downmix ac3 to stereo to match output layout (#2774)

CHANGELOG.md

@@ -18,6 +18,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Use configured searching log level on startup, instead of the default log level of `Information`
 - MySql: fix searching for shows and seasons in schedule items editor
 - Fix 500 errors when serving XMLTV due to concurrent file reads and writes
+- Fix playback of AC3 audio when targeting stereo output and input layout changes mid-stream
 
 ## [26.1.1] - 2026-01-08
 ### Fixed

ErsatzTV.FFmpeg/Decoder/DecoderAc3Downmix.cs

@@ -0,0 +1,35 @@
+using ErsatzTV.FFmpeg.Environment;
+using ErsatzTV.FFmpeg.Format;
+
+namespace ErsatzTV.FFmpeg.Decoder;
+
+public class DecoderAc3Downmix(int sourceChannels, int desiredChannels) : IDecoder
+{
+    public string Name => "ac3";
+    public EnvironmentVariable[] EnvironmentVariables => [];
+    public string[] GlobalOptions => [];
+    public string[] FilterOptions => [];
+    public string[] OutputOptions => [];
+
+    public string[] InputOptions(InputFile inputFile)
+    {
+        if (sourceChannels >= 2 && desiredChannels == 2)
+        {
+            return ["-c:a", "ac3", "-downmix", AudioLayout.Stereo];
+        }
+
+        // unsupported downmix; hopefully the layout doesn't change
+        return [];
+    }
+
+    public FrameState NextState(FrameState currentState) => currentState;
+
+    public bool AppliesTo(AudioInputFile audioInputFile) => true;
+
+    public bool AppliesTo(VideoInputFile videoInputFile) => false;
+
+    public bool AppliesTo(ConcatInputFile concatInputFile) => false;
+
+    public bool AppliesTo(GraphicsEngineInput graphicsEngineInput) => false;
+
+}

ErsatzTV.FFmpeg/Format/AudioLayout.cs

@@ -0,0 +1,6 @@
+namespace ErsatzTV.FFmpeg.Format;
+
+public static class AudioLayout
+{
+    public const string Stereo = "stereo";
+}

ErsatzTV.FFmpeg/Pipeline/PipelineBuilderBase.cs

@@ -466,6 +466,18 @@ public abstract class PipelineBuilderBase : IPipelineBuilder
             }
         }
 
+        // workaround for ac3 audio layout changes
+        // downmix from decoder so filter doesn't need to reinit (it can't, it will fail)
+        // this only really works when the desired output format matches the minimal input format
+        foreach (var audioInputStream in audioInputFile.Streams.OfType<AudioStream>()
+                     .Where(s => s.Codec == AudioFormat.Ac3).HeadOrNone())
+        {
+            foreach (int desiredAudioChannels in audioInputFile.DesiredState.AudioChannels)
+            {
+                audioInputFile.AddOption(new DecoderAc3Downmix(audioInputStream.Channels, desiredAudioChannels));
+            }
+        }
+
         // always need to specify audio codec so ffmpeg doesn't default to a codec we don't want
         foreach (IEncoder step in AvailableEncoders.ForAudioFormat(ffmpegState, audioInputFile.DesiredState, _logger))
         {