A common protocol structure is to have a header containing a length field, followed by data of length bytes, followed by additional protocol fields such as a CRC. If the length field is variable, the position of subsequent URH-specified protocol fields defined by fixed position will not be in the correct location.

I do not know the best way to address this, but quick and easy way to support

... and the docstring is inappropriate too, therefore.

Either fix by removing argument, or implement decimation.

Probably just replacing the 1 in

        self.fft_filter_xxx_3 = filter.fft_filter_fff(1, stereo_carrier_filter_coeffs_0, 1)

with audio_decimation + writing a test case.