What does the compress opcode do?

>From a cursory look at the doc, it uses dB, and rather than calculating rms, 
looks at peaks.

Art Hunkins

----- Original Message ----- 
From: "Oeyvind Brandtsegg" 
To: "Csound mailing list address" 
Sent: Friday, September 20, 2013 4:11 AM
Subject: [Csnd] compression, logarithmic or linear


> Hi,
> I'm building a compressor for pedagogical purposes,
> and stumbled upon a question.
> When calculating the gain reduction, should we always work in dB or in
> a linear amplitude scale?
> For example, with a threshold of -20 and a ratio of 2:1, using an
> input signal at 0dB:
> - should I get a stable output at -10 dB,
> - or at amplitude 0.55 (-20 dB is amp=0.1, 0 dB is amp=1.0)
>
> I'm guessing it should work in dB,
> but figured it sensible to ask.
> It's not just the relation between parameter values and output level,
> the compressor will react differently to a dynamic input as well, so
> transients will be shaped differently.
> They sound differently, so of course there's no reason not to have
> both methods available, but it would be interesting to get insight
> into what is most common, what other implementations use, and what
> good old hardware boxes do.
>
> Below is a csd doing linear compression on the right channel and dB
> compression in the left.
>
> best
> Oeyvind
>
>
> 
> 
> -ocompressor1compare.wav    ; output to audio file
> 
> 
>
>    sr = 48000
>    ksmps = 1
>    nchnls = 2
>    0dbfs = 1
>
> ;***************************************************
> ;ftables
> ;***************************************************
>
>    giSine        ftgen    0, 0, 65536, 10, 1                ; sine
>
> ;***************************************************
> ; compare linear and dB based compression
> ;***************************************************
>    instr    2
>
>    iamp        = ampdbfs(p4)            ; Amp in -dB
>    kthresh        = p5                ; compression threshold
>    kratio        = p6                ; compression ratio
>
> ; audio generator
>    a1        oscili    iamp, 440, giSine                ; sine tone
>
> ; compressor (dB)
>    krms1        rms a1                            ; analyze amplitude
> of input signal
>    krms1        = krms1 * 1.42                        ; normalize the
> rms measurement
>    krms_dB1    = dbfsamp(krms1)                    ; convert to dB scale
>    kovershoot1    = kthresh - krms_dB1                    ; how much
> over the threshold are we?
>    ktarget1    = kthresh - (kovershoot1*(1/kratio))            ;
> target output level (with current input level, threshold and ratio)
>    kampMod_dB1    =  ktarget1 - krms_dB1                    ;
> difference from target = adjust amount
>    kampMod_dB1    limit kampMod_dB1, -150, 0                ; do not
> adjust unless negative
>    kampMod1    = ampdbfs(kampMod_dB1)                    ; convert
> back to normalized scale
>    acomp1        = a1*kampMod1                        ; apply
> amplitude modification
>
> ; compressor (linear)
>    krms        rms a1                            ; analyze amplitude
> of input signal
>    krms        = krms * 1.42                        ; normalize the
> rms measurement
>    krms_dB        = dbfsamp(krms)                        ; convert to dB 
> scale
>    kovershoot    = krms - ampdbfs(kthresh)                ; how much
> over the threshold are we?
>    ktarget        = ampdbfs(kthresh) + (kovershoot*(1/kratio))
> ; target output level (with current input level, threshold and ratio)
>    kampMod        =  ktarget/krms                        ; get
> adjustment factor as ratio between target and input amp
>    kampMod        limit kampMod, 0, 1                    ; do not
> adjust if over 1
>    acomp        = a1*kampMod                        ; apply amplitude
> modification
>
> ; audio out
>    outs        acomp1, acomp
>    endin
> ;***************************************************
>
>
> 
> 
>
> ;compression
> ;     start    dur    amp    thresh    ratio
> i2    0    3    -0    -20    2
> i2    ^+4    3    -0    -20    3
> i2    ^+4    3    -0    -20    6
> 
> 
>
>
> -- 
>
> Oeyvind Brandtsegg
> Professor of Music Technology
> NTNU
> 7491 Trondheim
> Norway
> Cell: +47 92 203 205
>
> http://flyndresang.no/
> http://www.partikkelaudio.com/
> http://soundcloud.com/brandtsegg
> http://soundcloud.com/t-emp
>
>
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