Hi
> > > Mikael Adle originally asked:
> > > Could someone explain the difference between peak and rms setting on
> > > a compressor and when to use what?
in response to which Kool Musick in part wrote:
> > Rule of thumb ... peak for percussion, RMS for pretty much everything else.
> > Break this rule often and you'll be a good mix engineer!!
unfortunately for Kool Musick it wasn't over yet because blue alien THEN
wrote:>
>Why would you compress a signal and then expand? Don't "pro" engineers
>often use these types of effects in this order during mastering?
Yes indeed, "pro" engineers do use these various types of effects in that
order when mastering -- although occasionally also the opposite.
First off, I am not a "pro engineer". Just an enthusiastic amateur without
a single hit or even a professional mix job that someone has paid me for to
my name. I just enjoy messing around with stuff in my sitting room and
garage. So ... please take the little I know with the proverbial pinch of
salt. Hopefully, someone who knows a LOT more will correct anything I say
that's wrong.
The trouble with compressing and/or expanding -- in fact with ANY signal
processing -- is that you never affect just the one thing you set out
trying to affect (or effect). Sound is much more organic or holistic than
that, and you affect everything. You might, for example, want to use your Q
setting on your eq to hit that horrible sound you are hearing at say 6Khz,
but what you in fact do is hit everything else -- vocals, bass guitar, that
nice acoustic etc -- that's also sitting in your mix at around that
frequency. All you can do then is use your ears to dowse your nightmare
sound in the best way you can, while still leaving enough of everything
else present to make it sound "good". For example, after you've done your
cutting you might use a parametric to boost the signals you do want a
little bit on either side of where you've just cut and so that the sounds
you DO want bleed back in and so fill up the hole you just created. Problem
here is, of course, as you all know, that you are very likely going to
reintroduce at least a little more of that nightmare sound you were trying
to get rid of in the first place!!
OK. So music's organic and you can't really isolate anything.
The compressing/expanding issue deals -- as far as I know -- with much the
same kind of terrain. If you're going to compress, then you're going to
darken the signal a bit. By the very nature of the compression, the aural
energy of the lower frequency sounds tends to be brought into increased
dominance by the whole compressing process. Trebles end up having pretty
much the same ratio as basses. The whole sound can therefore lose quite a
bit in its 'sparkle' or definition. When using the compressor you have to
balance this up against the dynamic effect you are looking to create.
Once you've done your compressing, you may then decide that your mix has
lost a bit of its 'sheen' -- that sheet of high frequency transients that
made the original sound more alive -- even though the original contained
that dynamic range you wanted to suppress. The problem then is ... how do
you regenerate some of those high frequency transients without also
reintroducing the dynamic peaking associated with those transients ... and
that probably made you want to compress in the first place?
Some very expensive compressors (and plug-ins) try to address this by
putting in some very nifty pieces of circuitry which are effectively high
frequency harmonic content generators so while you are actually
compressing, you can add back in to the sound you are creating some of
those 'aurally pleasing' high frequency detail contents ... but still
achieve the squashing or compressing effect you want.
Sometimes, also, you want to compress, and to create your wet sound ... but
then add that compressed signal back in to the original dry sound ... and
yet STILL not get your transient distortions etc. This would be the case if
you wanted a nice fat sounding bass, for example. You would isolate the
bass frequency, compress it, and then add it back to the original with not
really much danger of distortion etc, because the sound you are bringing
back is probably not going to affect your high transients over much. It's
possible to compress, alter by EQ, and then add back in all simultaneously.
If you are using a compressor, you are probably also using a gate -- and
immediately you have the problem of 'swallowing' those high transients. You
thus lose some of your reverb tails and nice decays. Lots of things can happen.
Effectively, therefore, the process of compressing and then gating is an
attempt to overcome the problems caused by compression. The signal you are
left with after compression is dynamically acceptable when compared to the
overall dynamic reference level you are dealing with, but on the
equalization side it may be a bit suspect. But ... the sound is probably
'safe' in that you have taken out some of the high frequency transients.
Therefore, if you now expand, you are working with a controlled and
controllable piece of sonic input, and you can try to re-introduce your
high frequency sheen effects without losing the overall dynamic effect you
wanted.
I hope this makes sense.
I have tried to be clear and also explain my reasoning so that if it's
faulty we'll all know where and why in about five minutes time when some
'pro' mastering engineer steps up to the plate and tells you all what a
complete drongo I am and how I don't know nothing. But ... that's MY
understanding of what's going on, and that's the theory I use behind my own
work.
HTH
Kool Musick
Keep Musick Kool
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