Module 45

Updated: 07/10/2005

Wrapping
Up Audio

Audio Level Control Devices

Although manually maintaining audio levels is generally the best approach, there are some automatic devices that can help-and even do some things that you can't do manually.
  

AGC Circuits

We'll start with a simple audio control circuit, one that is built into most consumer audio equipment. If the average audio level is low, an AGC (automatic gain control) circuit will raise it; if the average level is too high, the circuit will bring it down.

Even though AGC circuits can free you from having to worry about manually controlling audio levels, they can't intelligently respond to different audio needs.

For example, in an attempt to bring an audio level up to a standard setting when no other sound is present, AGC circuits will make annoying background sounds louder. During pauses in dialogue you may notice that the loudness of background noise increases. If subsequent audio processing circuits (in editing equipment, for example) have AGC circuits, the problem gets progressively worse as each piece of equipment further increases background noise.

AGC circuits can also introduce a reverse problem. Since they respond to loud noises by quickly pulling down audio levels, this means that words can be lost when an AGC circuit reacts to a loud sound, such as someone bumping the microphone.

In professional camcorders audio levels can be manually controlled, but in many nonprofessional camcorders the AGC circuit can't be switched off.

Because of the effect of the AGC circuit in bringing up sound levels during a period of silence, the first few seconds of audio may be distorted until the AGC sets the proper level.

To get around this problem, many videographers (stuck with an AGC circuit they can't switch to manual control) have the on-camera talent say a few words just before the actual start of the segment. This can be simply counting, "5, 4, 3, 2, 1," to allow the AGC to adjust proper audio level. This countdown is simply deleted during editing.

Compressors

Audio compressors also bring up low amplitude sounds and pull down the amplitude of loud sounds-but they are much more sophisticated than AGC circuits.

Unlike AGC circuits, compressors can be adjusted so that many of the negative effects of automatic control go unnoticed. Program audio that has been compressed seems louder to the ear than non-compressed audio-a feature that hasn't escaped the attention of the producers of TV commercials.

Compressors typically have three controls:

• threshold, which establishes the audio level were compression begins
   
• compression ratio, which determines the amount of compression (which would be like expanding or narrowing the area on the right side of the illustration above)
   
• gain, which is simply the maximum output level

Some compressors have only two controls: input and output levels.

By raising the input level while keeping the output the same, a greater compression is achieved-at least until major distortion becomes evident. The compressor shown here has VU meters for monitoring the input and output levels.

Compressors and AGC circuits can create problems with music. Although AM rock radio stations of the 1960s and 1970s may have preferred a maximum-loud sound, the artists often complained that their carefully balanced audio levels were destroyed. Everything in the recording, whether intended to be loud or soft, came out sounding about the same.
  

Limiters, Peak Limiters

A basic audio limiter isn't as sophisticated as a compressor or even an AGC circuit. As the name suggests, limiters simply keep the audio from exceeding a set level-generally 0dB.

By setting a limiter at 0dB, for example, you can be assured that a sudden loud noise, such as a door slamming, will not "pin" the VU meter and cause major audio distortion (and possibly jar listeners out of their seats!).
  

Audio Expanders

Although they have more limited use, audio expanders increase the dynamic range of audio that has been overly processed. (Audio that has gone through satellite relays, for example, often ends up being overly compressed.) Expanders can restore the audio to its normal range and in the process, reduce noticeable background noise.

Audio Filters

An audio filter can be used to cut or attenuate audio frequencies above or below certain points.  For example, you may need to reduce or eliminate the low rumble of air conditioning or the hum of alternating current.  In both cases a filter that eliminates frequencies below about 120Hz may solve the problem.

On the other end of the frequency range, you may want to try to eliminate upper range frequencies associated with the rustle of clothes or paper.  For this you can try cutting off everything above about 8,000Hz.

By cutting all frequencies below about 2,000Hz, you can simulate the sound of a telephone conversation-or possibly a radio or TV in the background of a dramatic scene. You can use a graphic equalizer to do this, or on some audio boards, switch a filter into an audio channel.

Production Communication Systems

PL Systems

Since a live, multi-camera TV production typically involves the closely coordinated efforts of numerous people, reliable behind-the-scenes communication links are critical.

Using a PL (private line or production line) headset such as the ones shown here, production personnel can talk to each other and receive instructions from a director. Most PL or intercom systems are wired together on a kind of party line. In this way, each member can hear and talk to everyone else.

Normally, the headset microphones are always on so that both hands can be kept free to operate equipment. But, for high-noise situations some PL headsets have a push-to-talk feature, which means that everyone's headset mic isn't on at the same time and contributing to the overall noise level.

Another feature that's useful in high noise situations is a large padded earphone, which will help screen out competing sound.

IFB Systems

In ENG (electronic newsgathering) and EFP (electronic field production) it may be necessary for a director to relay messages directly to on-air talent-even while they are on the air. This can be done if the talent uses a small earphone, or earpiece. This system is referred to as IFB for interrupted or interruptible feedback.

When switched to program audio (or the basic audio being recorded or transmitted during the production) IFB systems allow on-air talent to hear questions or comments from studio anchors.

Now that you are up to speed on audio, we can turn our attention to video in the next module.