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robin

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Posts: 60 Member Since: 31/07/2011

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Mar 13 17 10:31 AM

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I've been trying to measure and compare time constants and ratios of some compressors as part of an (amateurish) attempt to design a sidechain for an opto compressor. 

My method for the ratio and knee has been to use VST plugin analyzer for software, and drawing up graphs in Excel for the hardware.
I've been feeding the compressors a 1k sine wave that alternates betwen 10dB above and 10dBbelow the threshold, then using a wave editor to calculate the attack and release times.

A few questions  -

What percentage of the final value is considered the attack/release time? I've seen a few conflicting ideas. 
Is there any software that draw up transfer curves for the knee/ratio measurements (output v input)? I've been using excel, which is a bit of a pain.
Do you know of a similar alternative to VST plugin analyzer? it's really buggy and only works with 32bit plugs.

Tips appreciated :D

Cheers,
Robin.
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Mike Rivers

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Mar 13 17 11:38 AM

You're on the right track for measuring. Generally attack time is the time between the input going over-threshold and when gain reduction begins. Release time is the time between the input going below threshold and when gain returns to unity (or there's no more gain reduction). For consistency, you could define those boundaries yourself, say, when gain drops by 1 dB (attack) and recovers to -1 dB, but the caveat is that you need to have enough gain reduction to get past the knee. But some define it as the time to reach and fully recover from the full gain reduction.

I suspect that if there are front panel markings for attack and release times, they're based on time constants in the hardware or math in the software rather than actual measurements. It's why so many compressors have controls labeled "fast, medium, slow."



For a good time, call mikeriversaudio.wordpress.com

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dcollins

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Mar 14 17 1:48 AM

There is no standard for this. Sometimes 10%-90% like normal rise-rime, some do 60-something percent of the curve and use that number.


Confusion between log and linear domains also adds to the mystery.

https://pdfs.semanticscholar.org/f1b2/0a5681e6ef7080e5b5fbce81911c6873543c.pdf

Is a good paper with lots of info about technical compressor operation. They use 1-1/e as the value and some other Greek letters that must sound punchy.

Last Edited By: dcollins Mar 14 17 1:53 AM. Edited 2 times.

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Cirrus

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Posts: 46 Member Since:08/12/2014

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Mar 14 17 7:24 AM

mikerivers wrote:
Generally attack time is the time between the input going over-threshold and when gain reduction begins.

Let me preface this by saying I'm sure you know a hell of a lot more about this stuff than me...

But is this right? I thought gain reduction usually begins immediately, and attack time is how long it takes to get to the target gain reduction? (Eg if you fed it a perfect compressor with a 2:1 ratio a steady state signal 10 dB over the threshold assuming hard knee, the gain reduction would begin to ramp up immediately and the attack time would be the time it took to reach 5dB gain reduction [or a certain percentage of the way there, like, 80% or something like that... on this I am hazy.]

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zmix

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Posts: 4,066 Member Since:20/01/2011

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Mar 14 17 8:49 AM

Cirrus wrote:
mikerivers wrote:
Generally attack time is the time between the input going over-threshold and when gain reduction begins.

Let me preface this by saying I'm sure you know a hell of a lot more about this stuff than me...

But is this right? I thought gain reduction usually begins immediately, and attack time is how long it takes to get to the target gain reduction? (Eg if you fed it a perfect compressor with a 2:1 ratio a steady state signal 10 dB over the threshold assuming hard knee, the gain reduction would begin to ramp up immediately and the attack time would be the time it took to reach 5dB gain reduction [or a certain percentage of the way there, like, 80% or something like that... on this I am hazy.]
The wording from the AES paper Dave Collins posted:


The attack time defines the time it takes the compressor
to decrease the gain to the level determined by the ratio
once the signal overshoots the threshold. The release time
defines the time it takes to bring the gain back up to the
normal level once the signal has fallen below the threshold.


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Mike Rivers

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Posts: 2,512 Member Since:13/10/2012

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Mar 14 17 11:25 AM

zmix wrote:
The wording from the AES paper Dave Collins posted:


The attack time defines the time it takes the compressor
to decrease the gain to the level determined by the ratio
once the signal overshoots the threshold. The release time
defines the time it takes to bring the gain back up to the
normal level once the signal has fallen below the threshold.



I probalby should have re-read the article about compression that I wrote about 20 years ago rather than just tried to explain it. Time to reach the target gain reduction and return to no gain reduction (or maybe at the time 90% was considered "there enough") seems like a more useful thing to know. What's also useful, and what I think most contributes to the sound of a compressor is the shape of the attack and release curves with respect to time. This has to do with the difference in response time between light dependent resistors and their light sources, VCAs, tubes, and arithmetic.

Of course the transformers contribute to the sound of a compressor, too, but that's a different thing.



For a good time, call mikeriversaudio.wordpress.com

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seth

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Mar 14 17 12:21 PM

mikerivers wrote:
 ....and what I think most contributes to the sound of a compressor is the shape of the attack and release curves with respect to time. This has to do with the difference in response time between light dependent resistors and their light sources, VCAs, tubes, and arithmetic.

Of course the transformers contribute to the sound of a compressor, too, but that's a different thing.

And capacitors in the release circuit of some compressors, no?

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dcollins

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Posts: 2,343 Member Since:27/01/2011

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Mar 14 17 9:27 PM

seth wrote:
mikerivers wrote:
 ....and what I think most contributes to the sound of a compressor is the shape of the attack and release curves with respect to time. This has to do with the difference in response time between light dependent resistors and their light sources, VCAs, tubes, and arithmetic.

Of course the transformers contribute to the sound of a compressor, too, but that's a different thing.

And capacitors in the release circuit of some compressors, no?
There is a capacitor in the time-constant section of _all_ compressors.  Analog(ue) ones at least. The charging and subsequent discharge of the capacitor is where the attack and release times come from.  The type of the capacitor, per se, is not much of a sonic factor - if that's the question.

 davecollinsmastering.com


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robin

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Posts: 60 Member Since:31/07/2011

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Mar 16 17 6:09 AM

Thanks for the info! 
How about the static characteristic curves, for measuring knee/ratio. Do you guys know of any software that can run tests and display this?

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