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5.1 Why use compression?
Compression and limiting is a way to control the dynamics of a track or mix. It is used to keep the loudest signal peaks below a certain level. By keeping peaks under control, you're now able to turn the output level up, making the average level of the signal higher. Compression is useful on tracks where the level fluctuates a little too much... you tame the loud peaks, by making them a little quieter. This makes them sound a more consistent with the quieter passages, and makes the track sound more unified.
5.2 Types of Compressors:
There are quite a few good compressors in the sub-$500 range. The compressors in this price range are generally very usable, but not quite as transparent or open sounding as more expensive ones.
Look for used dbx 160's. They're a half rack wide and 2 spaces high. These are a very nice compressor (mono) and can be had for $600 or so used. A pair mount side-by-side in a rack, and they may be connected to a stero pair with a jumper between them. These are relatively expensive, and have quite a colored sound, making them less than ideal in some situations.
the dbx 161 is essentially 2 dbx 160's joined together. It works only as a stereo unit, and not as a dual mono (it only has one set of controls). Also, unlike the 160, the 161 is not balanced.
The dbx 1066 is a stereo compressor/limiter. It's very quiet, flexible enough to use on most tracks, and works surprisingly well.
The Aphex 651 Expressor is a mono compressor/expander. The expander focuses on restoring the high end from a heavily compressed track. This allows a little more squeezing than you could do with some compressors. (Although I don't necessarily recommend heavy compression in most instances)
Also look at the Aphex Compellor. This is available used for this price range. They're designed primarily for compressing mixes, and are very clean.
The Drawmer DL 241 is another good compressor that can be found used for around $500. It's a step above many of these others.
A hot new compressor is the RNC from FMR Audio. This compressor is around $200 and blows away most everything here. Very transparent sounding. Comes as a stereo unit in a 1/3 rack mount case. There's only one pair of controls, so it can not be used as a dual mono unit.
The Alesis 3630 is also a common choice amongst home studio owners. I have one, but honestly don't like it. I find it to pump quite a bit, and it clicks with bass notes. Your experience may be different, however.
5.3 Threshold and Ratio Settings:
The Threshold setting is the absolute maximum level you wish for your signal to reach. The ratio is basically how much you want your compressor to squish the signal while trying to keep it below the threshold. At lower ratio settings, the output will pass the threshold, but it is pulled down in level. The higher the ratio settings get, the less the signal is allowed to surpass the threshold. Lower ratios will generally produce a more open and natural sound, while higher ratios tend to sound squished.
At maximum ratio settings (infinite compression ratio) the compressor is said to be in limiting mode. This means that the signal level will never cross the threshold. As soon as the input level reaches the threshold, it is not allowed to get any louder. This is useful when trying not to overload a digital recorder, for example. You can put a limiter before your DAT deck, and set the threshold at -3dB, and the ratio at infinity. This will prevent the DAT deck from ever receiving a signal above -3dB, and totally avoid digital clipping.
5.4 Attack and Release settings:
Setting the attack speed of your compression helps determine exactly what the effect will sound like. Using a faster time will allow you to compress transients... these could be peaks at the beginning of words and phrases like hard consonant sounds (T's and popped P's). Having the ratio set too fast may muddy up the sound a little, by rounding out the inital sound, and making it unintelligible. In other words, use the longest attack time possible that will allow you to achieve the proper results.
For release times, you have to think of the nature of the sound again. If you need to quiet down a short burst or exclamation but not affect the following phrases of lower volume, then use a shorter release time. This will allow the compression to release quickly with out smashing the quieter portion. Do some phrases start at lower level and swell? For these, use a slower attack and a longer release. If your release time is too quick you;'ll notice a rapid swell in level when the compression ends.
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