Re: Audio Compressor

On Tue, 18 Aug 2020 at 14:41, Renato S. Yamane <[email protected]>
wrote:

> Hi,
>
> Is there an Audio Compressor available in Octave?
> https://www.mathworks.com/help/audio/ref/compressor-system-object.html
>
> I'm compressing my noises in the worst way, as you can see in the "while"
> condition on the example below.
>
> In my way, the peaks will be "clipped", and I would like to avoid my
> stupid workaround:
>
> ====================
> crest_factor = 6;
> typenoise = noise(10*44100, 1, 'pink');
> [z, p, k] = butter(4, [100/(44100/2), 500/(44100/2)]);
> sos = zp2sos (z, p, k);
> filtered = sosfilt(sos, typenoise);
> normalized = filtered / (rms(filtered) / 10^(-crest_factor/20));
>
> #{
> Now I'm making an stupid workaround, as the audio file must be in
> a range from -1 to +1:
> #}
>
> while (normalized(normalized > 1) || normalized(normalized < -1))
>   normalized(normalized > 1) = 1;
>   normalized(normalized < -1) = -1;
>   normalized = normalized / (rms(normalized) / 10^(-crest_factor/20));
> endwhile
>
> audiowrite ('AudioFile.wav', normalized, 44100);
>

I'm not aware there is an equivalent function on Octave to the
Matlab compressor, but if I might make a couple of comments on the code:

1. Your while loop looks very strange to me and I believe the identical
result would be achieved much faster by simply omitting the while and
endwhile statements.

2.  The statement:
         normalized = filtered / (rms(filtered) / 10^(-crest_factor/20));
     normalises the signal to an RMS level of -6dB below 1. An RMS below 1
does not of
     course guarantee there will be no samples greater than 1, but unless
your signal is very
     peculiar I would expect there to be very few or none.

Possibly therefore the following code would meet your needs without the
need for complex compression.

crest_factor = 6;
typenoise = noise(10*44100, 1, 'pink');
[z, p, k] = butter(4, [100/(44100/2), 500/(44100/2)]);
sos = zp2sos (z, p, k);
filtered = sosfilt(sos, typenoise);
normalized = filtered / (rms(filtered) / 10^(-crest_factor/20));
NumberOfClippedSamples = nnz( normalized > 1 |normalized<-1)
normalized(normalized > 1) = 1;
normalized(normalized < -1) = -1;

I hope this helps.

Cheers... Ian