Camcorder Sounds: Understanding What Your Audio Goes Through

Okay, let's be honest. How many of you videomakers out there know how sound gets from your camcorder's microphone to the finished video? Do you know what an AGC is? How about depth multiplexing? If you don't know, you're in good company. All too many videomakers focus their energies on understanding the visual aspect of their trade, while turning a deaf ear to the way in which a camcorder handles sound.

It is important to understand the way camcorders record audio. On the most basic level, this knowledge can help you buy the right gear. More importantly, it will help you to make better sounding videos by working with the hardware itself to get the best results.

In order to learn more about camcorder audio, let's follow the audio signal from the microphone to the audio output. We'll look at both the VHS or 8mm families, pointing out all the major differences as we go along.

Virtually all camcorders have a built-in mono or stereo mike. Most units also have a plug for connecting an external mike; this cuts off the built-in mike. One of the advantages of the external mike is that it will not pick up motor noises from the camcorder, even if it's attached to it.

Several methods help to get rid of motor noise from the built-in mike. Some manufacturers mount the motors in rubber casings and use rubber bushings on the motor mounts to help stop vibrations from reaching the mike. Brushless, very quiet running motors eliminate static and mechanical noises. Of course, higher priced camcorders tend to have more success in getting rid of motor noise. The manufacturer's ultimate goal is to have the mike pick up only desired sounds.

Once the mike turns sound into a changing electrical voltage, this signal runs down a wire to a microphone preamplifier circuit. If your camcorder is stereo, there is a second amp for the other mike's signal. The mike amp is usually a single IC (integrated circuit) chip. In fact, many camcorders use a single IC chip to handle most every audio chore in the camcorder.

The mike preamp boosts the audio signal level up to that required by the rest of the camcorder's audio circuitry. Mike signals are very weak, sometimes even in the order of micro volts (1/1,000,000 volt). That's too weak a signal for the rest of the circuitry to work with, so the amp boosts this signal up to around 0.7 or 1.0 volt.

This is a crucial stage for your sound, because the mike preamp has to boost the signal so dramatically. If the camcorder's mike preamp isn't of good quality, your audio will suffer from excess noise.

Some camcorders accept line level audio in VCR mode. These signals, usually from another VCR, come into the camcorder already amplified. The amplified signals from the mike path and the line level path meet at a switch that toggles between camera and VCR modes. In the VCR position, the switch bypasses the mike to record the sound from the line level inputs.

Okay, we're past the switch and now we come to the record amplifier circuitry. This area processes your sound signal for actual recording. The first section of this circuitry is where we adjust the signal level (you can think of it as audio signal strength) for proper recording. Setting the recording level is important. A level that is too low will become plagued with tape noise, and may be difficult to hear when played back. Too high a recording level will saturate the tape (record at a level higher then the tape can handle), resulting in distortion of the sound.

A number of circuits designed to assist in the record level process may or may not be included in the recording amplifier. The simplest are manual level controls. These consist of potentiometers (devices that create a variable electrical resistance) placed directly in the signal path. You turn the attached knobs to set the recorded signal level.

It's pretty much like controlling the flow of water at your sink by turning the faucet handle. If you turn it down, the flow gets weaker and vice versa. The signal, once adjusted by the level control, also drives a set of VU (volume unit) meters or LEDs (light emitting diodes) that give you a visual reading to set your levels by.

All consumer camcorders incorporate an automatic gain control (AGC). The AGC looks at the highest level of the incoming audio signal and compares it to a predetermined reference (generally a level just below tape saturation). The AGC then raises or lowers the gain of the incoming signal to match this level. In other words, it sets the levels for you.