Hey, I wanted to post a correction to this for anyone who might come
this. I had previously read a discussion from Tom R. and some other
GNU gods discussing the sign of the feedback taps needing to be negated;
when working with the test bench data, that is still the case.
Normally the gain reported from the feedback filters is subtracted, as
the equation that Mark posted above. However, when looking at the actual
function in iir_filter.cc it turns out that the feedback coefficients
acc = d_fftaps * static_cast<gr_complexd>(input);
for(i = 1; i < n; i ++)
acc += (d_fftaps[i] * static_cast<gr_complexd>(d_prev_input[latest_n +
for(i = 1; i < m; i ++)
acc += (d_fbtaps[i] * static_cast<gr_complexd>(d_prev_output[latest_m
This means you can either do one of two things when dealing with the
feedback. You can either negate the value of the reported coefficient
using it in the equation above, or change the operation of the equation.
Get your taps from fm_emph.py as above, and utilize them in the
y(n) = b0x(n) + b1x(n-1) + a1*y(n-1)
When doing this, the calculated values match what we printed to a file
while under operation.