65 lines
2.0 KiB
Python
65 lines
2.0 KiB
Python
"""
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Not all output rates are going to be supported by the system sound card.
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Choosing from a limited subset of preferred output rates is how we guarantee
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that the system will be able to play the output audio.
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Sample rates stolen from:
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https://en.wikipedia.org/wiki/Sampling_(signal_processing)
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"""
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supported_ouput_rates = [
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8000, # Telephone, P25 audio (sufficient for speech, some consonants unintelligble)
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16000, # Modern telephone/VoIP, good quality speech
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32000, # FM radio
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44100, # CD audio quality
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48000,
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50000, # Uncommon but supported
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88200,
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96000, # DVD/Blu-ray audio
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192000, # Too much.
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]
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def score(pair, target_output=32000, target_ratio=10):
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"""
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Heuristic for scoring input & output sample rates. The criteria are:
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- closest to 44.1 kHz is better;
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- closer to a ratio of 10 is better,
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- additionally penalising ratios lower than 10.
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Lower scores are better.
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This should result in selected sample rates that give good audio quality
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without wasting CPU cycles on a needlessly high input rate.
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"""
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# Give the worst score possible for impossible pairs.
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if pair[0] % pair[1] != 0:
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return float("inf")
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ratio = pair[0] // pair[1]
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return abs(pair[1] - target_output)/2500 \
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+ max(0, target_output - pair[1])/2500 \
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+ abs(ratio - target_ratio)**0.8 \
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+ max(0, target_ratio - ratio)**2
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def flatten(l):
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return [item for sublist in l for item in sublist]
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def flatten_dict(d):
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return [(key,value) for key,rates in d.items() for value in rates]
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def get_pairs(input_rate):
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return [
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(input_rate, rate)
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for rate in supported_ouput_rates
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if (input_rate % rate == 0)
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]
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def supported_sample_rates(supported_input_rates):
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return {
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in_rate: [out_rate for out_rate in supported_ouput_rates if in_rate % out_rate == 0]
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for in_rate in supported_input_rates
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}
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def preferred_sample_rates(supported_input_rates):
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return sorted(flatten_dict(supported_sample_rates(supported_input_rates)), key=score)
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