[SoX-users] FM radio compressor example bug

[Paul Jessop]

Hi,

SoX is an awesome tool and mostly I’m making very good use of it.

There are a few references in the archive here (*) and elsewhere to a problem with the example in the current (2014-12-31) documentation for mcompand:

play track1.wav gain −3 sinc 8000− 29 100 mcompand \
"0.005,0.1 −47,−40,−34,−34,−17,−33" 100 \
"0.003,0.05 −47,−40,−34,−34,−17,−33" 400 \
"0.000625,0.0125 −47,−40,−34,−34,−15,−33" 1600 \
"0.0001,0.025 −47,−40,−34,−34,−31,−31,−0,−30" 6400 \
"0,0.025 −38,−31,−28,−28,−0,−25" \
gain 15 highpass 22 highpass 22 sinc −n 255 −b 16 −17500 \
gain 9 lowpass −1 17801

I am looking to emulate the kind of compressor used in FM broadcasting and broadly understand the chain here: decrease level, some sort of filter (read on), multiband compander with crossovers at 100, 400, 1600 and 6400Hz, increase level, filter out very low frequencies, lowpass at 17.5kHz, increase level and lowpass at 17.8kHz.

The second instance of “sinc” seems to be correct but the first “sinc 8000− 29 100” throws an error (using 14.4.2) and (according to others who have looked at this) seems to retain the parameters of the old “filter” effect. That would make it a low pass filter with a corner at 8kHz and a window-length of 29 (the default being 128) and a beta of 100 (default being 16 for a Kaiser window). I wonder if 29 and 100 are the right way around even? They are very far from their defaults.

My simplistic attempts to emulate this with “sinc” or “lowpass” create all sorts of level problems and I’m trying to understand what this effect is intended to do! If it is indeed supposed to roll off the response of the overall effect at 6dB/octave above 8kHz, that doesn’t seem to be recovered further down the chain. Knowing what the chain emulates would help!

Has anyone solved this?

Best regards,

Paul


(*) Most recently Doug Lee at https://sourceforge.net/p/sox/mailman/message/34787636/

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