Fix iOS stereo microphone capture

PROGRESS.md

@@ -19,10 +19,11 @@ The source-of-truth layout is:
 - `native/media/` and `native/rnnoise/` — required Opus/RNNoise shim and vendored RNNoise.
 - `native/apple/broadcast/` — required ReplayKit upload extension and shared ring producer.
 
-The physical-device iOS voice path now supports ReplayKit fallback and stable Apple VPIO
-voice-chat capture using a paced 20 ms handoff. Stereo microphone routing remains an open device
-gate: the stereo preset currently receives duplicated mono samples despite selecting the built-in
-stereo capsule.
+The physical-device iOS voice path now supports ReplayKit fallback, stable Apple VPIO voice-chat
+capture using a paced 20 ms handoff, and true built-in stereo microphone capture. Stereo was
+verified on an iPhone 16 Pro Max with a two-channel AVAudioEngine input and distinct left/right
+samples; the managed Apple binding requires native use of its otherwise-unmapped stereo polar
+pattern constant.
 
 ## Release gates
 
@@ -30,8 +31,8 @@ stereo capsule.
   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. Verify distinct left/right capture with the stereo microphone
-  preset, including changing mono/stereo while joined, and Windows desktop/per-app stereo sharing.
+  ScreenCaptureKit paths on devices. Complete extended mono/stereo/voice-chat switching while
+  joined, and verify Windows desktop/per-app stereo sharing.
 - Complete Developer ID signing/notarization. The iOS host and ReplayKit extension have been
   distribution-signed and packaged locally; upload the IPA for Apple's server-side validation.
 - Run the published Linux container and a 30-minute-or-longer server soak.

clients/apple/VoiceCat.iOS/IosAudioEngine.cs

@@ -55,7 +55,7 @@ internal sealed class IosAudioEngine
 
     internal void StartMicrophone(uint streamId, int channels)
     {
-        Volatile.Write(ref microphone, new(streamId, Math.Clamp(channels, 1, 2))); Rebuild();
+        Volatile.Write(ref microphone, CreateMicrophoneRoute(streamId, channels)); Rebuild();
     }
 
     internal void StopMicrophone() { Volatile.Write(ref microphone, null); Rebuild(); }
@@ -70,10 +70,21 @@ internal sealed class IosAudioEngine
             // an in-flight callback could interpret its old converter buffer using the new width.
             DestroyGraph();
             if (current.Channels != channels) client!.Audio.SetCaptureChannels(current.StreamId, channels);
-            Volatile.Write(ref microphone, new(current.StreamId, channels));
+            Volatile.Write(ref microphone, CreateMicrophoneRoute(current.StreamId, channels));
         }
         Rebuild();
     }
+
+    private static MicrophoneRoute CreateMicrophoneRoute(uint streamId, int channels)
+    {
+        var route = new MicrophoneRoute(streamId, Math.Clamp(channels, 1, 2));
+        // Physical RemoteIO capture is commonly delivered in 100 ms bursts. Starting its 20 ms
+        // pacer with only the VPIO-oriented 60 ms cushion guarantees several underruns after every
+        // mono/stereo rebuild before the adaptive path catches up. Prime one full burst plus one
+        // frame for non-VPIO routes; VPIO remains at the proven low-latency three-frame cushion.
+        route.TargetFrames = IosAudioRouter.Shared.UsesVoiceProcessing ? 3 : 6;
+        return route;
+    }
     internal bool EnsureRunning()
     {
         if (!IsConnected || engine?.Running == true) return true;

clients/apple/VoiceCat.iOS/IosAudioRouter.cs

@@ -1,5 +1,6 @@
 using AVFoundation;
 using Foundation;
+using ObjCRuntime;
 using UIKit;
 
 namespace VoiceCat.iOS;
@@ -12,6 +13,8 @@ internal sealed record IosAudioDataSource(string Id, string Name, IReadOnlyList<
 
 internal sealed class IosAudioRouter
 {
+    private static readonly NativeHandle SupportedPolarPatternsSelector = Selector.GetHandle("supportedPolarPatterns");
+    private static readonly NativeHandle SetPreferredPolarPatternSelector = Selector.GetHandle("setPreferredPolarPattern:error:");
     internal static IosAudioRouter Shared { get; } = new();
     private readonly NSUserDefaults defaults = NSUserDefaults.StandardUserDefaults;
     private bool applying;
@@ -123,17 +126,26 @@ internal sealed class IosAudioRouter
         AVAudioSessionPortDescription? port = session.AvailableInputs?.FirstOrDefault(value => value.UID == SelectedInputId);
         if (CaptureChannels == 2) port ??= session.AvailableInputs?.FirstOrDefault(value => value.PortType == AVAudioSession.PortBuiltInMic);
         if (port is null) { if (CaptureChannels == 1) ClearStereoPolarPattern(session); return; }
+        if (CaptureChannels == 2)
+        {
+            // Polar-pattern discovery alone is insufficient on current iPhones: until a stereo
+            // orientation is requested, the built-in port can expose only its mono Bottom source
+            // and AVAudioEngine consequently binds a one-channel input node. This is Apple's
+            // dedicated switch for built-in stereo recording. Portrait matches this portrait UI;
+            // it also gives Core Audio an unambiguous left/right mapping before graph creation.
+            if (!session.SetPreferredInputOrientation(AVAudioStereoOrientation.Portrait, out NSError? orientationError))
+                throw new InvalidOperationException(orientationError?.LocalizedDescription ?? "Could not set the stereo microphone orientation.");
+        }
         AVAudioSessionDataSourceDescription? source = CaptureChannels == 2
-            ? port.DataSources?.FirstOrDefault(value => value.SupportedPolarPatterns?.Any(pattern => pattern.ToString().Contains("Stereo", StringComparison.OrdinalIgnoreCase)) == true)
+            ? port.DataSources?.FirstOrDefault(SupportsStereoPolarPattern)
             : port.DataSources?.FirstOrDefault(value => value.DataSourceID.ToString() == SelectedDataSourceId);
         if (CaptureChannels == 1 && source is null) ClearStereoPolarPattern(session);
         if (source is not null)
         {
             if (!port.SetPreferredDataSource(source, out NSError? portError)) throw new InvalidOperationException(portError.LocalizedDescription);
-            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))
+            if (CaptureChannels == 2) SetStereoPolarPattern(source);
+            else if (SelectedPolarPattern != AVAudioDataSourcePolarPattern.Unknown &&
+                !source.SetPreferredPolarPattern(SelectedPolarPattern, out NSError? patternError))
                 throw new InvalidOperationException(patternError.LocalizedDescription);
         }
         if (!session.SetPreferredInput(port, out NSError? inputError)) throw new InvalidOperationException(inputError.LocalizedDescription);
@@ -144,15 +156,47 @@ internal sealed class IosAudioRouter
             !session.SetInputDataSource(source, out NSError? sourceError))
             throw new InvalidOperationException(sourceError.LocalizedDescription);
         Console.Error.WriteLine($"VC_ROUTE_SELECT port={port.PortName} source={source?.DataSourceName ?? "none"} " +
+            $"stereoPattern={source is not null && SupportsStereoPolarPattern(source)} " +
             $"patterns={string.Join(',', source?.SupportedPolarPatterns?.Select(value => value.ToString()) ?? [])}");
     }
 
+    // Microsoft.iOS 27 exposes AVAudioSessionPolarPatternStereo as an NSString constant but its
+    // AVAudioDataSourcePolarPattern smart enum still has no Stereo member. Reading the native
+    // NSArray and setting the native NSString avoids silently mapping Stereo to Unknown.
+    private static bool SupportsStereoPolarPattern(AVAudioSessionDataSourceDescription source)
+    {
+        NativeHandle handle = NativeMethods.GetObject(source.Handle, SupportedPolarPatternsSelector);
+        NSArray? patterns = Runtime.GetNSObject<NSArray>(handle);
+        if (patterns is null) return false;
+        for (nuint index = 0; index < patterns.Count; index++)
+            if (patterns.GetItem<NSString>(index)?.IsEqualTo(AVAudioSession.PolarPatternStereo.Handle) == true) return true;
+        return false;
+    }
+
+    private static void SetStereoPolarPattern(AVAudioSessionDataSourceDescription source)
+    {
+        NativeHandle error = NativeHandle.Zero;
+        if (NativeMethods.SetObject(source.Handle, SetPreferredPolarPatternSelector,
+            AVAudioSession.PolarPatternStereo.Handle, ref error) == 0)
+            throw new InvalidOperationException(Runtime.GetNSObject<NSError>(error)?.LocalizedDescription ?? "Could not enable stereo microphone capture.");
+    }
+
+    private static class NativeMethods
+    {
+        [System.Runtime.InteropServices.DllImport("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSend")]
+        internal static extern NativeHandle GetObject(NativeHandle receiver, NativeHandle selector);
+
+        [System.Runtime.InteropServices.DllImport("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSend")]
+        internal static extern byte SetObject(NativeHandle receiver, NativeHandle selector, NativeHandle value, ref NativeHandle error);
+    }
+
     private static void ClearStereoPolarPattern(AVAudioSession session)
     {
+        session.SetPreferredInputOrientation(AVAudioStereoOrientation.None, out _);
         AVAudioSessionPortDescription? builtIn = session.AvailableInputs?.FirstOrDefault(value => value.PortType == AVAudioSession.PortBuiltInMic);
         if (builtIn is null) return;
         foreach (AVAudioSessionDataSourceDescription source in builtIn.DataSources ?? [])
-            if (source.SelectedPolarPattern.ToString().Contains("Stereo", StringComparison.OrdinalIgnoreCase))
+            if (source.SelectedPolarPattern == AVAudioDataSourcePolarPattern.Unknown && SupportsStereoPolarPattern(source))
                 source.SetPreferredPolarPattern(AVAudioDataSourcePolarPattern.Unknown, out _);
     }
 

tests/VoiceCat.Tests/PublishServerScriptTests.cs

@@ -124,6 +124,11 @@ public class PublishServerScriptTests
             router.IndexOf("SetActive(true", StringComparison.Ordinal));
         Assert.DoesNotContain("session.SetPreferredInputNumberOfChannels", router);
         Assert.DoesNotContain("MaximumInputNumberOfChannels", router);
+        Assert.Contains("SetPreferredInputOrientation(AVAudioStereoOrientation.Portrait", router);
+        Assert.Contains("SetPreferredInputOrientation(AVAudioStereoOrientation.None", router);
+        Assert.Contains("AVAudioSession.PolarPatternStereo", router);
+        Assert.Contains("SupportsStereoPolarPattern", router);
+        Assert.Contains("route.TargetFrames = IosAudioRouter.Shared.UsesVoiceProcessing ? 3 : 6", engine);
         Assert.Contains("SelectedDataSourceId = null; SelectedPolarPattern = AVAudioDataSourcePolarPattern.Unknown", router);
         Assert.Contains("ClearStereoPolarPattern(session)", router);
         Assert.Contains("CaptureChannels == 2\n            ? port.DataSources?.FirstOrDefault", router);