Whats the difference between AC3 and TrueHD AC3 core tracks

[preiner]

Hi,
My UHD disks come with Dolby TrueHD 7.1 (ATMOS) as well as AC3.

I need both because several of my playback systems dont have 7.1 yet and transcoding them in realtime causes extreme stuttering (even with my quadro card enabled)

When ripping the video, MakeMKV shows 3 audio tracks. They are (in order)

1 AC3
2 TrueHD 7.1
3 AC3 (core)

What is the difference between tracks 1 and 3? They SEEM to always contain the same audio stream.

I was guessing that somehow MMKV can rip out 5.1 from the 7.1 and maybe thats why its called "core".
But if thats correct, why show it? Every TrueHD released has to have an AC3 track already.

Is there any use case where I would need / prefer this track?

Lastly. Is there a selection string that would automatically deselect this track?

Thanks for any help here.

[Ezatoka]

I was guessing that somehow MMKV can rip out 5.1 from the 7.1 and maybe thats why its called "core".
But if thats correct, why show it? Every TrueHD released has to have an AC3 track already.

Yes, it does take the core audio from the higher quality version. But you are incorrect to say, that every disc would have a single AC3 track.

Avengers Endgame BluRay:

(https://i.imgur.com/TESf8p2.png if the picture doesn't load)

[preiner]

Thanks for the clarification.
Is there a reason why there would be an AC3 track listed as default as well as the AC3 core track when they both contain same content?

I would understand if one was alternate dialog or similar.

I ask because Im trying to reduce confusion during playback. All of my Emby devices ignore the audio track name that I assign (such as commentary) and instead only display AC3 or ATMOS.

Family ends up seeing multiple streams labeled AC3 with no description.

Additionally im not sure how to write a selection string that would remove the AC3 track. I dont think -sel:AC3 is valid.

In fact I cant find any list of valid tokens. Like is -sel:5.1 valid?

Argh.

[Ezatoka]

I cannot give a distinctive answer. All I can add is, that I had BluRays, where there was a 5.1 DTS track as well as a 5.1 PCM track of the same language. And they also contained the same audio content. I guess, the muxers might add those tracks for being the most compatible with all kind of players.

[dcoke22]

Someone else had a similar question recently: viewtopic.php?f=8&t=31340

I don't have any experience with Emby (I use Plex). I'm surprised that Emby can't find the lossy core of one of the lossless formats and use it without transcoding.