Fix stereo capture paths

PROGRESS.md

@@ -25,7 +25,8 @@ The source-of-truth layout is:
   20/40/60 ms buffering, duration-aware DRED/FEC, and mismatched input/output endpoints.
 - Complete NVDA and VoiceOver navigation/announcement passes.
 - Exercise iOS background/lock, interruption, Bluetooth, route-change, ReplayKit, and iOS 27
-  ScreenCaptureKit paths on devices.
+  ScreenCaptureKit paths on devices. Verify distinct left/right capture with the stereo microphone
+  preset, including changing mono/stereo while joined, and Windows desktop/per-app stereo sharing.
 - Complete Developer ID signing/notarization and iOS distribution signing.
 - Run the published Linux container and a 30-minute-or-longer server soak.
 

clients/apple/VoiceCat.iOS/IosAudioEngine.cs

@@ -14,9 +14,8 @@ internal sealed class IosAudioEngine
     private AVAudioSourceNode? source;
     private AVAudioFormat? outputFormat;
     private VoiceCatClient? client;
-    private uint microphoneStream;
+    private sealed record MicrophoneRoute(uint StreamId, int Channels, PcmRing Ring);
-    private int microphoneChannels = 1;
+    private MicrophoneRoute? microphone;
-    private readonly PcmRing microphoneRing = new(131_072);
     private readonly short[] microphoneFrame = new short[960 * 2];
     private readonly CancellationTokenSource microphoneStop = new();
     private readonly Task microphoneWorker;
@@ -39,11 +38,25 @@ internal sealed class IosAudioEngine
 
     internal void StartMicrophone(uint streamId, int channels)
     {
-        microphoneStream = streamId; microphoneChannels = Math.Clamp(channels, 1, 2); Rebuild();
+        Volatile.Write(ref microphone, new(streamId, Math.Clamp(channels, 1, 2), new(131_072))); Rebuild();
     }
 
-    internal void StopMicrophone() { microphoneStream = 0; Rebuild(); }
+    internal void StopMicrophone() { Volatile.Write(ref microphone, null); Rebuild(); }
-    internal void Reconfigure() { if (IsConnected) Rebuild(); }
+    internal void Reconfigure()
+    {
+        if (!IsConnected) return;
+        MicrophoneRoute? current = Volatile.Read(ref microphone);
+        int channels = IosAudioRouter.Shared.CaptureChannels;
+        if (current is not null && current.Channels != channels)
+        {
+            // Stop the old tap before publishing a route with a different sample width. Otherwise
+            // an in-flight callback could interpret its old converter buffer using the new width.
+            DestroyGraph();
+            client!.Audio.SetCaptureChannels(current.StreamId, channels);
+            Volatile.Write(ref microphone, new(current.StreamId, channels, new(131_072)));
+        }
+        Rebuild();
+    }
     internal bool EnsureRunning()
     {
         if (!IsConnected || engine?.Running == true) return true;
@@ -52,7 +65,7 @@ internal sealed class IosAudioEngine
 
     private void Rebuild()
     {
-        DestroyGraph(); IosAudioRouter.Shared.Apply();
+        DestroyGraph(); MicrophoneRoute? route = Volatile.Read(ref microphone); IosAudioRouter.Shared.Apply(route is not null);
         var next = new AVAudioEngine();
         outputFormat = new(AVAudioCommonFormat.PCMFloat32, 48_000, 2, false);
         source = new(outputFormat, Render);
@@ -61,13 +74,13 @@ internal sealed class IosAudioEngine
         if (OperatingSystem.IsIOSVersionAtLeast(27)) next.Connect(source, next.MainMixerNode, outputFormat, out connectionError);
         else next.Connect(source, next.MainMixerNode, outputFormat);
         if (connectionError is not null) throw new InvalidOperationException(connectionError.LocalizedDescription);
-        if (microphoneStream != 0)
+        if (route is not null)
         {
             AVAudioInputNode input = next.InputNode;
             input.SetVoiceProcessingEnabled(IosAudioRouter.Shared.UsesVoiceProcessing, out _);
             if (IosAudioRouter.Shared.UsesVoiceProcessing) input.VoiceProcessingAgcEnabled = IosAudioRouter.Shared.AutomaticGainControl;
             AVAudioFormat inputFormat = input.GetBusOutputFormat(0);
-            microphoneFormat = new(AVAudioCommonFormat.PCMInt16, 48_000, (uint)microphoneChannels, true);
+            microphoneFormat = new(AVAudioCommonFormat.PCMInt16, 48_000, (uint)route.Channels, true);
             microphoneConverter = new(inputFormat, microphoneFormat);
             uint capacity = checked((uint)Math.Ceiling(4_096 * 48_000 / inputFormat.SampleRate) + 64);
             convertedMicrophone = new(microphoneFormat, capacity);
@@ -85,8 +98,8 @@ internal sealed class IosAudioEngine
 
     private unsafe void Capture(AVAudioPcmBuffer buffer, AVAudioTime time)
     {
-        VoiceCatClient? owner = client; uint stream = microphoneStream;
+        VoiceCatClient? owner = client; MicrophoneRoute? route = Volatile.Read(ref microphone);
-        if (owner is null || stream == 0 || buffer.FrameLength == 0) return;
+        if (owner is null || route is null || buffer.FrameLength == 0) return;
         AVAudioConverter? converter = microphoneConverter;
         AVAudioPcmBuffer? converted = convertedMicrophone;
         AVAudioConverterInputHandler? provider = inputProvider;
@@ -95,7 +108,7 @@ internal sealed class IosAudioEngine
         converter.ConvertToBuffer(converted, out _, provider);
         if (converted.FrameLength == 0) return;
         nint samples = Marshal.ReadIntPtr(converted.Int16ChannelData);
-        if (samples != 0) microphoneRing.TryWrite(new ReadOnlySpan<short>((void*)samples, checked((int)converted.FrameLength * microphoneChannels)));
+        if (samples != 0) route.Ring.TryWrite(new ReadOnlySpan<short>((void*)samples, checked((int)converted.FrameLength * route.Channels)));
         pendingInput = null;
     }
 
@@ -110,12 +123,15 @@ internal sealed class IosAudioEngine
         using var timer = new PeriodicTimer(TimeSpan.FromMilliseconds(10));
         while (await timer.WaitForNextTickAsync(token).ConfigureAwait(false))
         {
-            int channels = microphoneChannels, required = 960 * channels;
+            MicrophoneRoute? route = Volatile.Read(ref microphone);
-            while (microphoneRing.Count >= required)
+            if (route is null) continue;
+            int required = 960 * route.Channels;
+            while (route.Ring.Count >= required)
             {
-                int read = microphoneRing.Read(microphoneFrame.AsSpan(0, required));
+                int read = route.Ring.Read(microphoneFrame.AsSpan(0, required));
-                VoiceCatClient? owner = client; uint stream = microphoneStream;
+                VoiceCatClient? owner = client;
-                if (read == required && owner is not null && stream != 0) owner.Audio.FeedPcm(stream, microphoneFrame.AsSpan(0, required), channels);
+                if (read == required && owner is not null && ReferenceEquals(route, Volatile.Read(ref microphone)))
+                    owner.Audio.FeedPcm(route.StreamId, microphoneFrame.AsSpan(0, required), route.Channels);
             }
         }
     }
@@ -142,7 +158,7 @@ internal sealed class IosAudioEngine
 
     internal void Stop()
     {
-        IsConnected = false; microphoneStream = 0;
+        IsConnected = false; Volatile.Write(ref microphone, null);
         if (client is { } owner) owner.Audio.MixedPcm -= ReceiveMixedPcm;
         DestroyGraph(); client = null; IosAudioRouter.Shared.Deactivate();
     }

clients/apple/VoiceCat.iOS/IosAudioRouter.cs

@@ -90,7 +90,7 @@ internal sealed class IosAudioRouter
         SelectedInputId ??= session.PreferredInput?.UID; Changed?.Invoke();
     }
 
-    internal void Apply()
+    internal void Apply(bool configureInput)
     {
         if (applying) return; applying = true;
         try
@@ -102,8 +102,9 @@ internal sealed class IosAudioRouter
             AVAudioSessionMode mode = CaptureChannels == 2 ? AVAudioSessionMode.Default : MicMode == IosMicMode.Raw ? AVAudioSessionMode.Measurement
                 : BluetoothMode == IosBluetoothMode.BuiltInMicA2dp ? AVAudioSessionMode.VideoRecording : AVAudioSessionMode.VoiceChat;
             if (!session.SetCategory(AVAudioSessionCategory.PlayAndRecord, mode, options, out NSError? categoryError)) throw new InvalidOperationException(categoryError.LocalizedDescription);
-            session.SetPreferredSampleRate(48_000, out _); session.SetPreferredIOBufferDuration(0.02, out _); ApplyInput(session);
+            session.SetPreferredSampleRate(48_000, out _); session.SetPreferredIOBufferDuration(0.02, out _);
             if (!session.SetActive(true, AVAudioSessionSetActiveOptions.NotifyOthersOnDeactivation, out NSError? activeError)) throw new InvalidOperationException(activeError.LocalizedDescription);
+            if (configureInput) ApplyInput(session);
             session.OverrideOutputAudioPort(ForceSpeaker && BluetoothMode != IosBluetoothMode.BuiltInMicA2dp ? AVAudioSessionPortOverride.Speaker : AVAudioSessionPortOverride.None, out _); RefreshRoutes();
         }
         finally { applying = false; }
@@ -116,8 +117,24 @@ internal sealed class IosAudioRouter
         if (port is null) return; session.SetPreferredInput(port, out _);
         AVAudioSessionDataSourceDescription? source = port.DataSources?.FirstOrDefault(value => value.DataSourceID.ToString() == SelectedDataSourceId);
         if (CaptureChannels == 2) source ??= port.DataSources?.FirstOrDefault(value => value.SupportedPolarPatterns?.Any(pattern => pattern.ToString().Contains("Stereo", StringComparison.OrdinalIgnoreCase)) == true);
-        if (source is null) return; port.SetPreferredDataSource(source, out _); session.SetInputDataSource(source, out _);
+        if (source is not null)
-        if (CaptureChannels != 2 && SelectedPolarPattern != AVAudioDataSourcePolarPattern.Unknown) source.SetPreferredPolarPattern(SelectedPolarPattern, out _);
+        {
+            if (!port.SetPreferredDataSource(source, out NSError? portError)) throw new InvalidOperationException(portError.LocalizedDescription);
+            if (!session.SetInputDataSource(source, out NSError? sourceError)) throw new InvalidOperationException(sourceError.LocalizedDescription);
+        }
+        if (session.MaximumInputNumberOfChannels < CaptureChannels)
+            throw new InvalidOperationException($"The selected input supports at most {session.MaximumInputNumberOfChannels} channel(s), not {CaptureChannels}.");
+        if (!session.SetPreferredInputNumberOfChannels(CaptureChannels, out NSError? channelError)) throw new InvalidOperationException(channelError.LocalizedDescription);
+        if (source is not null)
+        {
+            AVAudioDataSourcePolarPattern pattern = CaptureChannels == 2
+                ? source.SupportedPolarPatterns?.FirstOrDefault(value => value.ToString().Contains("Stereo", StringComparison.OrdinalIgnoreCase)) ?? AVAudioDataSourcePolarPattern.Unknown
+                : SelectedPolarPattern;
+            if (pattern != AVAudioDataSourcePolarPattern.Unknown && !source.SetPreferredPolarPattern(pattern, out NSError? patternError))
+                throw new InvalidOperationException(patternError.LocalizedDescription);
+        }
+        if (session.InputNumberOfChannels != CaptureChannels)
+            throw new InvalidOperationException($"The selected input activated with {session.InputNumberOfChannels} channel(s), not {CaptureChannels}.");
     }
 
     private void SaveAndReconfigure()

clients/windows/VoiceCat.Windows/AudioControls.cs

@@ -14,8 +14,9 @@ public sealed partial class VoiceCatClient
         StreamInfo? stream = null;
         try
         {
-            stream = core.StartStreamAsync((StreamKind)kind, label).GetAwaiter().GetResult();
+            int captureChannels = kind == VcStreamKind.Mic ? 1 : 2;
-            local[stream.StreamId] = new(stream, external);
+            stream = core.StartStreamAsync((StreamKind)kind, label, captureChannels).GetAwaiter().GetResult();
+            local[stream.StreamId] = new(stream, external, captureChannels);
             if (!external && backend is not null) captures[stream.StreamId] = Capture(stream);
             return (VcResult.Ok, stream.StreamId);
         }

clients/windows/VoiceCat.Windows/VoiceCatClient.cs

@@ -20,12 +20,12 @@ public sealed partial class VoiceCatClient : IDisposable
     private readonly Dictionary<uint, string?> devices = [];
     private readonly Dictionary<uint, StreamSummary[]> remoteStreams = [];
     private readonly Dictionary<uint, bool> talkState = [];
-    private sealed class LocalStream(StreamInfo info, bool external)
+    private sealed class LocalStream(StreamInfo info, bool external, int captureChannels)
     {
         internal readonly uint Alias = info.StreamId;
         internal StreamInfo Info = info;
         internal readonly bool External = external;
-        internal int CaptureChannels = 1;
+        internal int CaptureChannels = captureChannels;
         internal int Restarting;
     }
     private readonly ConcurrentDictionary<uint, LocalStream> local = new();

tests/VoiceCat.Tests/WindowsManagedClientTests.cs

@@ -55,4 +55,42 @@ public class WindowsManagedClientTests
         Assert.Equal(VcResult.Ok, alice.StopStream(a.StreamId));
         Assert.Empty(alice.ManagedClient.LocalStreams);
     }
+
+    [Fact]
+    public async Task WindowsScreenAudioPreservesDistinctStereoChannels()
+    {
+        await using var fixture = new ServerFixture();
+        using (var accounts = new AccountStore(Path.Combine(fixture.Directory, "voicecat.db"))) await accounts.CreateAccountAsync("Admin", "secret", true);
+        using var alice = new Client("Alice", "test", tofuStorePath: Path.Combine(fixture.Directory, "alice-screen.pins"));
+        using var bob = new Client("Bob", "test", tofuStorePath: Path.Combine(fixture.Directory, "bob-screen.pins"));
+        await Login(alice, fixture, true); await Login(bob, fixture);
+        await alice.ManagedClient.RequestAsync(new() { CreateChannel = new() { Channel = new() { Name = "Stereo screen", ParentId = 1, Audio = AudioEngineTests.Stream(20, true).Audio } } });
+        await Until(alice, () => alice.ListChannels().Any(c => c.Name == "Stereo screen"));
+        uint channel = alice.ListChannels().Single(c => c.Name == "Stereo screen").Id;
+        Assert.Equal(VcResult.Ok, alice.JoinChannel(channel)); Assert.Equal(VcResult.Ok, bob.JoinChannel(channel));
+        await Until(alice, () => alice.ListUsers().Count(u => u.ChannelId == channel) == 2);
+        Assert.Equal(VcResult.Ok, alice.JoinVoice()); Assert.Equal(VcResult.Ok, bob.JoinVoice());
+        var screen = alice.StartStreamExternalFeed(VcStreamKind.ScreenAudio, "Screen audio");
+        Assert.Equal(VcResult.Ok, screen.Result);
+        await Until(bob, () => bob.ManagedClient.Users.Any(u => u.Id != bob.ManagedClient.Authentication!.Self.Id && u.Streams.Any(s => s.Kind == StreamKind.StreamScreenAudio)));
+        long separation = 0; int receivedChannels = 0;
+        bob.ManagedClient.Audio.StreamPcm += (_, _, pcm, channels) =>
+        {
+            receivedChannels = channels;
+            if (channels == 2) for (int i = 0; i < pcm.Length; i += 2) Interlocked.Add(ref separation, Math.Abs((int)pcm[i] - pcm[i + 1]));
+        };
+        short[] stereo = new short[1920];
+        for (int i = 0; i < 960; i++)
+        {
+            stereo[2 * i] = (short)(8000 * Math.Sin(i * Math.PI * 880 / 48000));
+            stereo[2 * i + 1] = (short)(8000 * Math.Sin(i * Math.PI * 1320 / 48000));
+        }
+        for (int i = 0; i < 40; i++)
+        {
+            Assert.Equal(VcResult.Ok, alice.StreamFeedPcm(screen.StreamId, stereo, 960, 2));
+            alice.PumpEvents(); bob.PumpEvents(); await Task.Delay(20);
+        }
+        Assert.Equal(2, receivedChannels);
+        Assert.True(Interlocked.Read(ref separation) > 100_000);
+    }
 }