meeting-assistant/MeetingAssistant/Recording/AcousticEchoCanceller.cs

namespace MeetingAssistant.Recording;

public interface IAcousticEchoCanceller
{
    void Reset();

    AudioChunk CleanMicrophone(AudioChunk microphone, AudioChunk systemAudio);
}

public interface IAcousticEchoCancellerFactory
{
    IAcousticEchoCanceller Create();
}

public sealed class AdaptiveFilterAcousticEchoCancellerFactory : IAcousticEchoCancellerFactory
{
    public IAcousticEchoCanceller Create()
    {
        return new AdaptiveFilterAcousticEchoCanceller();
    }
}

public sealed class NoopAcousticEchoCanceller : IAcousticEchoCanceller
{
    public void Reset()
    {
    }

    public AudioChunk CleanMicrophone(AudioChunk microphone, AudioChunk systemAudio)
    {
        return microphone;
    }
}

public sealed class AdaptiveFilterAcousticEchoCanceller : IAcousticEchoCanceller
{
    private const int DefaultFilterLength = 256;
    private const double DefaultAdaptationStep = 0.25;
    private const double Epsilon = 1.0;
    private readonly double[] coefficients;
    private readonly double[] renderHistory;
    private int historyPosition;

    public AdaptiveFilterAcousticEchoCanceller()
        : this(DefaultFilterLength)
    {
    }

    public AdaptiveFilterAcousticEchoCanceller(int filterLength)
    {
        var effectiveFilterLength = Math.Max(16, filterLength);
        coefficients = new double[effectiveFilterLength];
        renderHistory = new double[effectiveFilterLength];
    }

    public void Reset()
    {
        Array.Clear(coefficients);
        Array.Clear(renderHistory);
        historyPosition = 0;
    }

    public AudioChunk CleanMicrophone(AudioChunk microphone, AudioChunk systemAudio)
    {
        if (microphone.Pcm.Length == 0 ||
            systemAudio.Pcm.Length == 0 ||
            microphone.SampleRate != systemAudio.SampleRate ||
            microphone.Channels != 1 ||
            systemAudio.Channels != 1)
        {
            return microphone;
        }

        var byteCount = Math.Min(microphone.Pcm.Length, systemAudio.Pcm.Length);
        byteCount -= byteCount % sizeof(short);
        if (byteCount <= 0)
        {
            return microphone;
        }

        var cleaned = new byte[microphone.Pcm.Length];
        for (var byteIndex = 0; byteIndex < byteCount; byteIndex += sizeof(short))
        {
            var render = BitConverter.ToInt16(systemAudio.Pcm, byteIndex) / 32768.0;
            var capture = BitConverter.ToInt16(microphone.Pcm, byteIndex) / 32768.0;
            renderHistory[historyPosition] = render;

            var estimatedEcho = 0.0;
            var energy = Epsilon;
            for (var tap = 0; tap < coefficients.Length; tap++)
            {
                var sample = renderHistory[HistoryIndex(tap)];
                estimatedEcho += coefficients[tap] * sample;
                energy += sample * sample;
            }

            var error = capture - estimatedEcho;
            var updateScale = DefaultAdaptationStep * error / energy;
            for (var tap = 0; tap < coefficients.Length; tap++)
            {
                coefficients[tap] += updateScale * renderHistory[HistoryIndex(tap)];
            }

            WriteSample(cleaned, byteIndex, error);
            historyPosition = historyPosition == 0 ? renderHistory.Length - 1 : historyPosition - 1;
        }

        if (byteCount < microphone.Pcm.Length)
        {
            Buffer.BlockCopy(microphone.Pcm, byteCount, cleaned, byteCount, microphone.Pcm.Length - byteCount);
        }

        return microphone with { Pcm = cleaned };
    }

    private int HistoryIndex(int offset)
    {
        var index = historyPosition + offset;
        return index >= renderHistory.Length ? index - renderHistory.Length : index;
    }

    private static void WriteSample(byte[] target, int byteIndex, double sample)
    {
        var scaled = Math.Clamp(
            (int)Math.Round(sample * 32768.0),
            short.MinValue,
            short.MaxValue);
        BitConverter.GetBytes((short)scaled).CopyTo(target, byteIndex);
    }
}