NGWave Audio/Sound/MP3 Editor: Compression Part II
This is a continuation of our compression tutorial.Typical Side Effects
With AGC, there are many side effects that make it especially unsuitable for music. One such effect is commonly referred to as Pumping. Pumping is where the Bass kick (or other loud sound) in a song is louder than everything else, and causes the AGC to lower the volume very rapidly whenever that sound is heard. Since this happens rythmically, it's an extremely noticable side-effect -- you can hear the audio level of everything else jump up and down as a result of the bass kick or bass line.Breathing is another common problem. This typically occurrs when compressing vocals. As the vocalist enhales, the AGC is bringing the level up (since the audio program is below the threshold), and the enhalation ends up being almost as loud as the singing or speaking. This effect is often simply considered over compressed.
The Noise Floor is the level of background noise when no signal is present, eg, when not speaking into the microphone. AGC tends to raise the noise floor, since it is increasing the gain when no usable signal is present. If your Threshold is set very low (and your Gain very high), a period of silence will continue increasing the level, and you'll hear the background noise increase. This is typically heard as a hiss sound, gradually becoming louder. Once a loud sound kicks the compression into effect, the hiss suddenly drops down, and this is often very noticable.
This effect sometimes comes from noisy equipment. Cheap microphones, cabling, or other equipment often adds to the noise floor. It's best to use known quality equipment, and good, shielded and balanced XLR microphone cables to keep the noise level to a minimum. Still, however, too much compression (too low a Threshold + too much Gain) can lead to a nigh noise floor on its own.
Performing your processing in an all-digital realm helps as well; none of the processing steps will add any noise to your signal. But note that using consumer-grade audio equipment -- including most sound cards -- will lead to more noise. A high-end sound card, preferrably electrically isolated from the PC itself (such as a USB audio device) is best.
How Compressors Deal with these Issues
Dynamic Compression has come a long way since AGC first came about. While the typical AGC circuit is well suited for radio-frequency or brightness control applications, it tends to be too noticable with audio applications, especially music. However, many tricks have been implemented and pefected over the years.Side Chain
The Side Chain of a typical compressor is the signal that the compressor acts upon. Usually, this is the same signal that is being compressed -- a louder signal should cause the compressor to lower the level on that same signal. However, this is not necessarily the case. Sometimes, you may want to compress one signal based on some other signal.A typical use for the Side Chain is for ducking. This is where a voice-over causes the background music level to drop down. Typically the voice is fed into a compressor's Side Chain, with the background music being compressed as a result. The background music in this case doesn't affect the compression in any way. Only what is fed into the Side Chain -- the voice in our example -- affects the compression level of the background music.
Essentially you have one signal controlling another signal's output level.
However, another use is to feed in a copy of the audio that is being compressed, but slightly modified. For example, if Pumping is a serious problem, you may want to reduce the bass response before sending the audio into the Side Chain. The audio being compressed, however, is still the full, unmodified audio signal. This practice is referred to as Side Chain Filtering.
Essentially, you are reducing the compressor's sensitivity to low frequencies, without reducing its low frequency response. So pounding bass comes through, but doesn't kick out the audio level nearly as drastically. Obviously this isn't a perfect solution for every application, but it goes quite far in reducing how noticable the compression is.
