Digital Technical Theater Simplified

High Tech Lighting, Audio, Video and More on a Low Budget

Drew Campbell

• Publisher: Allworth
• Year: 2011
• ISBN:
  9781621533740
• Topic: Media & Performing Arts
• Subtopic: Theatre Stagecraft & Scenography

The theater is in the midst of a digital revolution! This book provides readers with an easy-to-understand overview of the digital technology currently available for the stage. In clear language, Digital Technical Theater Simplified explains digital technology in the fields of lighting, audio, video, and show control. All chapters contain do-it-yourself examples of how anyone can use these advanced technologies, as well as case studies of "How the Pros Do It."

 

 

Part 1: Audio

 

CHAPTER 1

Audio: An Introduction

Like lighting, you will need to have a general understanding of theatrical audio practice in order to understand what follows. If you feel like you’ve already got this knowledge in your pocket, skip ahead to chapter 2. If you aren’t sure if you are ready to enter the realm of high-tech audio, try the following sentence:

“I want to put a microphone into the mixer, but the only available channels are for line-level signals and don’t have XLR inputs.”

All clear on that? If not, read this chapter before you skip ahead.

There Is No Sound Without a Source

An audio system is composed of a string of devices, all of which add up to form a signal chain. The beginning of that chain, like the beginning of a great river, is the source.

Sources come in three varieties: microphones, everything else analog, and digital. Let’s do microphones first.

Microphones

Sound, when it is traveling around the real, nonelectronic world, is composed of waves of vibration in the air. If there is no air, there is no sound. That is why “in space, no one can hear you scream.” That’s not a scary thought, it’s just a practical one. Sound needs air.

When I slap my hand against the table, the impact of my hand causes the table to oscillate back and forth very quickly. When the table moves away from my hand, it creates an area of low pressure in the air immediately next to the table. When the table rushes back toward my hand, it squeezes the air, creating an area of high pressure. As the table continues to oscillate back and forth, it creates waves of differing air pressure that travel outward from the table, filling the room, just like throwing a stone into water creates ripples that travel outward, filling the pond. The ripples are composed of peaks of high water level separated by valleys of low water level. The sound waves are composed of peaks of high air pressure separated by valleys of low air pressure.

When those waves of pressure run into something, they cause that something to move. The high-pressure waves cause that thing to move away from the air. The low-pressure waves cause that thing to move back toward the air. In this way, the vibration of the table is carried through the air until it meets another object. If that object is very dense and heavy, like a concrete wall, it will vibrate very little. If that object is very light, like your eardrum, it will vibrate quite a bit. This is how we hear sounds. Our eardrum is extremely light and susceptible to the vibrations caused by air movements. When the waves of air pressure strike our eardrum, it vibrates at the same rate as the wave of air pressure. The eardrum, in turn, is attached to a number of nerve cells, which pick up those vibrations and transmit them to the brain, which figures out that you just “heard” something.

Think about that for a moment. When a week-old kitten across the room from you lets out a tiny little “meow,” the vocal cords of that newborn animal are vibrating a tiny amount of air and creating minuscule waves of air pressure that travel across the room and strike your eardrum, which is sensitive enough to not only register them but also pick out the pitch, timbre, and direction of the sound so that the brain can put them all together and conclude “kitten.” Truly, the human ear is a marvel.

That’s one reason why a microphone is designed a lot like a human ear. In essence, a microphone is an artificial ear, but instead of an eardrum, it has a tiny little strip of material called a diaphragm, which vibrates when struck by pressure waves in the air. The diaphragm is attached to a magnet, which moves whenever and however the diaphragm moves. The magnet, in turn, is floating inside a wire coil. The movement of the magnet inside the wire coil creates tiny fluctuations of electrical current, which are sent down a wire into an electronic device of some sort which “hears” the sound.

Microphones come in a dizzying variety and entire books are dedicated to this piece of technology alone.¹ For our immediate purposes, you need to know that the electric signal produced by a microphone—known as a miclevel signal—is extremely weak, and requires amplification before you can do anything with it. Hence, if you are going to plug it into a mixer or other audio device, you need to make sure that the device has a mic-level input, where the signal can be pumped up to a useful level by a preamp a small amplifier that raises the mic-level signal up to a line-level signal, the common signal strength that is used inside sound systems.

All microphones from midlevel on up use a three-wire cable and a three-pin plug, known as an XLR. Don’t use a mic that has any other kind of plug on it—it is low quality and will cause you problems sooner or later. Those other plugs, and the cables that go with them, have many uses, but mic signals are not one of them.

Mics use XLR cables because they can run balanced signals. Basically, a balanced output splits the audio signal into two signals, then flips one of them over, making a “negative” version of the original. The balanced output sends the two signals, one positive, one negative, through the cable in side-by-side wires. If some kind of interference, like a stray electrical signal from a power cable, hits the wire, it distorts both the positive signal and the negative signal equally. When the signals get to the other end, however, the balanced input flips the negative signal back to the right way, thus reversing the effect of the distortion. When the two signals are added back together, the distortion in the positive signal is cancelled out by the flipped-over distortion in the (previously) negative signal. Result: a clean signal.

Always use XLR outputs, cables, and inputs with microphones. This ensures that you are using a balanced signal. Otherwise, those tiny little mic-level signals will get wiped out by interference.

Line-Level Sources

Mics are not the only things that produce sound, but they are the only things that produce mic-level signals. Everything else that produces sound—CD players, iPods, DVD players, electronic keyboards, computers, cable boxes, and any other electronic device—produces a line-level signal.

Line-level signals use several different types of plugs, but the phono plug is most common. This is a long, barrel-shaped plug that comes in 1/8" and 1/4" versions. That plug on the end of your iPod headphones is an 1/8" phono plug. Also common is the RCA plug, which is often used for consumer electronic equipment like your DVD player. The RCA plug has a round collar, about 1/4" wide with a stubby little pin in the center. It is used for both audio and video at the consumer level.

Line-level signals can and sometimes do use XLR plugs, particularly on pro-level gear.

Both mic-level and line-level signals are actually not a single level; these terms refer to a range of levels. A high mic level is still less powerful than a low line level. Think of it this way: A high school teacher’s salary may fluctuate up and down over the years, but it is still in the range of high school teacher salaries. The CEO of General Motors has a much higher salary, which may also fluctuate up and down. The range of the teacher’s salary, however, will never be anywhere near the CEO’s. Which is just wrong, by the way.

Digital Sources

All of the items listed above, including microphones and all the line-level devices, produce analog audio, even though the devices themselves may be digital. Your MP3 player is actually storing and playing a digital music file, but the last thing the player does before it sends the signal to the outside world is a digital-to-analog conversion, where it turns the stream of digital 1s and 0s into a continuously varying, analog electrical signal. This is the signal that goes into your headphones.

There are an increasing number of devices, however, that put out a signal that is still fully digital. This kind of a signal won’t do you any good in your headphones (your ears are analog, after all), but it is a good way to pass a signal to another device without losing any signal quality.

Digital signals do not have a level—they are just a stream of 1s and 0s—so mic level and line level don’t apply. We will discuss the various types of digital signals in chapter 5 on digital gear.

Sources Must Be Mixed

Now that you have some sound, whether analog or digital, you usually have to combine, or mix, several sources together. Enter the mixer.

Of any piece of audio gear, the one that seems to inspire the most reverence (among technicians) and confusion (among nontechnicians) is the mixer. With its seemingly endless rows of buttons and knobs, the mixer can look fairly intimidating.

Mixers are primarily described by the number of inputs and outputs that they have. A twelve-by-two mixer has twelve places to plug something in and two places to take sound out. Any source that has stereo sound will require two inputs for left and right. A microphone takes a single input.

On a decent mixer, the microphone input will be an XLR and will be equipped with an input level control, which allows you to adjust how much preamplification the mixer will do to turn the mic-level signal into a line-level signal.

Line-level inputs will be phono jacks or the oh-so-clever combo jack, which can take either an XLR or a phono plug. Digital signals must plug into digital inputs—we’ll get to that in chapter 5.

Each input feeds into a mixer channel. Each channel has its own string of controls that allow you to change various aspects of the sound. Depending on the size and quality of the mixer, it can perform various operations on a signal, but it all really breaks down into four things:

Level

A mixer can change the volume, or level, of each channel. It can do that by changing the input level, by changing the channel level, and by changing the master level. You use the input level to get all the channels to be in the same general area. Then, you use the channel level to create the mix of sound that you want. Finally, you use the master level to control the overall level of sound coming from the mixer. At the channel level, there is usually a mute button to quickly shut the channel off and a solo button to shut all the other channels off, so that you can hear one channel all by itself.

Equalization

In the world of audio, the frequency of the audio wave—whether it is waves of pressure in the air or waves of electrons in a cable—determines its pitch. Every sound, except very pure “test tones,” is composed of a collection of different frequencies. Equalization, or EQ, got its name because it was originally designed to balance out or equalize all the frequencies of a sound, so that you had equal amounts of energy across the audio spectrum. What we really use it for is to adjust the “timbre” or “color” of a sound, increasing or decreasing various frequencies. There is a very simple form of equalization on your stereo: the bass and treble controls. The bass controls the lower frequencies, while the treble controls the higher frequencies.

A mixer channel will almost always have at least two EQ controls: high and low. The more expensive the mixer, the more EQ controls it will have. A very good mixer might have five or more controls, each of which can be set to increase or decrease a particular set of frequencies.

Routing

A mixer channel will often have the ability to send the signal to an auxiliary send, also known as an “aux” or a “send.” This is another output from the mixer, separate from the master output, which is used to send a portion of the signals somewhere else. Sometimes the signal is sent away for good, so that it can feed a monitor speaker, headset system, or other devices. Sometimes it is only sent away temporarily, to be processed by another device like an effects processor. In that case, it comes back into the mixer through a return and is added back into the mix. This is called an effects loop.

Signals might also be gathered together into a submaster. This is useful if you want to gather together a certain group of signals to control their collected volume. Rock-and-roll mixers for example, might gather together all the signals from the microphones that are pointed at a drum set. That way, they can control the entire drum set volume with one slider or knob.

Pan

We have two ears, so audio signals are frequently divided up into a stereo pair with one signal intended for each ear. The pan control on a mixer channel determines the relative strength of the channel output in the left and right outputs. If you pan the signal left, you increase the strength of the signal that is going to the left output, while decreasing the right one. Pan the signal to the right and you get the opposite effect.

The reason that mixers are covered in a blanket of knobs is that all of the controls for...