Dear all,
I am currently trying to emulate some of the sox effects by
scripting them in Reaper. I need those for exactly mirroring
an external effects chain in sox within a DAW.
For most of the effects reprogrammed, this works fine
(the results are bit-exact).
But I have significant problems with mcompand.
mcompand uses a 4-th order Linkwitz-Riley-filter for
crossovers. I am able to reproduce the crossover algorithm
quite faithfully, but nevertheless I am not convinced about
the filter coefficients.
sox biquad filters produce the following IIR coefficients
for a sample rate of 44.1kHz and a crossover frequency of
1kHz (the numbers gained by checking the source-code):
======================================
butterworth 2 biquad filters from sox:
======================================
lb = [ 4.604E-03, 9.208E-03, 4.604E-03 ]
hb = [ 9.042E-01, -1.808E+00, 9.042E-01 ]
a = [ 1.000E+00, -1.799E+00, 8.175E-01 ]
Those perfectly match the values from scipy.signal:
signal.iirfilter(2, 1000/22050, btype="lowpass")
signal.iirfilter(2, 1000/22050, btype="highpass")
==========================================
butterworth 2 crossover from scipy.signal:
==========================================
bLow2 = [ 0.004604, 0.009208, 0.004604 ]
bHigh2 = [ 0.9041522, -1.80830441, 0.9041522 ]
a2 = [ 1., -1.79909641, 0.8175124 ]
Using full debug mode for mcompand with a 1kHz crossover
frequency reveals the following coefficients:
=================================
butterworth 4 crossover from sox:
=================================
DBUG mcompand_xover:
lb=[2.119680195800917e-005 8.47872078320367e-005
0.000127180811748055
8.47872078320367e-005 2.119680195800917e-005]
DBUG mcompand_xover:
hb=[0.8174912065828001 -3.2699648263312 4.9049472394968
-3.2699648263312 0.8174912065828001]
DBUG mcompand_xover:
a=[1 -3.598192818969336 4.871772697390141 -2.9415672592774
0.6683265296879234];
This does _not_ match the output from scipy.signal:
==========================================
butterworth 4 crossover from scipy.signal:
==========================================
lb=[2.15209512e-05, 8.60838049e-05, 1.29125707e-04,
8.60838049e-05, 2.15209512e-05 ]
hb=[ 0.82999258, -3.31997033, 4.97995549, -3.31997033,
0.82999258 ]
a=[ 1., -3.6278442, 4.95122513, -3.01192428, 0.68888769 ]
They are near, but not identical. Is the coefficient
transformation for the iir filter cascade in
mcompand_xover.h possibly incorrect?
Best regards,
Thomas
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