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Compression : Peak & RMS

Discussion in 'Mixing & Song Critique' started by paradice, Aug 7, 2008.

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  1. paradice

    paradice Guest

    Hi all..
    can someone briefly explain the difference between peak and rms compression and its applications

    thank you
  2. GeckoMusic

    GeckoMusic Guest

    The Wikipedia explains it well.

    Drums signals are going to have a higher peak level than an RMS equivalent pad or bowed string.

    Also because RMS level is a root mean square average over time, the peak level detection will have a faster attack at the cost of some distortion.
  3. Link555

    Link555 Well-Known Member

    Mar 31, 2007
    North Vancouver
    Ah yep
    RMS is the Root mean square,

    To simplify things you can think of the rms level as a average level. Say you have 3 bank accounts. Each bank account has 3 different amounts of money.

    Bank 1 = $40
    Bank 2 = $50
    Bank 3 = $60

    The average amount of money per bank would be:

    (40 + 50 + 60) / The number of Banks (3) = 150/3 = 50

    Now the rms value is a bit different calculation:

    The rms average amount of money per bank would be:

    ((40^2 + 50^2 + 60^2) /3)^(1/2) = ((1600 + 2500 + 3600)/3)^(1/2) =2567^(1/2)


    Peak is the largest value. Either negative or positive. In electronics you will hear the term peak to peak. Think of a sine wave swinging around 0V. The distance from the OV mark to the crest, be it negative or positive is the peak value.
    Check this link for pretty pictures:


    In Simple Math ->
    You can convert overall peak values to overall rms just multiply by 0.707 =(1 /root 2).

    So.... in peak mode your compressor looks for peaks to act on. usefull for catching quick transients, tameing a ACU GTR for example. Peak compression when used with a large ratio and slow release times will be more noticeable. In this mode the RMS filter in the control path is removed.

    In rms mode the compressor responds to changes in the rms level.
    The overall level will be smoothed out. In this mode the rms filter in the control path is engaged, and the control signal (either the input (feedforward) or output (feedback)) is changed into an rms level. This dc level then controls the amout of attenuation in the gain control device (Typically a VCA or optical device).

    Hope this helps.
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