TDM Audio 32CL-2 v.2 - Manual (Page 14)

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 14
the highest possible threshold to accomplish your goal. This will result in the most natural sound.
When used properly, you should not be able to hear when the compressor is working.
The range of compression ratios commonly used varies from about 4:1 to about 20:1. Ratios
outside this range are useful for achieving special effects, but they rarely sound natural. For
speech, background music, and general program material, somewhere between 4:1 and 8:1 should
be about right. If you are inexperienced, try closer to 4:1 first. If you notice that you have to set
the threshold too low (more on setting the threshold later), raise the ratio up to about 6:1 and
then raise the threshold and see if that works. After some time you should get to the point where
you have a pretty good idea of what compression ratio you need for a particular application.
After selecting a compression ratio, adjust the output level so that the quietest parts of the signal
are the right volume. Then slowly lower the threshold until the loudest parts of the signal are
reduced in volume so that they, too, are the right volume. You can see when the signal passes the
threshold and compression begins because the compression light will come on. If you have to
lower the threshold a really long way after you first see the compression light come on, and you
notice that the sound is unnatural, try moving the threshold back up, raising the compression ratio
a little bit (not too much), and then lowering the threshold again. Usually, you will end up with a
threshold that is not much lower than about -10 dBm. If it is a lot lower than this, you might need
a higher compression ratio. Generally, the Compression light should only come on during the
louder parts of the program. If it is staying on constantly, try using a higher ratio and a higher
threshold.
Fast attack and release times are sufficient for most applications. A slow release time is sometimes
used for speech or program material. A slow attack time is generally used to solve problems
associated with having a slow release time.
Basic Limiting
Limiting is not very difficult. It’s a matter of finding the absolute maximum allowable level and
setting the limiter’s threshold to that level. Generally, this is done by raising the signal to an unac-
ceptably high level, then lowing the limiter threshold until the signal level is as high as is
permissible, but no higher. For example, if you wanted to use the limiter to prevent clipping in an
amplifier, you would raise the input signal until the amplifier began to clip, then lower the thresh-
old of the limiter until the clipping stopped. You should then see the limit light on the limiter
flashing on the same program peaks that originally caused clipping.
Compressing Speech
When compressing speech, it is of utmost importance that the resulting sound is natural and intel-
ligible. A compression ratio of about 4:1 is sufficient for most speakers.
One problem that is common when compressing speech for multiple speakers is that the quietest
speakers need a lot of gain to be heard, so the loudest speakers are constantly being compressed.
This can become very noticeable because with a loud voice, every time the speaker pauses for a
breath, the compressor releases the gain reduction so that the first syllable of the next phrase is
unnaturally loud. Then you hear the compressor reduce the gain to a reasonable level again. This
can become very annoying. The basic problem is that you generally want only the loudest parts of