SOLVED: AC3 Mono won't play or transcode unless app is set to downmix to stereo

[coffeetablecandle]

• Jellyfin Server: 10.10.7ubu2404
  

• Affected clients: Android TV (0.18.8, Bitstream Dolby Digital Audio is enabled) and Android (2.6.3)
  
• TV Model: Sony Bravia 7, set to prioritize passthrough but it makes no difference whether it's the TV speakers or Soundbar
  
• Soundbar: Samsung Q800D
  
• Additional notes: Windows 11 Media Player also cannot play these files, neither can my TVs media player from USB (doesn't see video or audio codec or any details at all) but VLC on any platform can play them without issue, including the one on Android TV.
  
• Here's all the media info Jellyfin gives for most of the files: https://pastebin.com/v5M4CsDu
  

Any videos with AC3 Mono audio won't play at all unless the Android TV app is set to downmix to stereo. It loads to a black screen and appears paused. I can hit play but only the symbol changes. Interestingly, Jellyfin Dashboard shows that the video is playing, and says it is 100% compatible and Direct Playing. On Android I just can't get it to play, as I can't find an option to downmix.

No attempt is made to transcode it, and I don't understand why it would need to, but I ain't that smart haha. I have plenty of other media with AC3 codec, but it's all Stereo instead of Mono, and works without issue.

When Audio Output is set to Direct, the logs are basically empty: https://pastebin.com/Kd4ezNm2

You can see I waited a minute between pressing play and exiting. It simply sat at 0:00.

When Audio Output is set to Downmix to Stereo, the logs look like this

• General Log: https://pastebin.com/tBg7QgbL
  
• FFmpeg Log: https://pastebin.com/aaiiwaNU
  

[coffeetablecandle]

I have a debug log. Lots of:

"Jellyfin.Api.Auth.CustomAuthenticationHandler: AuthenticationScheme: "CustomAuthentication" was successfully authenticated."

When I find the section where I press play on a video, I see:

"isEligibleForDirectPlay: True, isEligibleForDirectStream: False"

and a bit further down it does say PlayMethod is DirectPlay. Then there's a mix of the CustomAuthentication" messages and:

"MediaBrowser.MediaEncoding.Transcoding.TranscodeManager: PingTranscodingJob PlaySessionId="IRedactedThis" isUsedPaused: False"

over and over until I press stop.

Does that mean it thinks it's direct playing but also think it needs to transcode, and therefore waiting for transcoding that will never happen?

[gnattu]

The problem could be that your device does not really support mono channel AC3 audio but the device profile says it does. Currently we don't have custom device profile config which means you have to enforce this downmix to stereo option to avoid compatibility issues like this.

[coffeetablecandle]

The problem could be that your device does not really support mono channel AC3 audio but the device profile says it does. Currently we don't have custom device profile config which means you have to enforce this downmix to stereo option to avoid compatibility issues like this.

I created a plain mono AC3 track, and my PC, phone, TV media player, and Jellyfin have no issues. I created a MKV with the same info (minus subtitles), no issues again.

If I transcode the issue media with Handbrake and keep everything the same (at least everything VLC Codec Info shows me), there's also no issues anywhere.

So I believe my systems support AC3 Mono, and I'm stumped at where to look next. Does it seem likely that an original encoding error is at fault, and is there actually a way to investigate that?

[gnattu]

So I believe my systems support AC3 Mono, and I'm stumped at where to look next. Does it seem likely that an original encoding error is at fault, and is there actually a way to investigate that?

It could be. If that is the case, probably the best thing you can do is to re-encoding the audio part. Can you try to do that manually and see if it fixes the problem for you? If so, it is indeed the original encoding has some quirk.

[theguymadmax]

I bet there's something wrong with the container. Just do a straight remux to fix any container errors. Use MKVToolNix or ffmepg.

[coffeetablecandle]

I bet there's something wrong with the container. Just do a straight remux to fix any container errors. Use MKVToolNix or ffmepg.

I think this must be the answer. I used ffmpeg to do a straight copy to a new MKV and the new file plays everywhere without issue. It did set the subtitles to "Default: Yes" but my server automatically sets the default to off, so I'm not sure if that could be the issue. No logs have ever complained about the subtitles either.

I know this is no longer a Jellyfin issue, but do you have any advice for diagnosing the exact issue with the original file? The mystery is really bugging me now.

[theguymadmax]

You can use mkvalidator to check the mkv containers. I'm sure it'll come back with errors vs the remuxed version. Besides checking manually, I know of no other way to determine this, other than noticing the video has errors when played.