RMS in compressors; how is period of time determined?

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1796 posts since 15 Apr, 2004 from Capital City, UK

Post Wed Oct 10, 2018 12:56 pm

Sam-U wrote:
Wed Oct 10, 2018 12:12 pm
CinningBao wrote:
Tue Oct 09, 2018 4:27 pm
how do you decide how much time to RMS before the volume is altered?
Trial and error is one method. I think I read some take a look at outboard, try to measure it and compare to their own implementation to find the "right" length.

I'd also simply go by ear, what gives the expected results or how it behaves if the rms window gets altered. Maybe that way you find a sweetspot fitting fast, slow or dynamic material. Or you recognize a pattern which can be setup with adapting rms windows.

But I never read in a compressor manual that it follows some standard. Even in metering a good rms meter has an adjustable rms length. That is another option: take a look at the loudness standard like EBU. It defines how to measure for loudness, maybe you can adapt that for compressor rms.
Thanks Sam-U - Yeah, I figured some developers just tweak until it either matches the gear they're emulating or "it sounds right". Others, but not many by the sounds of it, allow the user to adjust as they see fit; pro-ears only. I should do more metering..

4665 posts since 22 Jan, 2005 from Sweden

Re: RMS in compressors; how is period of time determined?

Post Wed Oct 10, 2018 11:15 pm

If window feels like being to short, or does not sound how you want it, one way to make it longer might be to cascade many compressors of the same type. Very common approach...not for RMS as such, just a general way to use compressors.

Waves L3 use a lot of lookahead compared to L2, and either does not work the best on any type of audio. So guess that goes for other compressors as well, they don't work well on any type of material.

2364 posts since 17 Apr, 2005 from S.E. TN

Re: RMS in compressors; how is period of time determined?

Post Thu Oct 11, 2018 9:50 am

CinningBao wrote:
Wed Oct 10, 2018 12:51 pm
Aaaaaah, now THAT makes sense! Thanks JCJR :)

And I look at what you've said, about how hard it is respond to both low and high frequency material without compromising, and wonder if splitting the signal up and using clever maths to work out how to combine the outputs to create a more responsive envelope. Is that reasonable? Do developers already do it?
Hi CinningBao. Over the years compressor designs have used myriads of strategies. I could be casually overestimating, but there are probably enough details to fill out a good-sized book on all the things that have been done in different devices. Its hard to judge because even tic-tac-toe looks like a deep game if one is simple-minded enough. :) It would be a labor-intensive book to research and write with probably not much of a market.

Am guessing by now books have been written just about USING compressors or dynamics processors in general, regardless of the technical innards. Didn't go looking for examples.

More complex because the design depends on intended use, and even narrowing focus to the "compressor/limiter" subset of dynamics processors, different folks use em for different purposes and have different expectations.

Some people PREFER some amount of compressor distortion while others avoid distortion even if maybe a little bit of distortion might turn out to sound good.

Some people want to compress, reduce dynamic range, without the audio "sounding compressed". They want the result to sound as natural, un-affected possible, just with a more-controlled dynamic range. Other folks want the effect easily audible and "in your face".

The guy who wants transparent compression will resent using a pumping breathing monster and the guy who wants an over-the-top pumping R&B or dance mix won't like the transparent compressor which miraculously manages to "near invisibly" control dynamics.

It is possible to make "pretty fast" envelopes which manage to control ripple and audio-frequency intermodulation distortion. But the audible effects named "pumping" or "breathing" are actually slowed-down intermodulation, intermodulation below audible frequencies. I think there are several different compression artifacts which might be called "pumping" but am not an expert on the topic.

So if you have a nice fast low-distortion envelope, you might decide you need to slow it down even more than necessary to avoid distortion, to avoid some slower artifacts which might offend the ear. Ferinstance, maybe each kick drum hit jerks the level of the hats/cymbals "too fast and nervous" even if otherwise the audio is clean with no IM distortion.

Sometimes too-slow recovery is annoying to the ear. Sometimes too-fast recovery annoying to the ear. Horses for courses.

I was always in the "transparent compression" camp, thinking that the ideal compressor would compress without the listener knowing that anything has been done to the signal. Folks in that camp have worked on multi-stage envelopes, various kinds of adaptive envelopes, so that maybe the compressor reacts fast to some kinds of audio but slow to other kinds of audio, etc. Or multi-band compression. I've been interested in multi-band processing but haven't done much experimentation with it.

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Aleksey Vaneev
3540 posts since 7 Sep, 2002

Re: RMS in compressors; how is period of time determined?

Post Thu Oct 11, 2018 11:15 am

In Soniformer's RMS mode the RMS uses running RMS (leaky integrator over square of the signal) tuned to 500 Hz (effectively about 1ms). Why 1ms? To reduce impact of high-frequency oscillations. Using longer RMS windows is similar to using higher attack time in a compressor, actually difference is not serious to just increasing the attack time. Difference between peak and RMS estimation in compressor is like using higher percentile or median of the signal (that's not exactly the same, but just an example). There are differences, but they are neigther good or bad - all depends on the sound material.

1796 posts since 15 Apr, 2004 from Capital City, UK

Re: RMS in compressors; how is period of time determined?

Post Sun Oct 14, 2018 9:29 am

Thanks you guys, you've opened my eyes, and ears, to the problems and panoply of solutions when it comes to dynamics management with RMS, and why devs choose what choices, where.

Understanding the situation in this level of detail will also help me tune my ears a little better to how my compressors are responding to the material. And what to use, where.

cheers :)

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