"Jim Carr" <newsgroups@azwebpages.com> wrote in message
news:HdGFh.31536$OY.4088@newsfe20.lga...
> "Jim" <askme@beforeyousend.com> wrote in message
> news:12ue9uf9i7ftq6c@corp.supernews.com...
>
>>> That's not correct. A compressor is a device that decreases the dynamic
>>> range of a signal by lowering the gain above a certain threshold. For a
>>> more detailed explanation, see
>>> www.azwebpages.com/bass/basscompression.htm
>>
>> Technically correct. But you should know that guitarists often use them
>> with increased output level. This results in a tighter range signal at a
>> higher level, giving artificial sustain. That's how a lot of guitar
>> players use them.
>
> I do know that. They use make-up gain (almost everyone does with a
> compressor), which affects the *entire* signal. The threshold is used as a
> trigger to reduce again above a level. It does not lower the signal below
> the threshold.
While that last sentence is true, what I mean to say was that it does not
raise the signals below the threshold in any special way. A compressor
squashes the signal above the threshold. The signal below the threshold is
left untouched. If you played in such a way that the signal never crossed
the threshold, the dynamics would be left intact. Suppose the signal exceeds
the threshold by 4dB and your ratio is 4:1. The compressor will attenuate
that signal so that it's only 1dB above the threshold.
Compressors come equipped with makeup gain which means you can boost the
entire signal. You don't have to use the makeup gain, but as a practical
matter that's what is usually done. If you didn't use it, you could add the
gain at a later stage. It's important to think of the makeup gain as a
separate element so that you get a good understanding of what the compressor
itself is actually doing.
What some people think is that a compressor raises the signal below the
threshold. It doesn't work that way. Suppose you had a line where some of
the notes were above -10dB and some were below. Set the threshold to -10dB
and the ratio to 2:1 with no makeup gain. A note at -16dB is still -16dB. A
note at -6dB will be affected so that it's only -8dB. Softer notes are
unaffected by compression.
Let's suppose now that you add 6dB of makeup gain. The -16dB notes are now
at -10dB. At first blush it seems that the softer notes are now louder.
Well, you could have done that without the compressor just by adding more
gain. The louder note is now at -2dB. It's louder than it was before, but
not as loud as it would have been without the compressor.
This is the important difference. If you're using the compressor to make
softer notes louder, what you're really doing is making louder notes softer,
then bringing up the entire signal. In our example if you also had notes
at -24dB, they would still be 8dB softer than your -16dB notes even after
the makeup gain. By contrast a -2dB note would only be 2dB louder than
the -6dB note because of the compression. Relatively speaking you have less
in the way of dynamics above the threshold compared to below the threshold.
Also the difference between your softest note and the loudest note above the
threshold is smaller. If none of the notes exceed the threshold, you've done
nothing but add some additional gain.
Using a compressor for sustain is in effect making an "artificial" sustain.
In an unmodified signal such as plucking a string, you have a peak followed
by the signal dropping off rather quickly. As time goes on the signal drops
off more and more slowly:
http://www.carlmartin.com/Images/comp%20manuel%20fig1.gif
When you use a compressor to create sustain, you end up lopping off much of
the attack. The steep drop-off in level is no longer so steep. This is a
somewhat extreme version of what it might look like:
http://www.carlmartin.com/Images/comp%20manuel%20fig%209.gif. You can adjust
the attack time of the compressor to let some of the transient through, but
you get the idea.
You don't have to know all this. If you twiddle some knobs and get the sound
you want, that's great. However, if you don't know what's going on, you
might find yourself fighting the compressor while you're playing. If you
think of it as affecting only softer notes, you might find yourself plucking
harder and harder wondering why it's not getting as loud as you expect.