Finding Compressor gain reduction (transfer) curves ??

Discussion in 'Compressors / Limiters (analog)' started by kylen, Feb 20, 2005.

  1. kylen

    kylen Guest

    Can you folks tell me how compressor gain reduction curves are generated like this one: Mu/gr chart.pdf

    In other words there is a calibrated piece of test equipment that generates some type of test signal at various levels (I assume), the signal is applied to the compressor inputs, a piece of test equipment is attached to the compressor outputs that record or notes the various output levels (I assume). I'm thinking someone doesn't sit there with a pad of paper and plot X/Y points on a piece of paper - but I don't know.

    These curves might also be know as a gain transfer curve - or IN/OUT transfer graph or diagram. I'm not having any luck finding out about much of this so I thought I'd ask you folks.

    I'm not trying to build a compressor or convolve one - just trying to experiment a bit and satisfy myself concerning the amount of compressor character that can be attributed to the gain transfer curves measured at the input and output of the entire device.

    What tools or test equipment or procedures are used to divine the compressor gain reduction curves? Any links or sites describing this or some cool curves would be appreciated also.

  2. Kev

    Kev Well-Known Member

    Nov 13, 2001
    some pro gear with big price tags

    there is some cool test software like AudioTester that can acheive this sort of stuff

    there are others

    can be done by measuring descete points and then ploting the results with soimething like Excel
  3. Sanity Inn

    Sanity Inn Guest

    here's anither one i found
    1st level isfree, seems to do quite a bit

    MKH 800's

    i typed " audio test software" is google, lots of choices


  4. kylen

    kylen Guest

    OK - thanks for the replies!

    Looks like folks do just sit down and chart these things out in Excel. I suppose there's a way to capture the data and throw it into Excel but doing it by hand goes pretty quickly once you get the hang of it.

    So without knowing any better I ran a 1KHz sine wave into RMS Buddy to get the peak/rms inputs, calibrated the compressor under test to zero out the in/out delta if any, then made various ratio and threshold settings, then watched another instance of RMS Buddy to get my output peak/rms outputs.

    After plotting a few gain reduction transfer curves what I suspected is beginning to prove itself. Compressors that I have been using to fix certain problems lately are of the type that are not flat below the threshold but continue to compress. I'd like to say that it's some kind of deep knee but I'm not sure - I think there's more of a large parabolic shape to the curve. At any rate it is a lot different sounding than a simple flat transfer curve below the threshold (eg 1:1).

    Now I can begin catagorizing my compressors into the 'beefy' ones (non-linear transfer curve below threshold) and the ones that are just limiters dressed up in compressor clothing (linear transfer curve below threshold). It's easier to grab the right tool for the right job knowing a little bit about the compression curves (I think) - I could listen to them and hear something was different but I wanted to know why that was...

    Any other comments and ideas are appreciated also!
  5. anxious

    anxious Guest

    Well, unless you want to invest in some $20K+ hardware (, etc), the easiest way to do this kind of thing is with a scope and a tone burst generator.

    There are two main issues to be mindful of... the attack and decay times, and the steady-state gain.


  6. kylen

    kylen Guest

    OK - Thanks ! The link gives me some more info & buzzwords.

    My Plan:
    Step1 - Find out how manufacturers generate the compression or gain transfer curves.

    Step2: Figure out a way to do it myself as cheaply as possible.

    In the curves I drew up yesterday I wasn't bursting just using steady state 1KHz tone. There used to be a 1176 burst test procedure on the internet somewhere but the link is gone now so that's next on the list - bursting.

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