio is defined as the relationship to the width respective of the image height. In todays commercial environment, video’s high definition resolution (1920×1080) seems to have gained the upper hand. Doing a little math that comes out to a 1.78:1 ratio aka 16:9. The other popular aspect ratio 16:10 (1.6:1) which seems to be used mostly in schools. In the computer world this represents WXGA and all of it’s derivitives.

If your dealing with someone who hasn’t jumped into HD, the standard definition aspect ratio was / is 1.33:1 and for those designing a presentation space for movies you may want to consider cinemascope which is 2.35:1

If your designing a home theater you’ll often want to go with a cinemascope screen. It should be noted that cinemascope is wider and narrower than an HD image. So you’l probably want to decide whether to go with a constant width or constant height system for your projection screen and add masking as appropriate. If you decide on a constant height then you’ll want side masking for displaying HD content. Conversely, if you go with constant width, then top and bottom masking would be the wise choice for those wide screen videos.

So, in the conference room discussed above where the image height was 60" and you are looking for a 16:9 (1.78:1) image size you would multiply the image height (60") by 1.78. The image width would be roughly 106.8"

Viewing Angles

To determine the viewing angle, imagine that a member of the audience is sitting in line with the center of the projection screen. That viewer is considered on-axis. Now imagine that the projection screen represents one axis and a second axis is drawn from the center of the projection screen through the on-axis viewer, creating a right angle. If the screen specified for the project has a viewing angle of 60 degrees, then a 60 degree angle can be drawn to both the right and left of the center viewer. These two angles define what is commonly referred to as the viewing cone. No seats should be planned outside of the viewing cone, because the image that the people in those seats will have of the screen will be distorted.

Projection screens today are manufactured to offer a wide range of viewing angles. A 60-degree viewing angle is considered especially wide. A 45-degree viewing angle is considered moderate. A narrow viewing angle will measure about 30-degrees.

Screen Surface

Because the screen surface goes hand in hand with the selection of the projector and I won’t be discussing projector selection until part 2 of Projection Geometry, I’ll hold off the discussion until then but I wanted it clear that screen surface is a very important area of discussion when selecting your screen and wanted to mention it.

Physical Characteristics

The final step in specifying a projection screen is to decide upon the different physical characteristics of the screen. These characteristics need to match the particular needs or uses of the projection screen and the presentation space.

Front or Rear Projection
There are two projection methods: front and rear projection. Front projection occurs when the projector is placed out in front of the projection screen. The projector shoots its visual image toward the projection screen, which then reflects the image back into the eyes of the audience.

With rear projection, the projector is positioned behind the projection screen. Instead of using the projection screen to reflect the image back into the audience, a rear projector transmits its visual message through the rear projection screen and into the audience.

Each of these projection methods has it’s strengths and limitations. A brief compare and contrast of these two projection methods reveals that the real estate requirement is perhaps the largest difference between front and rear projection. In front projection, the projector sits in the same room as the audience, either perched on a conference table, on a cart in the middle of the room, or suspended overhead. Rear projection requires that a dark room exist behind the projection screen that provides enough room for the projector to project an image of the necessary size. The larger the image, the deeper the dark room must be to support it.

However, rear projection allows you to remove the noise of a projector outside of the room. Rear projection also makes it impossible for the presenter to walk in front of the projectors light beam, nearly blinding themselves and blocking out the visual image in the process. Rear projection also manages ambient light better. A front projection screen cannot differentiate between the light from the projector and the light from overhead, it reflects all of it back into the eyes of the audience. With rear projection, the dark room behind the screen absorbs ambient light like a black box. The ambient light passes through the screen into the dark space, while the projected image passes through the projection screen into the auditorium, classroom, or conference room.

Electric or Manual Projection Screens
Today, there are a variety of ways to control both the small and large projection screens electrically. It can be as simple as touching a button on the wall to lower and raise a projection screen; as convenient as a handheld infrared or radio frequency remote control; or as intricate as a fully integrated control system with touch panel. With most integrated solutions, it is possible to touch only one button to make the room presentation ready. The projector can drop out of the ceiling turn itself on, the projection screen descends from its recessed hiding place and the lights dim to a pre-selected level.

Because of the convienance of integrated control systems, manual screens are rarely used any more but you will run into them on some legacy projects.

Wall-Mounted, Ceiling-Mounted, or Recessed
Mount projection screens on a wall or a ceiling for easy access and then tuck them away in their attached protective case when not in use. If the space demands something a little more discrete, projection screens can be easily recessed into the ceiling, completely removing it from the visual field when not being used.

There are a few applications ceiling or recessed mounting would not be a fitt. For example, recessing a projection screen in an application with an especially high ceiling, such as the sixty foot ceilings in some churches, is not an especially viable option. When the desired screen size is only a 9′ x12′, it does not make any sense to attach enough fabric to it to drop the projection screen to its desired position 30 feet below.

Portable or Fixed Frame
Is the projection screen being shared between multiple rooms or should it be an aesthetic fixture in the space? Both are possible with today’s technology.

Portable screens are available with black backing. This black backing reduces the amount of light interference incurred from the other side of the screen, improving the quality of the image projected on these mobile presentation screens. The portable case is made small and light so it is easier to move projection screens from one presentation to the next.

Fixed frames are good fits for rooms that host presentation after presentation and where the clients do not want to put the screen away or rooms with extremely high ceilings. Mounted to a wall like a picture, this type of frame does not need to be hard-wired into the space like its electric counterparts. Fixed frames accommodate both front and rear projection methods.

Screen Borders
Human eyes focus better onto a particular image when it is bordered. Bordered images are also perceived to have more contrast, brighter colors, and be, overall, a sharper image. That fundamental feature of our visual system is responsible for frames around art and margins on paper. Borders around a projection screen also enhance the professionalism of the presentation by allowing the projected image to, in print terms, "bleed-off" of the screen.

To provide the best viewing environment for an audience, specify that black borders be included on the projection screens in the space. Without the clearly defined boundary of the screen, the images can get lost as audience members try to disseminate the edge of the fabric from the supporting wall behind the screen.

Drop
The term drop in the projection screen industry refers to extra fabric that is added to the top or bottom of the screen to position the screen surface within normal viewing heights. An example of when this may be used is in a building with high ceilings. If there is a 15 foot ceiling and a 6′ x 8′ projection screen is going to be mounted so that the bottom of the screen is 4 feet or 48 inches off of the ground, the project demands 5 feet of drop to be added to the top of the screen and mounted to the ceiling.

Which comes first the projector or the projection screen? Thats like asking which came first the chicken or the egg. For the most part, I make those determinations simultaneously as most the time you considering the same information for each. Although, I usually start with the screen sizing and work from there. In Part 2 we will discuss the decision making process at greater length and work through more of the projection geometry.

Signal to Noise

November 9, 2013 in AV Design Tips, The Basics by Sam Davisson

signal-to-noiseI’m still not quite sure why but when I started the basic series with "Power and Grounding" most of the comments, both public and private where that I was setting the stage to talk about audio. Having a solid ground plane affects everything but I’ll concede that in this digital age, it affects audio the most since it is still basically distributed in analog.

The second most requested topic was signal to noise. The signal to noise ratio, or SNR, of a signal is a measurement of the power in the signal fundamental relative to the RMS sum of the energy of all in-band noise components excluding harmonics.

From an integration perspective I never give the SNR (signal to noise ratio) a second thought. The reason being is that IF you have a solid grounding system and IF you’ve built your cables correctly and IF you have minimized the distance of your unbalanced analog audio runs, then the worst case scenerio for SNR (and it’s cousins THD & SND) should be that as the lowest SNR rated equipment specified in the system. In other words, it’s a design consideration not an integration issue. But IF it’s not integrated properly, your going to be able to hear it, even with slightly poor hearing. As a consultant, my specifications do not even require testing for these.

From a design perspective you need to answer the question, for yourself, of how much noise is too much noise for the system in design and then specify products that meet those requirements. For me, if I’m designing a performing arts center or music recording studio then I’m looking at a SNR of 110db or greater. A classroom or conference room, typically a SNR around 75db and a executive board room or auditorium which will support the spoken word, a SNR above 80db.. But these are almost arbitrary numbers that I’ve come up with based on customer interaction and satisfaction. There is no science behind them.

Constant Voltage Speaker Systems

November 3, 2013 in AV Design Tips, The Basics by Sam Davisson

overhead-speakerWhile this might be a great reminder of how much technology has changed, I’m not sure why when I searched for an interesting image of overhead speakers this image came up. Can I assume that in Google’s eyes this instructor is teaching something that is way over the heads of the students?
Or, perhaps even, that should something catastrophic happen and we had to revert back to "old tech" people would be completely lost? But this has nothing to do with furthering today’s topic, Constant Voltage Speaker Systems.

Electrical power companies figure out a long time ago that the best, most efficient way to distribute power was to step up the voltage at the power station and then to step down at your house. The audio industry took this model for distributing audio to large numbers of speakers or speakers no where close to the amplifier which gives us the constant voltage system we currently use or over use but I’ll probably discuss that at some later date.

The term “constant-voltage” is somewhat misleading causing some confusion. In electronics, two terms exist to describe two very different power sources: “constant-current” and “constant-voltage.” Constant-current is a power source that supplies a fixed amount of current regardless of the load; so the output voltage varies, but the current remains constant.

Constant-voltage is just the opposite: the voltage stays constant regardless of the load; so the output current varies but not the voltage. Applied to distributed sound systems, the term is used to describe the action of the system at full power only, an important point in understanding. At full power the voltage on the system is constant and does not vary as a function of the number of loudspeakers driven. As long as you do not exceed the maximum power limit of the amplifier you may add any number of speakers and the voltage will remain constant.

The other thing that is “constant” is the amplifier’s output voltage at rated power – and it is the same voltage for all power ratings. Several voltages are used, but the most common in the U.S. is 70.7 volts rms (100 volts is common outside the US). The standard specifies that all power amplifiers put out 70.7 volts at their rated power. So, whether it is a 100 watt, or 500 watt or 10 watt power amplifier, the maximum output voltage of each must be the same 70.7 volts.

Advantages of 70V Systems

A 70V line reduces power loss due to cable heating. That’s because the loudspeaker cable carries the audio signal as a low current. Consequently you can use smaller-gauge loudspeaker cable, or very long cable runs, without losing excessive power.

Another advantage of 70V operation is that you can easily provide the amplifier with a matching load if you’re connecting hundred of loudspeakers. With a single 8-ohm amplifier output it can be difficult to wire the loudspeakers in a series-parallel combination having a total impedance of 8 ohms and if one loudspeaker fails, all of the loudspeakers in series are lost. This changes the load impedance as seen by the power amplifier.

Conversely, with a 70V system you can hang hundreds of loudspeakers in parallel on a single amplifier output. In addition, a 70V distributed system is relatively easy to design and allows flexibility in power settings meaning different speakers can be set to have differing volume settings.

Disadvantages

Simply, your audio quality suffers, especially in the low frequency range, transformers can degrade the frequency response and add distortion. Also, designers have gotten lazy. Ceiling speakers are thought of only in a 70V configuration and used to fill a room with noise rather than doing a proper LCR (left, center right) 8 ohm sound system with 8 ohm ceiling speakers for speech reinforcement.

Yes it may be cheaper but isn’t the goal natural sound.

Opinion

This is my strictly my opinion. Constant voltage systems are great when used as intended, as a means of connecting a large number of speakers where needed or in remoting speakers a long way from the amplifier.

But this technology is being over used because of simplicity. A design engineer doesn’t need to understand how to fill a room with sound. Clients and the industry suffers because of this.

I’ll leave you with this little chart on cable requirements I borrowed from Belden Cable’s article on this 70V systems:
Speaker Cable Distance Chart

Nominal & Load Impedance

October 26, 2013 in AV Design Tips, The Basics by Sam Davisson

Why we love speakers

AKA Impedance Part II

Ok, before I jump into this I’ll apologize to anyone who is offended by the cartoon. It’s a bit of an inside joke with a good friend of mine and I couldn’t resist the urge

Nominal impedance in electrical and audio engineering refers to the approximate designed impedance of an electrical circuit or device.

Actual impedance varies with frequency changes. It is possible for that variance to be quite large. Despite that, it is normal to speak of nominal impedance as if it were a constant resistance with no reactive components. Which is often far from the truth. Nominal impedance is referring to a specific point on the frequency response of the circuit being considered.

Loudspeaker impedance’s are kept relatively low so that the required audio power can be transmitted without the use of high voltages. The most common nominal impedance for loudspeakers is 8 Ω. Also used are 4 Ω and 16 Ω.

The once common 16 Ω is now mostly reserved for high frequency compression drivers since the high frequency end of the audio spectrum does not usually require much power to reproduce the sound.

Diagram showing the variation in impedance of a typical mid-range loudspeaker. Nominal impedance is determined at the lowest point after resonance.

Diagram showing the variation in impedance of a typical mid-range loudspeaker. Nominal impedance is determined at the lowest point after resonance.

The impedance of a loudspeaker is not constant across all frequencies. In a typical loudspeaker the impedance will rise with increasing frequency from its dc value (see diagram) until it reaches a point of mechanical resonance. Following resonance, the impedance falls to a minimum and then begins to rise again. Speakers are usually designed to operate at frequencies above their resonance, and for this reason it is the usual practice to define nominal impedance at this minimum.

The ratio of the peak resonant frequency to the nominal impedance can be as much as 4:1. It is, however, possible for the low frequency impedance to actually be lower than the nominal impedance. A given audio amplifier may not be capable of driving this low frequency impedance even though it is capable of driving the nominal impedance, a problem that can be solved with the use of crossover filters.

Note that is also may be possible to solve the problem above by underrating the amplifier. This is not a solution I would recommend but it is a possibility

Speaker Basics

Getting very basic here, our use of the word SPEAKER is the shortened form of the word LOUDSPEAKER and it refers to a device that converts electrical signals into sound waves that we can hear. A loudspeaker has several parts:

  • Cabinet – It should be noted that the cabinet performs a much greater function than to simply house the components that make up the loudspeaker. The discussion of which is well beyond the scope of this post. But if you truly want to understand loudspeakers and how they function to produce audiophile quality sound it is a subject worth delving into.
  • Driver (aka speaker) – Converts electrical energy into sound waves (more on this to come)
  • Crossover Network – As we will discuss, speakers are designed to reproduce specific ranges of frequencies. The crossover network filters the frequencies sending only the frequencies that can be used to a specific driver. Passive crossovers will typically be installed inside of the cabinet. Speakers meant for high end audio use will either have switches allowing for the passive network to be bypassed or have the passive crossover removed altogether. In these instances an external crossover, typically built within the systems digital signal processor (DSP), will be used.

Speakers (drivers) are constructed much in the same manner as a musical instrument in the fine tolerances and attention to detail make all the difference to the sound quality. Large speakers are suited for low frequencies, small speakers – high frequencies. Each speaker can only function efficiently and with linearity within 3 octaves of it’s design. Theoretically a single speaker would have to change diameter from 1" – 24′ to maintain a similar level and dispersion over the complete frequency spectrum.

Speaker cross section

The majority of drivers consist of paper or plastic molded into a cone shape, loosely suspended in a frame so as to easily move back and forth to vibrate the air. Glued to the back of the cone is a coil of wire aka voice coil suspended within a strong magnetic field. Passing electricity through the wire causes a magnetic field around the wire which attracts or repels which in turn causes the cone to move back and forth. The larger the magnet and voice coil the greater the power and efficiency of the driver. Because of the tight tolerances no two speakers are truly identical.

Load Impedance

For the load impedance discussion I’ll use the Ω symbol for resistance. Z is the symbol for impedance. As we have discussed impedance could be stated as resistance that varies with frequency.

Load impedance is the load speakers represent to the amplifier. The maximum power rating of an amp is always in reference to the load impedance. For example if the amplifier is rated for 100W (watts) of power into a 4Ω load it is only capable of producing 100 watts with a 4Ω load. If you connect an 8Ω load, the amplifier will only be capable of producing 50W of power. The higher the load impedance the cooler and more reliable the amplifier will be, and the lower the internal distortion. But it will produce less power.

Conversely, if you present a 2Ω load the amplifier will attempt to deliver 200W of power which could quite possibly damage the amplifier. If you don’t see smoke and flames the amplifier will run much hotter and less reliably.

Calculating Speaker Impedance with Multiple Speakers

Parallel, Series and Series Parallel Speakers

Calculating speaker impedance follows the same laws of electronics as for purely resistive circuits. For speakers in parallel divide the speakers nominal impedance by the number of speakers. Therefore, 2 8Ω speakers have a nominal impedance of 4Ω and 4 8Ω speakers have a nominal impedance of 2Ω.

For speakers connected in series, add the nominal impedance’s of the speakers. Therefore 2 8Ω speakers nominal impedance is 16Ω.

You should only connect speakers in a series-parallel configuration with an even number of speakers. First calculate the nominal impedance for the series speakers. Then calculate the 2 series pairs in parallel. In other words, 2 8Ω speakers in series is 16Ω. The 2 sets of series speakers in parallel would then represent a nominal impedance of 8Ω

Connecting speakers in series and series parallel may be essential in certain applications where many speakers are required to be connected to one amplifier. However connecting speakers in series causes the distortion of each speaker to be reflected into the others. But connecting speakers in series does not effect the reliability of the speakers or amplifier.

Amplifier Classes

October 20, 2013 in AV Design Tips, The Basics by Sam Davisson

6v6_top_sideAmplifier Classes

When I first started this article I fully intended to finish up Impedance by discussing amplifiers, speakers and the effects of impedance on audio quality. That plan changed. I think amplifier classes deserve to be a post of their own and need to be discussed before any meaningful continuation of the impedance discussion. Besides, thinking about all this along with trying to research to find out exactly what a Class G and Class H amplifier was has given me a splitting headache. 😉

This picture looks eerily like one of my early Heath Kit assemblies and if I remember correctly it taught me the value of keeping a fire extinguisher nearby when working with electronic components you were not familiar with. For those who may not know about Heath Kit, they were the originator of DIY electronic projects. You could by a box of parts and instructions from them to build just about any electronic device of the time.

Amplifier Classes

There are complete books on amplifiers and the differing classes of amplifiers. Im not going into that depth here this is the basics after all. Also, don’t expect a discussion of every class of amplifier in existence. Classes C, E and F are more typically found in RF applications than audio applications.

Class A
In a Class A amplifier, the output devices are continuously conducting for the entire cycle, or in other words there is always bias current flowing in the output devices. This topology has the least distortion and is the most linear, but at the same time is the least efficient at about 20%

Class B
This type of amplifier operates in the opposite way to Class A amplifiers. The output devices only conduct for half the sinusoidal cycle (one conducts in the positive region, and one conducts in the negative region), or in other words, if there is no input signal then there is no current flow in the output devices. This class of amplifier is obviously more efficient than Class A, at about 50%, but has some issue with linearity at the crossover point, due to the time it takes to turn one device off and turn the other device on. In other words, it is not distortion free.

Class AB
This type of amplifier is a combination of the above two types, and is the most common type of power amplifier in use. Here both devices are allowed to conduct at the same time, but just a small amount near the crossover point. Hence each device is conducting for more than half a cycle
but less than the whole cycle, so the inherent non-linearity of Class B designs is overcome, without the inefficiencies of a Class A design. Efficiencies for Class AB amplifiers is about 50%. This amplifier remains the amplifier of choice for the true audiophile.

The above amplifiers are known as linear amplifiers.

Class D
This class of amplifier is a switching or PWM amplifier. In this type of amplifier, the switches are either fully on or fully off, significantly reducing the power losses in the output devices. Efficiencies of 90-95% are possible. The audio signal is used to modulate a PWM carrier signal which drives the output devices, with the last stage being a low pass filter to remove the high frequency PWM carrier frequency.

The primary differences between linear (Class A and Class AB) amplifiers and switching (Class D) digital amplifiers is the efficiency. This is the whole reason for the invention of Class D amplifiers. Linear amplifiers are inherently very linear in terms of performance (therefore very distortion free), but are also very inefficient when it comes to power, whereas a Class D amplifier is much more efficient from a power standpoint but suffer from added noise and distortions due to the switching nature and additional filters.

Classes G & H
Honestly, before I started writing this I had heard of these classes and understood they were "improvements" to the Class AB amp in that they are supposed to provide distortion free audio with much greater power efficiencies. In doing the research for this article I found that I probably had about as clear of an understanding as anyone else.

The general consensus seems to be that Class-G runs from a low voltage rail until the signal goes beyond a certain voltage, and then a higher rail (or rails) is switched in. Class-H refines this to use a variable higher voltage rail (or rails), also known as a modulated rail or use a ‘bootstrap’ capacitor that lifts the rails as needed, but cannot maintain them at the full voltage for more than a few cycles.

Multiple rail amplifiers usually allow significantly higher efficiency than single-rail Class-AB design. The more rail levels are used, the higher the efficiency, assuming they’re spaced properly. While in all reality it is impossible, efficiencies can reach 100 percent with an infinite number of voltage rails.

Multiple rail amplifiers typically use only two voltage rails, which amounts to more than 80 percent theoretical efficiency at maximum power. Using more rails becomes impractical due to added power supply complexity, but with four rail voltages, efficiency at maximum power can reach 90 percent theoretically.

Impedance, Part 1

October 5, 2013 in AV Design Tips, The Basics by Sam Davisson

Resistance / ImpedancePart 1: Transmission Line (Characteristic) Impedance

Impedance has been the most requested subject since I started the “Basic’s” series. So I thought it about time to address it. Problem is, no one actually mentioned what area of impedance they were confused about and was looking for clarification on. Perhaps some are wondering why when you measure across the center conductor and braid of a 75ohm coax cable you don’t read 75ohms. Or how in the world you would ever know if a connector was a 75ohm connector or one of the 50ohm variety? Does all that mumbo jumbo have anything to do with audio amplifiers and the speaker connected to it.

Every signal input, and every output, has an impedance, this "impedance" represents the relationship between voltage and current which a device is capable of accepting or delivering. Electricity is all about the flow of electrons in wire. "Voltage" is a measure of how hard the electrons are pressing to get through, it’s like water pressure in a pipe. "Current," measured in amps, is a measure of the rate at which the electrons are flowing. It’s like the gallons-per-minute flow in a pipe. Total power delivery, in an electrical circuit, is measured in watts, which are simply the volts multiplied by the amps. A number of watts may represent a very high voltage with relatively low current (such as we see in high-tension power lines) or a low voltage with very high current (such as we see when a 12-volt car battery delivers hundreds of amps into a starter).

An output circuit can’t supply just any combination of voltage and current we want. Instead, it’s designed to deliver a signal into a specific kind of load ("load," here, simply meaning the device, such as the TV input that the signal is being delivered to). The "impedance" of the load represents the opposition to current flow which the load presents.

The impedance of the load is expressed in ohms, and the relationship between the current and the voltage in the circuit is controlled by the impedance in the circuit. When a signal source sees a very low-impedance circuit, it produces a larger than intended current; when it sees a very high-impedance circuit, it produces a smaller current than intended. These mismatched impedance’s redistribute the power in the circuit so that less of it is delivered to the load than the circuit was designed for, because the nature of the circuit is that it can’t simply readjust the voltage to deliver the same power regardless of the rate of current flow. What happens in an impedance mismatch between a source and load; power isn’t being transferred properly because the source circuit wasn’t designed to drive the kind of load it’s connected to. In some electronic applications, this will burn out equipment. A radio transmitter must be able to deliver its power into an antenna load that presents the proper impedance or it will self-destruct, and an audio amplifier can possibly be destroyed by attaching it to speakers of the wrong impedance.

Hopefully that is a rare occurrence. So why do we really care about impedance mismatches? The reason is that when impedances are mismatched, the mismatch causes portions of the signal to reflect. This can happen at the source, at the connectors, at any point along the cable, or at the load and when a portion of the signal bounces backward down the line, it combines with and interferes with the portions of the signal that follow it. This is why, in the case of a impedance mismatch your audio quality suffers. With digital video these reflections can cause a "sparkle" effect in your picture or a complete loss of picture.

So, when I say that the input impedance of HD SDI input jack is 75 ohms, that’s what I mean. But what does it mean to say that the impedance of the cable between the source and display is 75 ohms?

Well, first, it doesn’t mean that the cable itself presents a 75 ohm load. If it did, the total load would now be 150 ohms, and you’d have an impedance mismatch. Furthermore, if the cable itself constituted a 75 ohm load, that load would be dependent on length. So a cable twice as long would be 150 ohms, a cable half as long would be 37.5 ohms, and so on. In case it’s not obvious by now, another thing that it doesn’t mean is that the resistance of the cable will be 75 ohms. Resistance, which also confusingly happens to be measured in ohms, has nothing to do with characteristic impedance, which can’t be measured by using a VOM.

When I say that the characteristic impedance of a cable is 75ohms, or 50, 110, 300, or what-have-you, what I mean is that if we attach a load of the specified impedance to the other end of the cable, it will look like a load of that impedance regardless of the length of the cable. The object of a 75 ohm cable is simply to "carry" that 75 ohm impedance from point A to point B, so that as far as the devices are concerned, they’re right next to one another. If we take a hundred feet of 300-ohm television twin-lead cable, solder it to RCA connectors, and stick that in between the display and an an analog device, the load, as "seen" by the analog device, will not be 75 ohms. How bad the mismatch is, and what the consequences of it are, will depend on a variety of factors, but it’s fair to say that this sort of mismatch needs to be avoided.

Transmission line impedance is critical in some applications, and not so critical in others. In analog (line level) audio, impedance has become a non-factor as designers of these circuits dispensed with the idea of matched impedance’s completely and use what is called voltage matching instead.

The idea here is to engineer the equipment to have the lowest possible output impedance and a relatively high input impedance. The difference between them must be at least a factor of ten, and is often much more. Modern equipment typically employs output impedance’s of around 150ohms or below, with input impedance’s of at least 10kohms or above. With the minuscule output impedance and relatively high input impedance, the full output voltage should be developed across the input impedance.

Relatively high-impedance inputs such as these are called bridging inputs. They have the advantage that several devices can be connected in parallel without decreasing the impedance to any significant degree. The voltage developed across each input remains high and the source does not need to supply a high current. As an example, a mixing console output is feeding two tape machines. Each machine now has an input impedance of 30kohms. Connecting two in parallel will only reduce the combined input impedance to 15kohms, which is still substantially higher than the 150ohm output impedance of the console. Therefore the input voltage will be virtually unaffected. I calculate a loss of 0.04dB. Even connecting a third device to the output, the impedance would only fall to 10kohms and the level would fall by a further 0.05dB, which would not be audible. Because bridging inputs make studio work much easier, the idea of voltage matching is now employed universally in line-level audio equipment, irrespective of the actual reference signal levels used.

Back on topic now, the behavior of cables changes as signal frequencies increase. This is so because as frequency increases, the electrical "wavelength" of a signal becomes shorter and shorter. As the length of a cable becomes closer to a large fraction of the electrical wavelength of the signal it carries, the likelihood of significant reflections from impedance mismatch increases. The whole cable can resonate at the wavelength of the signal, or of a portion of the signal, and the impact on signal quality will be anything but good. Many signals are complex, occupying not a single frequency, but a whole range of frequencies. This is why we so often speak of the "bandwidth" of a signal, and so a mismatch will affect different parts of the signal differently.

Because the effects of impedance mismatch are dependent upon frequency, the issue has particular relevance for digital signals. Where analog audio or video signals consist of electrical waves which rise or fall continuously through a range, digital signals are very different. They switch rapidly between two states representing bits, 1 and 0. This switching creates something close to what we call a "square wave,", a waveform which, instead of being sloped like a sine wave, has sharp, sudden transitions. Although a digital signal can be said to have a "frequency" at the rate at which it switches, electrically, a square wave of a given frequency is equivalent to a sine wave at that frequency accompanied by an infinite series of harmonics, that is, multiples of the frequency. If all of these harmonics aren’t faithfully carried through the cable and, in fact, it’s physically impossible to carry all of them faithfully, then the "shoulders" of the digital square wave begin to round off. The more the wave becomes rounded, the higher the possibility of bit errors becomes. The device at the load end will, of course, reconstitute the digital information from this somewhat rounded wave, but as the rounding becomes worse and worse, eventually there comes a point where the errors are too severe to be corrected, and the signal can no longer be reconstituted. The best defense against the problem is, of course, a cable of the right impedance: for digital video or SPDIF digital audio, this means a 75 ohm cable like Belden 1694A; for AES/EBU balanced digital audio, this means a 110 ohm cable like Belden 1800F.

Fortunately, for most applications, it’s very easy to choose the right impedance cable. All common analog video standards and HD SDI use 75ohm cable, as do coaxial (unbalanced) digital audio connections. If you have balanced AES/EBU type digital audio lines, you’ll want 110 ohm AES/EBU cable. There are a few others you may bump into, however, and it’s good to be aware of them. RG-58 coax, such as is often used for CB or ham radio antenna lines and CATV, is 50 ohms, not suitable for video use. Twin-lead cable, the two wires separated by a band of insulation that used to be the most common way to hook up a TV antenna is a 300 ohm balanced line.

Connectors have impedance, too, and should be matched to the cable and equipment. Many BNC connectors, especially on older cables, are 50ohm types, and so it’s important to be sure that you’re using 75 ohm BNCs when connecting video lines. RCA connectors can’t quite meet the 75 ohm impedance standard because their physical dimensions just aren’t fully compatible with it, but there are RCA plugs which are designed for the best possible impedance match with 75 ohm cable and equipment.

Coming Impedance Part II, Speakers, Amplifiers and Nominal Impedance

Balanced / Unbalanced

September 28, 2013 in AV Design Tips, The Basics by Sam Davisson

UnbalancedWhen I did the first grounding post people seemed to think I was setting the groundworkk for talking about audio. I was actually setting the foundation for all AV. Although its true that any audio problems are related to grounding issues. Balanced and unbalanced interconnect systems, while mostly related to audio, are prevalent in all AV cabling.

RS-232 is an unbalanced interconnect system. RS-422 or RS-485 are balanced. Analog video and SDI are unbalanced. Analog audio, the pro format is balanced. The consumer format, unbalanced. Digital audio employs both types of signal interconnects as well.

Definition: Impedance –
A measure of the opposition to the flow of alternating current through a circuit. Impedance is measured in ohms. The resistance of a circuit to direct current (also measured in ohms) is generally not the same as its impedance, due to the effects of capacitance and induction in and among the components of the circuit.

Balance is defined in terms of the impedance of the two signal conductors with respect to a reference, which is usually ground. If these impedances are equal and non-zero, the system is balanced. If the impedances are unequal the system is unbalanced. A signal conductor with a grounded return conductor is, therefore, an unbalanced.

A small, common-mode, 60 Hz noise, voltage can exist between the chassis of two AC powered devices regardless of whether they are safety grounded (use a three-wire plug) or not. An isolated ground system is designed to minimize this.

Unbalanced Interconnections

An unbalanced interface uses only two conductors to carry the signal from one device to another, one conductor carries the signal and the other is the grounded return. In consumer audio systems this usually consists of a cable with a center conductor and a shield terminated in an RCA Phono plug.

The “RCA Phono” plug was developed by RCA many years ago to be used for short interconnects between a turntable and amplifier inside a phonograph. This unbalanced
interconnect system is simple and inexpensive, but as with many other connector systems has been adopted for uses other than originally intended. It has become the de-facto connector for consumer audio/video equipment. Note, however, that the design was original intended only for short cable runs within the same piece of equipment. Being an unbalanced system it is susceptible to common-mode noise voltages.

A problem occurs when there is a ground voltage (common-mode voltage) between the two interconnected devices. Because of this voltage, a small current will flow down the cable shield between the devices (sometimes referred to as common-mode current, or most commony as a ground loop current). If the cable shield were ideal (zero impedance) this current would not cause a problem. However, since the shield has a finite resistance, a small noise voltage will appear across the length of the cable shield. The magnitude of this voltage will equal the common-mode current times the shield resistance. This voltage is in series with the signal voltage and will add directly to it at the receiver. In other words, an unbalanced interconnect system has no ability to reject common-mode noise voltages.

This coupling can be referred to as common-impedance coupling, and is the result of the fact
that in an unbalanced two-wire system the shield is performing two functions. It is a shield carrying the common-mode noise current, but it is also one of the signal conductors carrying the return signal current

Consider a typical case of the interface between two grounded (3-prong AC plug) pieces of audio equipment. This example is typical some cases would be better and some worse. Also, note that calculating signal to noise is a future discussion.

The shield resistance of a fifteen-foot cable typically will be 0.25 ohms, depending on cable type used. If the 60-Hertz shield current is 250 uA, the voltage developed across the shield will be 62.5 uV. For consumer audio products the reference signal level is 300 mV (-10 dBV). The signal to noise ratio will therefore be 74 dB. In a high end consumer audio sysem, we would most likely be able to hear some 60-Hertz hum in quiet passages of the program material.

You could conclude, at this point, that ungrounded equipment, those using a 2-prong AC plug, might solve this problem by eliminating the ground connections. This sometimes helps, but it does not necessarily eliminate the problem.

For ungrounded equipment, the common-mode ground current can still flow through the inter-winding capacitance of the power transformer. The impedance of this capacitance will normally reduce the current to 100 uA, or less. However audible noise may still be present.

The impedance of the inter-winding capacitance is frequency dependent, more current will flow at higher frequencies (harmonics of 60 Hz) than at the fundamental frequency (60 Hertz). Therefore, the interference will consist of a high frequency buzz which is more audible by the human ear than the 60 Hz hum.

Despite its shortcomings, this unbalanced system works well. Especially in cases with short cable runs, and the equipment within the same rack so there is very little, or no, 60 Hz voltage between the chassis of the interconnected devices.

Trouble shooting and eliminating audio issues will come in future posts but understanding the issue at hand should help you develop techniques of eliminating problems before they occur making troubleshooting unnecessary.

Advantages of Twisted Pair Wire

Twisted pair wiring, even when unshielded, is very effective in reducing magnetic field coupling to and from the wire pair. There are only two conditions necessary for this to be true. First, the signal must flow equally, and in opposite directions, on the two conductors.

Secondly, the length of the twist must be less than one twentieth of a wavelength at the frequencies of concern. (One twist per inch will be effective up to about 500 MHz).

The above is true whether the terminations are balanced or not. In addition, if the terminations are balanced, twisted pair wiring will also be effective in reducing electric field coupling to and from the wire pair. Even though field coupling is not the primary noise coupling mechanism in audio systems, it is still a good practice to always twist the signal and return conductors in a cable. (Twisting is especially important in the case of microphone cables.)

Balanced Interconnections

For a balanced interconnect system both of the signal conductors have an equal, and non-zero, impedance to ground. Therefore, three conductors are required, signal+, signal-, and ground or shield.

Professional audio installations require long cable runs of 500 feet or more and have to be able to operate at microphone signal levels of 3 mV (-50 dBV) as opposed to a line level of 300 mV for consumer audio. As a result he maority of professional audio equipment is designed using a three-conductor balanced interface (two signal conductors,signal and signal return, and a grounded shield), using XLR connectors, or in some cases 1/4 inch phone plugs.

This three conductor balanced interconnect system avoids the problem of the shield having to serve two purposes. The signal is now carried on the two internal conductors (usually twisted together) and the shield only acts as a shield and not also as a signal return line. The penalty for this improvement in performance is a more complicated and hence more expensive system.

A 60Hz shield current flowing between two interconnected devices will still produces a voltage drop in the shield, but this noise voltage is not in series with the signal. Rather it will be coupled equally (as a common-mode noise voltage) into both signal conductors.

Since the receiver looks at the difference between the two signal conductors (not the voltage between one of them and ground), the common-mode noise voltage cancels out and is not seen by the receiver.

A balanced interface theoretically would be completely immune to noise and interference. But, in the real world, nothing is perfect. Even if you attempt to make the impedance of the two signal conductors to ground the same there will be some difference, if only a fraction of a percent, and this will limit the degree of common-mode voltage rejection and therefore the maximum noise suppression possible.

For this example I’ll assume that the impedance balance is such that the circuit can provide
60 dB of common-mode noise rejection, a very conservative assumption, and that the other
parameters are the same as in the unbalanced system above. A well designed balanced interface can have as much as 80 to 100 dB of common-mode noise rejection.

Conclusions

When using unbalanced interconnects between audio equipment the primary noise-coupling mechanism is due to common-impedance coupling. The cable shield resistance
is the common-impedance, and any small 60 Hz AC potential between the equipment chassis is the noise source producing a common-mode noise current on the cable shield (ground loop
current).

This problem can be minimized, or eliminated, by any one of the following approaches:

  • Minimize the AC voltage between the two pieces of equipment.
  • Minimize the cable shield resistance.
  • Block the common-mode noise current by using a signal isolation transformer.
  • Use a balanced interconnection system.

Power and Grounding

September 7, 2013 in AV Design Tips, The Basics by Sam Davisson

Isolation, the key to a good grounding structure

This is the first of an upcoming series on “The Basics” of designing the AV systems that high end clients expect and the average client will respect.

Quality AV systems don’t happen by accident. They happen by design. Just like building a house, you’ll never reach that true quality threshold without a proper and strong foundation. With all electronic systems, especially AV, that foundation is the grounding structure.

Most designers and installers of AV systems think of grounding as black magic. How often have you or someone you know said that a cable is “picking up” noise or that the solution is “better” shielding or floating the shielding? (Note: In very high end audio systems floating the shield can be essential but in the typical AV installation, if grounding is done properly, should not be required.) Equipment manufacturers are typically of no help as they don’t have a clue as to what’s going on because they design and test their equipment in a defined, never changing environment.

The basic rules of physics are overlooked, ignored, and / or forgotten. Electrical engineering courses rarely even mention practical issues of grounding. As a result, myth and misinformation have become the normal

The preferred grounding technique for AV equipment, which contributes to this sense of mystery, is isolated grounding. The reason for this is noise interference and particularly what is known as common mode noise. In AC power systems, the difference in potential between neutral and ground is one form of common mode noise, since any change in neutral potential relative to ground also affects all of the other power circuit conductor potentials to ground. A more troublesome form of common mode noise is the differences in ground potentials throughout an electrical system. When AV devices are interconnected byway of audio, video or control cables, any difference in ground potentials between the interconnected pieces of equipment is common mode noise to the audio, video or control circuits.

Properly Conceived Isolated Grounding System

Properly Conceived Isolated Grounding System Through a 3 Phase Transformer

There is much confusion over what an “isolated ground” (IG) is, how an isolated ground technique is implemented, and why it is used. Isolated ground is allowed in the U.S. by the National Electrical Code (NEC) and in Canada by the Canadian Electrical Code (CEC). But it should be noted that in both cases, isolated ground is an exception to the standard grounding requirements. NEC 250-74 and 250-75 allow IG wiring only where required for the reduction of electrical noise on the grounding circuit. In practical terms, what this means is that not all electrical contractors or electricians are versed in building isolated ground systems.

Common Myths About Earth Ground and Wires

Unfortunately, as electronic equipment developed the term “ground” became sort of a generic term. In AC power systems ground refers to a common connection point, typically earth ground. In a DC power system, such as what is powering my laptop, ground also refers to a common connection point for power return. Thus, the very meaning of the term ground has become vague, ambiguous, and often quite fanciful but make no mistake these grounds should never have thier common connection point in common.

Some electricians have a strong urge to reduce unwanted AC voltage differences by “shorting them out” with massive conductors, the results are most often disappointing. Other electricians think that system noise can be
improved experimentally by simply finding a “better” or “quieter” earth ground. Many indulge in wishful thinking that noise currents can somehow be skillfully directed to an earth ground, where they will disappear forever!

Here are some common myths about grounding:

  • Earth grounds are zero volts – presumably with respect to some “mystical absolute” reference point. This leads to fanciful ideas about lots of ground rods making system noise disappear. The fact is soil resistance between ground rods is much higher, often significantly than a wire between them.
  • Wires have zero impedance (is the apparent ac resistance of a circuit containing capacitance and/or inductance in addition to pure resistance) – therefore they can extend a zero-voltage reference to many locations in a system, eliminating voltage differences. In fact, wires are quite limited:
    • The DC resistance of a wire applies only at very low frequencies and is directly proportional to its length.
    • The inductance of a wire is nearly independent of its diameter (gauge) but is directly proportional to its length and increases at bends or loops.
    • A wire resonates (becomes an antenna) when its physical length is a quarter wavelength

Something to consider, are earth grounds even necessary for low noise system operation?

Equipment Rack Treatment for Isolated Ground SystemUpdate 9/11
To answer the above question: no earth grounds are required for safety, not for low noise system operation!

But what I really wanted to update here today, something I left out of my post, was the equipment rack treatment an AV integrator needs to do in order to maintain a nice clean isolated ground system.

Unless your are installing the equipment rack on a wood floor, the rack must be isolated from the floor. There are a number of ways to successfully accomplish this. While not called out, the drawing above used wooden 2×4’s underneath the rack but many equipment rack manufactures also sell ground isolation kits made from polycarbonate strips that work perfectly fine.

If your bolting the rack down you’ll need to use a nylon washer to keep the bolt itself from making the electrical path. If your simply setting the rack on the floor with no requirement to bolt it down be sure to include leveling feet with rubber covers to provide your isolation. Finally, if the rack is portable, the rubber wheels should do a good job of providing the required isolation.

Install a copper bus bar in each rack. This bus bar is connected to the isolated ground system (at the isolated ground sub panel) and then connected to the equipment rack via a 12 gauge stranded wire. The bus bar must be isolated from the equipment rack at all other points. Additionally most AV equipment provides a grounding lug on the back of the equipment. Again use a 12 gauge stranded wire to connect equipment chassis to the copper bus bar. Note: some older equipment may strap signal ground to chassis ground. If this is the case be sure and remove the strapping mechanism.

All AV conduits coming into the rack must be isolated from the rack. Most electrical supply outlets provide isolation bushings for this. In addition your equipment rack vendor may be able to provide isolation knockout panels. Finally, be sure and use an approved IG power strip.

Following these guidelines will ensure that your IG system remains so.

As I discuss future AV basic design topics grounds and dealing with grounding issues will be a constant theme. As I said, we are simply setting the foundation at this point, not only of your AV system but for future posts as well.

Isn’t Technology Grand!

August 20, 2012 in AV Design Tips by Sam Davisson

Technology for Technologies SakeI’ll start with a little site notice first, an upgrade from the previous site. I’ve added a download page where I will be adding design tools, sample drawing packages and just about anything else I develop or find that I think is cool enough to add to my own site to share. The link is obviously on top but I also linked a little of the text above mostly for google and friends to enjoy and spider. This blog is mostly about AV system design tips and issues so it only seems appropriate that I share what helps me in the design process.

One of the things an AV design engineer has to be careful with letting technology and not client needs rule their design. Technology is coo and I would assume that most of us got into this business because of our love of technology. I often see pieces big and small that I would love to find a project I could design them into. Seriously, this is really cool but I doubt I will ever have a client who needs this

Maybe that is a little over the top when it comes to an example… or maybe it is way over the top so how about I come up with another one. In one of my LinkedIn groups the following design requirements were posted: Shooting club wants to add a camera 1500′ down range that will feed a display at the shooters location. The club doesn’t have any real budget and is hoping to do this themselves and are just looking at what they may need.

The solution, to me, seems pretty simple and pretty old school. You are only feeding a display, no recording devices and no one cares if the video meets broadcast specifications. Just mount the camera, run a good coax cable and be done with it. If the picture isn’t quite good enough add a distribution amp. The focus of the whole thing is for the shooter to be able to see the target and where his bullet hit it. But it is an old school solution where the only quality factor you are taking into account is if there is enough signal to make a viewable picture on the screen.

Or, if your not quite sure that is going to work there are analog baluns that will do 1500′ over a UTP (Unshielded Twisted Pair aka Cat 5e cable). Personally I’d opt for testing the coax cable solution because I could always reuse a hunk of cable that long on a different project so a very small risk is involved. He could even teach them to terminate the cable ends themselves or take 15 minutes of his time to terminate them.

But, my suggestion was in the minority. The majority seemed to be split between differing UTP and fiber solutions. I’ll be honest here, I didn’t analyze the various UTP solutions to see if they would work. A lot of UTP stuff is distance limited to 330′. I’m sure some of the ones suggested were and some were not but I just didn’t see a reason to evaluate them.

Is there any question that fiber would be the best in technology solution. None what so ever but… single mode fiber (which is what is needed for over 1000′) is kind of expensive. Fiber transmitters and receivers are very expensive. Add in the fiber splice and terminating the ends and you’ve got a solution of around 10k as opposed to less than 2k and a fiber solution is not a DIY solution for people outside of the industry.

In summary, technology is great. It is fun to keep up with and always makes me want to drool and use it on my next design but technology is a tool and it is the design engineers job to come up with the right set of tools for the job and the job is satisfying the clients needs.

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