Compressor

  • Author or source: ur.liam@cnihsalf
  • Type: Hardknee compressor with RMS look-ahead envelope calculation and adjustable attack/decay
  • Created: 2004-05-26 16:02:59
notes
RMS is a true way to estimate _musical_ signal energy,
our ears behaves in a same way.

to making all it work,
try this values (as is, routine accepts percents and milliseconds) for first time:

threshold = 50%
slope = 50%
RMS window width = 1 ms
lookahead = 3 ms
attack time = 0.1 ms
release time = 300 ms

This code can be significantly improved in speed by
changing RMS calculation loop to 'running summ'
(keeping the summ in 'window' -
adding next newest sample and subtracting oldest on each step)
code
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void compress
    (
        float*  wav_in,     // signal
        int     n,          // N samples
        double  threshold,  // threshold (percents)
        double  slope,      // slope angle (percents)
        int     sr,         // sample rate (smp/sec)
        double  tla,        // lookahead  (ms)
        double  twnd,       // window time (ms)
        double  tatt,       // attack time  (ms)
        double  trel        // release time (ms)
    )
{
    typedef float   stereodata[2];
    stereodata*     wav = (stereodata*) wav_in; // our stereo signal
    threshold *= 0.01;          // threshold to unity (0...1)
    slope *= 0.01;              // slope to unity
    tla *= 1e-3;                // lookahead time to seconds
    twnd *= 1e-3;               // window time to seconds
    tatt *= 1e-3;               // attack time to seconds
    trel *= 1e-3;               // release time to seconds

    // attack and release "per sample decay"
    double  att = (tatt == 0.0) ? (0.0) : exp (-1.0 / (sr * tatt));
    double  rel = (trel == 0.0) ? (0.0) : exp (-1.0 / (sr * trel));

    // envelope
    double  env = 0.0;

    // sample offset to lookahead wnd start
    int     lhsmp = (int) (sr * tla);

    // samples count in lookahead window
    int     nrms = (int) (sr * twnd);

    // for each sample...
    for (int i = 0; i < n; ++i)
    {
        // now compute RMS
        double  summ = 0;

        // for each sample in window
        for (int j = 0; j < nrms; ++j)
        {
            int     lki = i + j + lhsmp;
            double  smp;

            // if we in bounds of signal?
            // if so, convert to mono
            if (lki < n)
                smp = 0.5 * wav[lki][0] + 0.5 * wav[lki][1];
            else
                smp = 0.0;      // if we out of bounds we just get zero in smp

            summ += smp * smp;  // square em..
        }

        double  rms = sqrt (summ / nrms);   // root-mean-square

        // dynamic selection: attack or release?
        double  theta = rms > env ? att : rel;

        // smoothing with capacitor, envelope extraction...
        // here be aware of pIV denormal numbers glitch
        env = (1.0 - theta) * rms + theta * env;

        // the very easy hard knee 1:N compressor
        double  gain = 1.0;
        if (env > threshold)
            gain = gain - (env - threshold) * slope;

        // result - two hard kneed compressed channels...
        float  leftchannel = wav[i][0] * gain;
        float  rightchannel = wav[i][1] * gain;
    }
}

Comments

My comments:

A rectangular window is not physical. It would make more physical sense, and be a lot cheaper, to
use a 1-pole low pass filter to do the RMS averaging. A 1-pole filter means you can lose the bounds
checks in the RMS calculation.

It does not make sense to convert to mono before squaring, you should square each channel
separately and then add them together to get the total signal power.

You might also consider whether you even need any filtering other than the attack/release filter.
You could modify the attack/release rates appropriately, place the sqrt after the attack/release,
and lose the rms averager entirely.

I don't think your compressor actually approaches a linear slope in the decibel domain. You need
a gain law more like

double gain = exp(max(0.0,log(env)-log(thresh))*slope);

Sincerely,
     Frederick Umminger

To sum up (and maybe augment) the RMS calculation method, this question and answer may be of use...

**********

music-dsp@shoko.calarts.edu writes:
I am looking at gain processing algorithms. I haven't found much in the way of reference material on
this, any pointers? In the level detection code, if one is doing peak detection, how many samples
does one generally average over (if at all)? What kind of window size for RMS level detection?
Is the RMS level detection generally the same algo. as peak, but with a bigger window?

The peak detector can be easily implemented as a one-pole low pass, you just have modify it,
so that it tracks the peaks and gently falls down afterwards. RMS detection is done squaring
the input signal, averaging with a lowpass and taking the root afterwards.

Hope this helps.

Kind regards

Steffan Diedrichsen

DSP developer

emagic GmbH

**********

I found the thread by searching old [music-dsp] forum posts. Hope it helps.

How would you use a 1-pole lowpass filter to do RMS averaging? How do you pick a coefficient to use?

Use x = exp(-1/d), where d is the time constant in samples. A 1 pole IIR filter has an infinite
impulse response, so instead of window width, this coeff determines the time when the impulse
response reaches 36.8% of the original value.

Coeffs:
a0 = 1.0-x;
b1 = -x;

Loop:
out = a0*in - b1*tmp;
tmp = out;

-- peter schoffhauzer

I am looking at gain processing algorithms£º
There are too such sentences :
double att = (tatt == 0.0) ? (0.0) : exp (-1.0 / (sr * tatt));
double rel = (trel == 0.0) ? (0.0) : exp (-1.0 / (sr * trel));

can you tell me something about the exp (-1.0 / (sr * tatt))?

New day ~~
thanks

This is a useful reference, however the RMS calculations are pretty dodgy. Firstly there is a
bug where is calculates the number of samples to use:

int sr, // sample rate (smp/sec)
...
double twnd, // window time (ms)
...
// samples count in lookahead window
int nrms = (int) (sr * twnd);

The units are mixed when calculating the number of samples in the RMS window. The window time needs
to be converted to seconds before multiplying by the sample rate.

As others have mentioned the RMS calculation is also really expensive, and in my tests I found it
was pretty innacurate unless you use a LOT of samples (you basically need a (sample rate)/2 window
of samples in your RMS calculation to accurately measure the power of all frequencies).

I ended up using the 1 pole low pass filter approach suggested here, and it is a good cheap
approximation of power. I did, however, end up mulitplying it by root(2) (the RMS of a sine wave,
which seemed like a reasonable normalisation factor) in order to get it between 0 and 1, which is
a more useful range.

Another slightly more accurate way to caculate the RMS without iterating over and entire window
for each sample is to keep a running total of the squared sums of samples.

for( i = 0; i < NumSamples; ++i )
{
  NewSample = Sample[i];
  OldSample = Sample[i - RMSWindowSize];

  SquaredSum = SquaredSum + NewSample * NewSample;
  SquaredSum = SquaredSum - OldSample * OldSample;

  RMS = sqrt( SquaredSum / RMSWindowSize );

  // etc...
}

Calculating the power in the signal is definately the awkward part of this DSP!