Camcorder Sounds: Understanding What Your Audio Goes Through (page 2)

Manual or AGC?

There are advantages and disadvantages to AGCs and manual controls. One advantage of AGCs is that they will set fairly decent levels if you are in a hurry. Their main drawback is that they're try to bring every sound up to the perfect signal level. If your talent stops speaking for a second, the AGC will crank up any sound it can find--be it a droning refrigerator motor or a sniffling cameraman.

AGC circuits are also somewhat slow to respond to level changes, so a sudden blast of noise may cause severe distortion before the AGC reacts. Or it may respond to a loud background noise and cut, for a few agonizing seconds, the overall level of the sound you are trying to capture.

In most camcorders, you're stuck with AGC. This makes it all the more important to use earphones to check the overall sound during the shoot. Be sure that what you want to record balances well with the background sound.

With manual control, you can eliminate most of the problems associated with AGC. You can also fine tune the recording levels and adjust the balance between stereo channels. Many expensive high-level camcorders use manual controls because in professional settings, control is paramount.

A serious disadvantage of manual input level controls is that each new scene or sound source requires that the settings be readjusted. It is very difficult to reset your levels while shooting--can you imagine trying to twist knobs on the camcorder and keep it steady at the same time? Manual controls demand a sound check, a luxury you may not be given.

Another level control is the limiter. A limiter circuit simply reduces any part of a signal stronger than a pre-determined level to prevent distortion. This is usually a point just below tape saturation. The limiter's main disadvantage: when it kicks in, you can really hear it squashing your audio.

Camcorders with manual level controls often include a limiter. While the AGC is the primary audio control of consumer camcorders, many industrial camcorders use all three of these level control methods, allowing the user to switch between them in different combinations.

Signal Processing

Let's move on down the wires to the processing section of the record amplifier. This circuitry changes the signal based on the method used to record it on tape. It is here that record equalization takes place. To understand record equalization, we have to consider the effect that different tape speeds have on audio recording. At the higher tape speed (SP for VHS), the changing audio signal is spread out over a longer section of tape than when recorded at slower speeds. This makes it easier for the heads to read during playback.

At the slower recording speeds (EP), high audio frequencies tend to get lost because they're packed onto a small section of tape. The tape is just not moving fast enough to properly record or playback these frequencies. The equalization circuit boosts the high frequencies at the slower speed to compensate. This tends to improve the recording quality of the higher frequencies because they are now more dominant on the tape.

Record equalization tends to boost any high frequency noise along with the original signal. But the advantage of better high frequency response outweighs this.

Now we'll look at the major differences in how each format--VHS or 8mm--actually records audio.

Recording VHS

VHS camcorders, by design, have always had a linear audio track. It's called a linear track because if it were visible to the eye, it would run in a straight line down the edge of the tape. It's recorded by a stationary record/playback head in much the same way a reel-to-reel or audiocassette deck records. At first this track was mono, but when Hollywood began to release movies on VHS tape the linear track split in half to become linear stereo.

The linear track has never been very high in fidelity. It offers, at the most, only 50 to 11KHz signal response at the fastest SP speed. Most consumer decks top out at closer to 5KHz. This is about the same sound quality as a cheap mono audiocassette recorder. With the audio track cut in half for stereo, the quality only got worse as the space for each track got smaller.

Along came the VHS hi-fi standard, a successful effort to improve the audio quality of the VHS family. But where did engineers put the hi-fi signals? In the original VHS design, the linear audio track, video track and control track covered the full width of the tape. Having no place left to go on the surface of the tape, the developers of VHS hi-fi decided to record under the video tracks using a process called depth multiplexing.