Why is AV1 HDR10 being transcoded to HEVC on Fire TV (causing tone mapping and HDR loss)?

[slevin7 86]

Hi everyone,

I’m trying to understand why Emby is transcoding the video in this case, and maybe I’m overlooking something.

The source file is:

• Video: AV1, 4K, HDR10 (10-bit, bt2020, smpte2084)

• Audio: DTS-HD MA (German 5.1)

• Subtitles: internal SRT + PGS

I fully understand that DTS isn’t supported on the Fire TV Stick, so transcoding the audio to AC3 makes perfect sense.
What I don’t understand is why the video stream is also being transcoded from AV1 to HEVC.

My Fire TV Stick normally has no issues playing AV1 + HDR10 directly, so I was expecting Emby to:

Direct Play / Direct Stream the AV1 video,
and only
transcode the DTS audio track.

However, Emby does a full video transcode, and because of that, tone mapping is applied – which means I lose HDR10 and end up with a tone-mapped version instead of the original HDR output.

So I’m wondering:

• If the only listed reason is TranscodeReasons: AudioCodecNotSupported, why is the video being transcoded at all?

• Is this expected behavior due to HDR10 + AV1 + subtitles + Fire TV limitations?

• Or is there possibly something I’ve overlooked or misconfigured?

Any insight or explanation would be really appreciated. Thank you!

ffmpeg-transcode-524db88e-3218-4b2b-bf75-3a0b8b6978c9_1.txt

[Luke 42077]

Hi there, did you explore the stats feature in the video player to learn why it was transcoding?

[slevin7 86]

Hi Luke, thanks for your reply!

Yes, I checked the playback stats — I’ve attached two screenshots to show the difference.

Screenshot 1 (Transcoding):

• Source: 4K HDR10 AV1

• Audio: DTS-HD MA 5.1

• Emby transcodes both audio (to AC3) and video (AV1 → HEVC)

• Because of that, tone mapping (Hable) is applied, so HDR10 is no longer passed through.

Screenshot 2 (Direct Play):

• Same movie, same device

• Only difference: I manually converted the audio track to EAC3 beforehand

• Result: Direct Play for both video and audio → HDR10 works perfectly, no tone mapping

So to summarise my question:
If the only unsupported part is the DTS audio track, why doesn’t Emby just direct stream the AV1 video and only transcode the audio?
Is there a technical limitation that forces video transcoding when audio needs conversion?
Or is there perhaps a setting or detail I’ve overlooked?

I’d really appreciate any insight — especially since AV1 + HDR10 plays fine on this Fire TV Stick when the audio is already in EAC3.

Thanks again for your help!

[Neminem 1518]

I guess you have enabled this feature.

[slevin7 86]

I do

[Neminem 1518]

ok that's your answer.

The stream need to be dismantled and combined again.

I order to alter the audio, if Enable HVEC is not enabled it would fall back to h264 / x264 codec.

And delivered to you client.

Edit.

And since does not of yet transcode in AV1 it either h264 or h265

Edited November 6, 2025 by Neminem

[slevin7 86]

Thanks for the explanation! I understand that the stream needs to be demuxed to convert the DTS audio.
But wouldn’t it still be possible to copy the AV1 video stream and only transcode the audio (instead of fully re-encoding the video to HEVC)?

Especially since the same file does play in Direct Play when I manually convert the audio to EAC3 beforehand.
So the client (Fire TV) can handle AV1 + HDR10 just fine.

 

Edited November 6, 2025 by slevin7

[speechles 2055]

HLS is used to transcode. Emby uses the TS container inside the m3u8. HLS only supports AV1 when the fMP4 container is used. When/If Emby eventually uses fMP4 this would indeed be possible. But today it is not.

[Luke 42077]

Not all platforms can handle av1 in HLS, so we'll have to look at it for android. Thanks.

[slevin7 86]

Thanks a lot for the explanations – that really helped me understand what’s going on behind the scenes.

So just to recap:
AV1 + HDR10 would technically be fine on the Fire TV, but as soon as audio transcoding kicks in (DTS → AC3), Emby switches to HLS with TS segments. Since TS doesn't support AV1, the video gets transcoded too – and tone mapping/HDR loss comes along for the ride. Totally makes sense now.

That said… may I politely submit a tiny feature wish?
It would be amazing if in the future there were an option like:

“Transcode audio only, keep video stream intact (Direct Stream), even when using HLS – as long as the client supports it.”

Or even better:
Support for fMP4/CMAF HLS with AV1, so Fire TV / Android devices could keep their shiny HDR10 untouched while only the DTS track gets converted.

No pressure of course – just planting the seed
And again, thanks for the detailed responses and all your work!

[FrostByte 5392]

1 hour ago, slevin7 said:

 

That said… may I politely submit a tiny feature wish?
It would be amazing if in the future there were an option like:

“Transcode audio only, keep video stream intact (Direct Stream), even when using HLS – as long as the client supports it.”

 

You could try turning off video transcoding only in user settings.  It's just an all or nothing option though and you would need to keep flipping it if you had stuff which needed video transcoding.

[slevin7 86]

Turns out it wasn’t Emby after all – it was me

I had the option
“Convert unsupported surround audio to Dolby Digital”
enabled in the Fire TV app settings.
Once I disabled it, Emby started direct playing DTS just fine – no transcoding, no tone mapping, HDR10 stays intact.

So… lesson learned:
Sometimes the real transcoder was inside us all along.

Big thanks again for your help and explanations – and I’ll still happily vote for fMP4/HLS + AV1 support in the future!