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Audio 101: Audio for Video Dummies

What they don’t tell you in the manual about all those plugs

No doubt you’ve got it patched together and it sounds OK. Like they say, if it ain’t broke, don’t fix it. But if you’ve got a little distortion around the fringes, you might want to pay a visit to the local Radio Shack and change some things. First, time for some basic audio electronics. While what we’re really talking about is moving electrons from one atom to another within a wire in a pattern analogous to sound waves (thus “analog” audio), one of the best analogies to use is that of flowing water.

TERMS

Here are some common terms you need to know:

CURRENT: This is the sheer volume of electrons (not loudness) that is flowing from the source to the receiver. It’s like looking at a river: the mile-wide Mississippi carries a lot more water downstream than does, say Turkey Run, down the street here. Current is expressed in Amperes (Amps), and in audio we deal with very small amounts of current, compared to, say, your air conditioner or kitchen stove.
VOLTAGE: This is the PRESSURE of the electron flow. Current and voltage are related; as you know when there is a lot of current (as in a flood), the water runs much faster (pressure) – so much so that it can sweep a house off its foundation. But when current and pressure are low, the same stream can be a calm and peaceful sight. That’s why you don’t want to plug your speaker directly into a wall socket!
AMPLIFIER: This is the “pump” that boosts the voltage and current to usable levels, the way a water pump would do to supply your house with water. Amplifiers come in all sizes, and the audio signal goes through at least 3 before it gets to your speakers. More about that later.
RESISTANCE: Resistance is exactly that. It’s the natural resistance to the flow of electrons, like rocks in the river. In electronics, unless you have an absolute-zero superconductor, all materials have some resistance to the flow of electricity. The amplifier (pump) has to be able to push hard enough to get the electrons (water) out the other end with enough useful strength. Resistance is expressed in Ohms.
CAPACITANCE: This is the ability of certain devices (capacitors) to hold a charge – the way a water tank could hold pressurized water and thus maintain pressure when the pump’s pressure goes down. Capacitance is expressed in Farads
INDUCTANCE: The closest water analogy to inductance is a double boiler. One pot of water heats another pot of water that's immersed in the first. The internal container separates the two volumes of water, but one "induces" the other to heat up. Inductance has to do with the way electrons behave when a magnetic field passes through a coil of wire. In fact it’s how electricity is generated in the first place. Rotating magnets near coils of wire “induce” the electrons to move. In our back yards, and in electronic components, the most typical inductors are “transformers”. In a transformer there are two coils of wire in close proximity. One is connected to the source, the other to the “load”. With alternating current, the source current “induces” voltage into the second coil, so alternating current goes to the load. Depending on the number of turns of wire in the two coils, the voltages can be changed without directly coupling the two sources (a la the double boiler). Inductance is expressed in Henries.*
REACTANCE Capacitance and Inductance do strange thing to AC signals. It’s known as “reactance”, and it’s best analogous to the eddies and whirlpools that you might find in flowing water. Depending on frequency, reactance will increase or decrease current flow, which is how "Equalizer" or tone control circuitry works. Wow, that’s too detailed for this course!
IMPEDANCE is a measure of the combined effects of Resistance and Capacitive and Inductive Reactance to the flow of electrons in a circuit. It’s measured in Ohms. In terms of water, impedance is the size of the pipe. The smaller the pipe, the higher the impedance, and the less "water" it takes to fill it up. So a 600-ohm professional line is a much bigger pipe than a 10k-ohm consumer line.
LEVEL: Audio is like water filling a pipe and keeping it filled just right – not too much, not too little. There is a range of quantity and pressure that the pipes and valves and everything can support. That varies depending on the kind of piping we’re talking about. In audio things are referenced to a standard voltage, that being 0.775 volts of a sine wave at 1khz. That’s known as 0db (decibels). Consumer electronics are usually designed to operate at –10db (0.0775v), and professional electronics at +4 or +6db. (There’s also mic level which is –40 to –60dB, but that’s a topic for audio 301).

Now why is all this important?

The Hose Analogy

Analogies are never perfect, but say we have an arcade game at the state fair. Instead of rifles at the counter we have two hoses, a 6” fire hose, and a ¾” garden hose. Across the way we have two target funnels – one has a 6” outlet, and the other has a ¾” outlet. Water flows from the hoses at the same pressure (voltage), but obviously, the quantity of water (current) is greater from the fire hose, and if you look out back, there's a much bigger pump hooked to the fire hose. The object of the game is to keep the output pipe from the funnels filled without overfilling (and for the sake of our analogy, we’ll say our aim is always dead on.)

So we first start with the funnel that has an outlet the size of a garden hose. When we point the garden hose at it, the water washes down the pipe at the same rate it comes in, and all is happy. Likewise when we point the fire hose at the big funnel, the same thing happens.

But if we try to use the fire hose to fill the small funnel, the water quickly overwhelms the funnel and splashes all over the place. If we could, we might try to constrict the fire hose to slow the quantity, but when we do the pressure goes up and it's hard to control and it causes the pump to overheat. If we slowed the pump, we might be successful in keeping it just filled, but then the pump doesn't work at peak efficiency. (Voltage and Current are related this way. The formula is called "Ohm's Law.")

Similarly, if we shoot the garden hose into the fire hose-sized funnel, the funnel never fills. We're losing on our goal to keep the pipe full as there's lots of air getting in, so let's crank up the little pump and see what it's got! If we get the pressure up high enough to start to fill the funnel, before long the little pump that could will become the little pump that couldn't, since it's not designed to fill fire hoses.

Moral to this story: When the output matches the input, things are great! When there's a mismatch, you got problems.

Going from pro pipes to consumer pipes:

A 600-ohm, +4dB professional circuit has fire-hose sized pump when compared with a 10k-ohm –10dB consumer funnel. So when you try to push a 600-ohm output into a 10k input, you can easily overwhelm the input creating distortion. Worst case, this can damage the 10k input because of too much current. You can turn down the output of the professional circuit so it won’t distort, and if this works for you, fine, but it’s not perfect.

Going from consumer to pro:

A 10k-ohm -10dB consumer circuit has a tiny pump and can’t fill the 600-ohm pipe. So the pump goes into overdrive and you get a distortion called ‘clipping’. And you have to turn up the gain on the +6dB input because it’s expecting a loud signal and not getting one. This raises the “noise floor”, causing hiss, like the air that gets into our big funnel. Again, worst case you burn out your tiny driver amplifier or pump.

In either case, even if you don’t hear audible distortion, the differences in impedance can “color” the frequency response of the system, causing inaccurate reproduction. (Though, for most video applications on tiny TV speakers, that’s not a big deal.)

What’s the solution to this?

The principle is simple: like at our carnival attraction, MATCH the impedances and levels. Here are four suggestions on how to do that:

Most professional video machines have back panel switches to set impedance and/or level. If not, there may be a sofware switch in the menu. If only one of your units has a switch, set it to match the other one. (If everything has consumer-grade RCA plugs, don't worry about it. Patch away!)

A simpler solution is to use a balancing transformer. One side of the transformer is 600-ohm (usually balanced XLR connector), and the other side is 10k-ohm (usually a ¼” plug). Just connect this in line between the two components and you’ll be set. They’re on the rack at Radio Shack - though better ones can be found at pro audio dealers. A balancing transformer will match impedances, but not levels. They're best used when going from pro to consumer, that way you can turn down the pro "pump" to match levels.

At a pro audio dealer you'll find something called a "direct box." This is a little box that does both jobs, matching levels and impedance. You'll need two for stereo. Be careful in shopping because some work at "mic" level and others work at "line" level. Ask you pro dealer for help.

And a 4th option is to use a mixer. Most mixers have balanced 600-ohm XLR inputs and unbalanced 10k-ohm ¼” or RCA line level inputs on each channel. They also have outputs in both flavors. Choose the inputs and outputs that match. Mixers will match both impedance and level.

Now if you have both pro and consumer connections available to you, I recommend using the professional levels with XLR balanced connectors. Here’s why:

DEFEATING THE NOISE ENEMY

Any time we connect two pieces of equipment we’re asking for trouble from electromagnetic interference from outside sources: hum from power cords, buzz from room light dimmers, or even interference from computers. To combat this there are lots of tools at our disposal, and this is the reason pro equipment has the big pipes:

HIGH LEVEL: LOW NOISE

Remember the Dolby system? It works by turning up the recorded levels of low level high frequency sounds, and then lowering them on playback, with the net effect of lowering the noise floor. If your system can support high-level sound, this keeps the noise floor low, and any induced noise picked up by the cables stays inaudible.

SHIELDS UP

Most audio cables use shielding of some kind. The idea behind shielding is that the shield will divert stray electromagnetic fields around the signal wire so it isn’t amplified. (A mini Farady Cage). The shield must be connected to an earth ground to carry away the charge.

UNBALANCED LINES

An unbalanced line sends the signal down a single wire surrounded by the shield. To complete the circuit, the signal returns to the source through the shield. For short runs, this is usually OK, but if the wire has to be run a long way, it can pick up enough interference to be audible. Most 10k-ohm –10dB (consumer) lines are unbalanced because the amplifiers (pumps) are too small to send the signals very far anyway. And they cost less.

BALANCED LINES

This has nothing to do with diet.

Balanced lines use 2 wires surrounded by a shield. The signal starts down the wire on the “high” side (usually XLR pin #2), and returns to the amplifier on the “low” side (pin #3). This creates a “floating” circuit within the grounded shield, so the interference that’s being carried away to ground doesn’t get into the audio signal at all. That, coupled with the “big pipes” of a 600-ohm +4 or +6dB circuit, means you can run a balanced line hundreds of feet with no noise or interference. For that matter, we commonly run mic cables that long with no noise for that very reason too.

UNBALANCED DIETS

The best way to convert from a balanced to unbalanced line is through a balancing transformer or a mixer or other active converter. But in a pinch you can unbalance a line. You do that by simply taking the “low” pin of the XLR plug and shorting it to ground (pin #1). This is NOT recommended, and you must be very careful with mixers that have “Phantom” power sources, because you can blow them up with one of these cables.

Well, that wraps up Audio 101 – audio for video dummies. If you found this article useful, please click here to let us know! And if you'd like more training of this nature, stay tuned to www.painfreepresentations.com for upcoming training events.

* Interesting Facts about Inductance:

[DON’T TRY THIS AT HOME, BUT – If you have a big power transmission line crossing your back yard, you can string wire across a couple of poles parallel with the transmission line and actually get enough electricity to power your house. But this is VERY dangerous – so don’t try it at home. It’s a large open air transformer.]

Another interesting phenomenon with coils or transformers: If no load is present on a transformer, a phenomenon called “back EMF” (electromotive force) actually blocks current flow through the transformer, effectively shutting it down. That's why your doorbell transformer can stay on for years and not cost anything to run.