Backport pull request #18195 from jellyfin/release-12.z

Apply stereo downmix filter consistently to progressive audio transcodes

Original-merge: 4d99715770afb0882b730fa78f0105bcea893a1d

Merged-by: crobibero <cody@robibe.ro>

Backported-by: Cody Robibero <cody@robibe.ro>

2 files changed, 70 insertions, 1 deletions

diff --git a/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs b/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs
index 6f010c0242..58d7495fa4 100644
--- a/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs
+++ b/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs
@@ -7832,7 +7832,9 @@ namespace MediaBrowser.Controller.MediaEncoding
 
             var channels = state.OutputAudioChannels;
 
-            var useDownMixAlgorithm = state.AudioStream is not null
+            // Must match the condition under which GetAudioFilterParam emits the downmix filter.
+            var useDownMixAlgorithm = channels == 2
+                                      && state.AudioStream?.Channels > 2
                                       && DownMixAlgorithmsHelper.AlgorithmFilterStrings.ContainsKey((encodingOptions.DownMixStereoAlgorithm, DownMixAlgorithmsHelper.InferChannelLayout(state.AudioStream)));
 
             if (channels.HasValue && !useDownMixAlgorithm)
@@ -7927,6 +7929,13 @@ namespace MediaBrowser.Controller.MediaEncoding
                 audioTranscodeParams.Add("-ar " + sampleRateValue.ToString(CultureInfo.InvariantCulture));
             }
 
+            // Without the downmix filter, -ac 2 alone drops the LFE channel.
+            var audioFilterParam = GetAudioFilterParam(state, encodingOptions);
+            if (!string.IsNullOrEmpty(audioFilterParam))
+            {
+                audioTranscodeParams.Add(audioFilterParam.TrimStart());
+            }
+
             // Copy the movflags from GetProgressiveVideoFullCommandLine
             // See #9248 and the associated PR for why this is needed
             if (_mp4ContainerNames.Contains(state.OutputContainer))
diff --git a/tests/Jellyfin.Controller.Tests/MediaEncoding/EncodingHelperTests.cs b/tests/Jellyfin.Controller.Tests/MediaEncoding/EncodingHelperTests.cs
index 2b009b4673..aca55fbcc1 100644
--- a/tests/Jellyfin.Controller.Tests/MediaEncoding/EncodingHelperTests.cs
+++ b/tests/Jellyfin.Controller.Tests/MediaEncoding/EncodingHelperTests.cs
@@ -261,6 +261,66 @@ public class EncodingHelperTests
         Assert.Contains("-ar 48000", args, StringComparison.Ordinal);
     }
 
+    [Fact]
+    public void GetProgressiveAudioFullCommandLine_StereoDownmix_AppliesDownMixAlgorithm()
+    {
+        // Issue #18194: -ac 2 alone drops the LFE channel, the configured downmix filter must be applied.
+        var state = BuildAudioState("aac", 48000);
+        state.AudioStream.Channels = 6;
+        state.AudioStream.ChannelLayout = "5.1";
+        state.OutputAudioChannels = 2;
+        var options = new EncodingOptions { DownMixStereoAlgorithm = DownMixStereoAlgorithms.Dave750, DownMixAudioBoost = 1 };
+        var args = CreateHelper().GetProgressiveAudioFullCommandLine(state, options, "/tmp/out");
+
+        Assert.Contains(
+            "-af \"" + DownMixAlgorithmsHelper.AlgorithmFilterStrings[(DownMixStereoAlgorithms.Dave750, "5.1")] + "\"",
+            args,
+            StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void GetProgressiveAudioFullCommandLine_NoDownmix_EmitsNoAudioFilter()
+    {
+        var state = BuildAudioState("aac", 48000);
+        state.AudioStream.Channels = 2;
+        state.OutputAudioChannels = 2;
+        var options = new EncodingOptions { DownMixStereoAlgorithm = DownMixStereoAlgorithms.Dave750 };
+        var args = CreateHelper().GetProgressiveAudioFullCommandLine(state, options, "/tmp/out");
+
+        Assert.DoesNotContain("-af", args, StringComparison.Ordinal);
+    }
+
+    [Theory]
+    [InlineData(1)]
+    [InlineData(6)]
+    public void GetProgressiveVideoAudioArguments_NonStereoOutput_KeepsChannelCount(int outputChannels)
+    {
+        // The downmix filter only applies to stereo output, so -ac must not be dropped otherwise.
+        var state = BuildAudioState("aac", 48000);
+        state.AudioStream.Channels = 6;
+        state.AudioStream.ChannelLayout = "5.1";
+        state.OutputAudioChannels = outputChannels;
+        var options = new EncodingOptions { DownMixStereoAlgorithm = DownMixStereoAlgorithms.Dave750 };
+        var args = CreateHelper().GetProgressiveVideoAudioArguments(state, options);
+
+        Assert.Contains("-ac " + outputChannels, args, StringComparison.Ordinal);
+        Assert.DoesNotContain("pan=", args, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void GetProgressiveVideoAudioArguments_StereoDownmix_UsesFilterInsteadOfChannelCount()
+    {
+        var state = BuildAudioState("aac", 48000);
+        state.AudioStream.Channels = 6;
+        state.AudioStream.ChannelLayout = "5.1";
+        state.OutputAudioChannels = 2;
+        var options = new EncodingOptions { DownMixStereoAlgorithm = DownMixStereoAlgorithms.Dave750 };
+        var args = CreateHelper().GetProgressiveVideoAudioArguments(state, options);
+
+        Assert.DoesNotContain("-ac ", args, StringComparison.Ordinal);
+        Assert.Contains("pan=stereo", args, StringComparison.Ordinal);
+    }
+
     private static EncodingJobInfo BuildAudioState(string audioCodec, int requestedSampleRate, string? outputContainer = null)
     {
         var audio = new MediaStream { Index = 0, Type = MediaStreamType.Audio, Codec = "flac", SampleRate = 96000 };