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keep intermediate sample rate down to a minimum

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Jakob Ketterl 2021-01-22 18:47:34 +01:00
parent bcab2b2288
commit 68a1abd37e
1 changed files with 6 additions and 11 deletions
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17
csdr/csdr.py
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@ -527,25 +527,20 @@ class dsp(object):
self.restart() self.restart()
def calculate_decimation(self): def calculate_decimation(self):
(self.decimation, self.last_decimation, _) = self.get_decimation(self.samp_rate, self.get_audio_rate()) (self.decimation, self.last_decimation) = self.get_decimation(self.samp_rate, self.get_audio_rate())
def get_decimation(self, input_rate, output_rate): def get_decimation(self, input_rate, output_rate):
decimation = 1 decimation = 1
correction = 1 target_rate = output_rate
# wideband fm has a much higher frequency deviation (75kHz). # wideband fm has a much higher frequency deviation (75kHz).
# we cannot cover this if we immediately decimate to the sample rate the audio will have later on, so we need # we cannot cover this if we immediately decimate to the sample rate the audio will have later on, so we need
# to compensate here. # to compensate here.
# the factor of 6 is by experimentation only, with a minimum audio rate of 36kHz (enforced by the client) if self.get_demodulator() == "wfm" and output_rate < 200000:
# this allows us to cover at least +/- 108kHz of frequency spectrum (may be higher, but that's the worst case). target_rate = 200000
# the correction factor is automatically compensated for by the secondary decimation stage, which comes while input_rate / (decimation + 1) >= target_rate:
# after the demodulator.
if self.get_demodulator() == "wfm":
correction = 6
while input_rate / (decimation + 1) >= output_rate * correction:
decimation += 1 decimation += 1
fraction = float(input_rate / decimation) / output_rate fraction = float(input_rate / decimation) / output_rate
intermediate_rate = input_rate / decimation return decimation, fraction
return decimation, fraction, intermediate_rate
def if_samp_rate(self): def if_samp_rate(self):
return self.samp_rate / self.decimation return self.samp_rate / self.decimation