[BUG] [SOLVED] Plex Server 1.11.1.4760 4k 5.1 DTS audio to AC3 wrong audio channel

When playing a 4k video that has DTS 5.1 the AC3 audio transcode maps center to the right audio channel.

Transcoding the audio to AC3 with ffmpeg fixes the issue. It only happens when the plex server does it on the fly.

Client is ATV 4k - 1.24 (8081)

Media XML is attached

Edit: added ATV 4k plex app version

And there I thought I was going crazy… my audio receiver seems to have issues when AAC is played so I tend to get plex to transcode to AC3 on the fly and I see the same thing! I thought I must’ve been doing something wrong!
I wonder whether it transcodes to AAC but in an AC3 container hence the swap?

@chaosmaker said:
And there I thought I was going crazy… my audio receiver seems to have issues when AAC is played so I tend to get plex to transcode to AC3 on the fly and I see the same thing! I thought I must’ve been doing something wrong!
I wonder whether it transcodes to AAC but in an AC3 container hence the swap?

No idea… I think its just a bug in how they are transcoding it on the fly. From my experience this issue is specific to the source audio being DTS-HD MA

I have the exact same issue: I have tested various media files and I see that both 6 channel AAC and DTS5.1 audio are being transcoded to AC3 with the center channel coming out of the right speaker.
Client: ATV 4k v1.23 (8012)
Server: 1.11.2.4772 (osx)

No response from plex?

I’ve noticed the same thing, and am filing an issue with the developers for this.

@DaveBinM do you know if this issue has a #number so I can look out for it if it was fixed in a server update?

@chaosmaker I’m still investigating this one. I’ll let you know once there’s more to share

I have the same problem. The strange thing is that it happens with some DTS files but not with others and I can’t see the difference between them.

Still the same with Plex 1.12, and tvOS 11.3b4.

Center mapped to right.

At one point in time, ffmpeg did not transcode AAC and DTS correctly to AC3, and the results were the same as reported here. I don’t pretend to understand all of the technical reasons, but basically, it was because the channels are mapped differently between the various formats. That is, the right channel on AAC and DTS are located in the slot where the center channel is located for AC3. You might not notice it, but I believe the rear channels are mapped slightly different as well.

Ffmpeg corrected the problem some time ago. I’m pretty sure Plex uses a modified version of ffmpeg in the background, and maybe they missed that update. That would explain why a file that you transcode yourself has the channels mapped properly, and the Plex transcoder does it wrong.

It goes without saying, that is just a wild guess. Just some armchair trouble shooting from someone who had the problem before I even heard of Plex and used ffmpeg to convert videos for storage on my computer.

Could you share a sample where the issue is more pronounced (for instance, with dialogue)? It’s difficult to tell what’s going on in the logo sequence you shared in https://forums.plex.tv/discussion/comment/1628771/#Comment_1628771

@rcombs said:
Could you share a sample where the issue is more pronounced (for instance, with dialogue)? It’s difficult to tell what’s going on in the logo sequence you shared in https://forums.plex.tv/discussion/comment/1628771/#Comment_1628771

Sure here is the new sample that is a gig. https://www.dropbox.com/s/1jzhupxy07zrfr8/Inception%20(2010)%20Remux-2160p.oth.mkv?dl=1

I’ve done some testing of my own and in my particular case it seems to be when the audio doesn’t have a proper channel layout. (I.e. it says there are 6 channels instead of 5.1) Running ffmpeg, which then guesses the channel order fixes my issues completely.
You can check the channel layout by doing

ffprobe -v error -select_streams a -show_entries stream=channel_layout -of default=noprint_wrappers=1:nokey=1 file

If it says unknown, I noticed Plex has issues to order the channels accordingly.

@chaosmaker said:
I’ve done some testing of my own and in my particular case it seems to be when the audio doesn’t have a proper channel layout. (I.e. it says there are 6 channels instead of 5.1) Running ffmpeg, which then guesses the channel order fixes my issues completely.
You can check the channel layout by doing

ffprobe -v error -select_streams a -show_entries stream=channel_layout -of default=noprint_wrappers=1:nokey=1 file

If it says unknown, I noticed Plex has issues to order the channels accordingly.

I think it is set corectly

ffprobe -v error -select_streams a -show_entries stream=channel_layout -of default=noprint_wrappers=1:nokey=1 Inception\ \(2010\)\ Remux-2160p.mkv 
5.1(side)

@chaosmaker said:
I’ve done some testing of my own and in my particular case it seems to be when the audio doesn’t have a proper channel layout. (I.e. it says there are 6 channels instead of 5.1) Running ffmpeg, which then guesses the channel order fixes my issues completely.

I think that tidbit of info has nailed the problem.

A quick google search will find that if the channel layouts aren’t defined, the default layout for multi-channel audio is:
FL, FR, FC, LFE, SL, SR
(that is also the standard channel location for WAV and FLAC audio, I believe)

and the default layout for multi-channel AC3 is:
FL, FC, FR, SL, SR, LFE

So if the Plex transcoder is confused by the ambiguous “6 Channel” flag and chooses the default channel layout, the transcoded AC3 file would end up like this:

CHANNEL #1 (Front Left)     >>  Front Left
CHANNEL #2 (Front Right)    >>  Front Center
CHANNEL #3 (Front Center)   >>  Front Right
CHANNEL #4 (LFE)            >>  Surround Left 
CHANNEL #5 (Surround Left)  >>  Surround Right
CHANNEL #6 (Surround Right) >>  LFE

I would guess that ffmpeg assumes all 6 channel audio is “5.1 (side)” and applies the correct channel layout, and the Plex transcoder simply transcodes to AC3 in the standard 6 channel layout. If you can follow my example, the surround and low frequency channels are probably messed up too. Unfortunately, I can’t find a file with a 6 channel flag to test.

You guys are good at sleuthing! Now we just need a fix to get coded in =)

At least you lot are getting audio I don’t get any sound from my LG B6 only when 4K MKVs with DTS HD 5.1/7.1 transcodes to eac3 5.1 and the ATV 4K ‘SURROUND SOUND’ is set to ‘best quality available’ or ‘DOLBY DIGITAL 5.1’, I only get sound when the ATV 4K ‘SURROUND SOUND’ is set to ‘STEREO’!

4K MKVs TrueHD transcodes to eac3 correctly having ATV4K set to best quality or Dolby

1080p MKvs h264 DTS transcodes to eac3 correctly also with ATV 4K set to best quality or Dolby

@leelynds no need to test - that is exactly what I found with the ones I have that say 6 channel and have an unknown layout - if you want I’ll grab a sample later on?
@WatchTowerPlex I’ll download your sample and have a look at how it looks on my system and will let you know shortly what I find

@Nik3sh I am a bit confused by your message sorry. Why are you using an ATV when the LG B6 has “now” a pretty good client? And I presume the ATV is connected via HDMI, and you have another HDMI connected to a home theatre system via ARC? On your TV is your sound set to pass through as “auto” or as “PCM”? Technically the B6 should be able to handle DTS HD unless you need to transcode it for your HT?

@chaosmaker said:
@leelynds no need to test - that is exactly what I found with the ones I have that say 6 channel and have an unknown layout - if you want I’ll grab a sample later on?
@WatchTowerPlex I’ll download your sample and have a look at how it looks on my system and will let you know shortly what I find

@Nik3sh I am a bit confused by your message sorry. Why are you using an ATV when the LG B6 has “now” a pretty good client? And I presume the ATV is connected via HDMI, and you have another HDMI connected to a home theatre system via ARC? On your TV is your sound set to pass through as “auto” or as “PCM”? Technically the B6 should be able to handle DTS HD unless you need to transcode it for your HT?

The question is why shouldn’t I use my ATV 4K? what’s so confusing about it? I want things to work! Why do you use one? Seriously What a mundane question to ask!

The LG B6 does not have a gigabit port so when playing high bit 4K videos it buffers. BtW I have a shield TV as well and everything direct plays or transcodes audio great on that, I hope you’re not going to ask me on why I don’t use that now. I’m just reporting on my ATV 4K issues.

Also I said 1080p MKV blu rays with DTS HD transcoding to eac3 when the ATV 4K surround setting is set to BEST Quality available, works! just not with 4K MKVS on my LG B6.

My ATV 4K is connected direct to the TV, I have no receiver, with LG B6 hdmi Sound set to AUTO.

You know ATV doesn’t support passthrough of DTS natively so of cause plex server needs to transcode to a suitable audio codec no matter how or what you hook your ATV 4K to, be it direct to the TV or if you have a receiver. The Infuse app on the otherhand can passthrough DTS and it works because their own app supports that codec.

Sorry If I sound annoyed but Plex has so many bugs on the ATV4K I shouldn’t have to resort to another Plex app because I have it, when I buy something I expect it to work or at least get some professional feedback.

My only guess is,for my TV, that there’s a problem with TVOS 11.3 beta. Just need a Plex employee to look at this, but haven’t heard anything.