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feat/sinc pcm resampler - #23
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dangusev merged 8 commits intoOct 13, 2026
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StreamResampler interpolated linearly. It did not filter, so it aliased when it downsampled: from 48 kHz to 16 kHz, a 12 kHz tone came out at 4 kHz at full level. It now uses a rubato windowed-sinc filter. StreamResampler::new takes a fixed input rate, output rate, and channel count, and rejects a frame with other settings. push and flush return a PcmFrame with the input channels. flush pushes the audio that the filter holds (128 input frames) out with silence. Equal rates copy the audio. LocalAudioTrack accepts PCM at LocalAudioTrackConfig::pcm_sample_rate (48 kHz by default) and averages its channels to mono. drain queues the audio that the resampler holds, for the end of an utterance. flush also drops that audio. write_pcm keeps the resampler lock until its audio is queued, so a flush cannot put audio from before it in the queue.
Users usually do not know the rate of their TTS audio, and a track does not change its rate after it is created. LocalAudioTrackConfig no longer has pcm_sample_rate: the first write_pcm frame sets the input rate of the track, and a later frame at another rate gets IllegalState. StreamResampler::new(out_rate, channels) cannot fail. The first push sets the input rate and builds the filter. If the filter rejects the rate, the rate stays unset.
When the queue holds less than one 20 ms frame, the pacer adds the audio that the resampler still holds and cuts the silence after it. The end of the written audio plays without the next write. LocalAudioTrack::drain is removed.
write_pcm and StreamResampler::push return RtcError::PcmRateMismatch { expected, actual } when a frame is not at the rate of the first frame, so a caller can match the error without its text. A frame with another channel count still gets IllegalState.
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aliev
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