ten31-transcripts/Ten31Transcripts/Audio/Resampler.swift

import AVFoundation

/// Converts arbitrary input PCM buffers to **16 kHz mono Float32**, maintaining
/// resampler state across calls. Reuse one instance per source stream so the
/// internal sample-rate converter stays continuous across buffers.
///
/// Not thread-safe: use one instance from a single thread. Both the mic and
/// system instances are driven exclusively from `AudioRecorder.ioQueue` (one per
/// source stream), kept continuous across buffers.
final class Resampler {
    /// The canonical Phase-1 audio format: 16 kHz, mono, Float32, deinterleaved.
    static let targetFormat = AVAudioFormat(
        commonFormat: .pcmFormatFloat32,
        sampleRate: 16_000,
        channels: 1,
        interleaved: false)!

    private var converter: AVAudioConverter?
    private var sourceFormat: AVAudioFormat?
    private var ended = false

    /// 16 kHz mono buffer for `input`, or nil if conversion produced nothing.
    func resample(_ input: AVAudioPCMBuffer) -> AVAudioPCMBuffer? {
        guard !ended, input.frameLength > 0 else { return nil }

        if converter == nil || sourceFormat != input.format {
            let c = AVAudioConverter(from: input.format, to: Self.targetFormat)
            // Highest-quality sample-rate conversion: best anti-aliasing on the
            // 48k→16k downsample, which avoids harsh artifacts on loud/bright speech.
            c?.sampleRateConverterQuality = .max
            c?.sampleRateConverterAlgorithm = AVSampleRateConverterAlgorithm_Mastering
            converter = c
            sourceFormat = input.format
        }
        guard let converter else { return nil }

        let ratio = Self.targetFormat.sampleRate / input.format.sampleRate
        let capacity = AVAudioFrameCount((Double(input.frameLength) * ratio).rounded(.up)) + 64
        guard let output = AVAudioPCMBuffer(pcmFormat: Self.targetFormat, frameCapacity: capacity) else {
            return nil
        }

        var consumed = false
        var error: NSError?
        let status = converter.convert(to: output, error: &error) { _, inputStatus in
            if consumed { inputStatus.pointee = .noDataNow; return nil }
            consumed = true
            inputStatus.pointee = .haveData
            return input
        }
        if status == .error || output.frameLength == 0 { return nil }
        return output
    }

    /// Flush the converter's internal tail at end of stream (call once on stop).
    func drain() -> AVAudioPCMBuffer? {
        guard !ended, let converter else { ended = true; return nil }
        ended = true
        guard let output = AVAudioPCMBuffer(pcmFormat: Self.targetFormat, frameCapacity: 8192) else {
            return nil
        }
        var error: NSError?
        let status = converter.convert(to: output, error: &error) { _, inputStatus in
            inputStatus.pointee = .endOfStream
            return nil
        }
        if status == .error || output.frameLength == 0 { return nil }
        return output
    }
}