Add AC4 downmix

Signed-off-by: gnattu <gnattuoc@me.com>

MediaBrowser.Controller/MediaEncoding/DownMixAlgorithmsHelper.cs

@@ -15,13 +15,18 @@ public static class DownMixAlgorithmsHelper
     public static readonly Dictionary<(DownMixStereoAlgorithms, string), string> AlgorithmFilterStrings = new()
     {
         { (DownMixStereoAlgorithms.Dave750, "5.1"), "pan=stereo|c0=0.5*c2+0.707*c0+0.707*c4+0.5*c3|c1=0.5*c2+0.707*c1+0.707*c5+0.5*c3" },
+        { (DownMixStereoAlgorithms.Dave750, "7.1"), "pan=5.1(side)|c0=c0|c1=c1|c2=c2|c3=c3|c4=0.707*c4+0.707*c6|c5=0.707*c5+0.707*c7,pan=stereo|c0=0.5*c2+0.707*c0+0.707*c4+0.5*c3|c1=0.5*c2+0.707*c1+0.707*c5+0.5*c3" },
         { (DownMixStereoAlgorithms.NightmodeDialogue, "5.1"), "pan=stereo|c0=c2+0.30*c0+0.30*c4|c1=c2+0.30*c1+0.30*c5" },
+        { (DownMixStereoAlgorithms.NightmodeDialogue, "7.1"), "pan=5.1(side)|c0=c0|c1=c1|c2=c2|c3=c3|c4=0.707*c4+0.707*c6|c5=0.707*c5+0.707*c7,pan=stereo|c0=c2+0.30*c0+0.30*c4|c1=c2+0.30*c1+0.30*c5" },
         { (DownMixStereoAlgorithms.Rfc7845, "3.0"), "pan=stereo|c0=0.414214*c2+0.585786*c0|c1=0.414214*c2+0.585786*c1" },
         { (DownMixStereoAlgorithms.Rfc7845, "quad"), "pan=stereo|c0=0.422650*c0+0.366025*c2+0.211325*c3|c1=0.422650*c1+0.366025*c3+0.211325*c2" },
         { (DownMixStereoAlgorithms.Rfc7845, "5.0"), "pan=stereo|c0=0.460186*c2+0.650802*c0+0.563611*c3+0.325401*c4|c1=0.460186*c2+0.650802*c1+0.563611*c4+0.325401*c3" },
         { (DownMixStereoAlgorithms.Rfc7845, "5.1"), "pan=stereo|c0=0.374107*c2+0.529067*c0+0.458186*c4+0.264534*c5+0.374107*c3|c1=0.374107*c2+0.529067*c1+0.458186*c5+0.264534*c4+0.374107*c3" },
         { (DownMixStereoAlgorithms.Rfc7845, "6.1"), "pan=stereo|c0=0.321953*c2+0.455310*c0+0.394310*c5+0.227655*c6+0.278819*c4+0.321953*c3|c1=0.321953*c2+0.455310*c1+0.394310*c6+0.227655*c5+0.278819*c4+0.321953*c3" },
         { (DownMixStereoAlgorithms.Rfc7845, "7.1"), "pan=stereo|c0=0.274804*c2+0.388631*c0+0.336565*c6+0.194316*c7+0.336565*c4+0.194316*c5+0.274804*c3|c1=0.274804*c2+0.388631*c1+0.336565*c7+0.194316*c6+0.336565*c5+0.194316*c4+0.274804*c3" },
+        { (DownMixStereoAlgorithms.Ac4, "3.0"), "pan=stereo|c0=c0+0.707*c2|c1=c1+0.707*c2"},
+        { (DownMixStereoAlgorithms.Ac4, "5.1"), "pan=stereo|c0=c0+0.707*c2+0.707*c4|c1=c1+0.707*c2+0.707*c5"},
+        { (DownMixStereoAlgorithms.Ac4, "7.1"), "pan=5.1(side)|c0=c0|c1=c1|c2=c2|c3=c3|c4=0.707*c4+0.707*c6|c5=0.707*c5+0.707*c7,pan=stereo|c0=c0+0.707*c2+0.707*c4|c1=c1+0.707*c2+0.707*c5"},
     };
 
     /// <summary>

MediaBrowser.Model/Entities/DownMixStereoAlgorithms.cs

@@ -1,8 +1,7 @@
 namespace MediaBrowser.Model.Entities;
 
 /// <summary>
-/// An enum representing an algorithm to downmix 6ch+ to stereo.
-/// Algorithms sourced from https://superuser.com/questions/852400/properly-downmix-5-1-to-stereo-using-ffmpeg/1410620#1410620.
+/// An enum representing an algorithm to downmix surround sound to stereo.
 /// </summary>
 public enum DownMixStereoAlgorithms
 {
@@ -13,16 +12,24 @@ public enum DownMixStereoAlgorithms
 
     /// <summary>
     /// Algorithm by Dave_750.
+    /// Sourced from https://superuser.com/questions/852400/properly-downmix-5-1-to-stereo-using-ffmpeg/1410620#1410620.
     /// </summary>
     Dave750 = 1,
 
     /// <summary>
     /// Nightmode Dialogue algorithm.
+    /// Sourced from https://superuser.com/questions/852400/properly-downmix-5-1-to-stereo-using-ffmpeg/1410620#1410620.
     /// </summary>
     NightmodeDialogue = 2,
 
     /// <summary>
     /// RFC7845 Section 5.1.1.5 defined algorithm.
     /// </summary>
-    Rfc7845 = 3
+    Rfc7845 = 3,
+
+    /// <summary>
+    /// AC-4 standard algorithm with its default gain values.
+    /// Defined in ETSI TS 103 190 Section 6.2.17
+    /// </summary>
+    Ac4 = 4
 }