Feature Request... Retain TrueHD 'AC3 Core'

[liquidskin76]

Hi,

I can see the logic in not retaining the TrueHD AC3 core (as TrueHD doesn't depend on the core like DTS-HD MA does), and instead providing it as a second additional audio track. However, in certain situations, it would make sense to be able to leave the AC3 core in place (like in a m2ts container), so you don't have to go through the process of selecting a different audio track.

Here's an example (which is what i come up against!)...

You have single networked source where your media files are stored. You have 2 htpc's that both connect to this source. The first htpc that can bitstream truehd to a truehd compatible receiver. No problems with not having core AC3.

The second htpc only has spdif and is connected to a receiver that can only decode ac3. The problem you find it that whilst mpc-hc, ffdshow, etc can decode truehd and output as 5.1 lpcm, spdif cannot handle the bitrate, and outputs as 2 channel.

Hence the wish to be able to retain the AC3 core, so you don't have to start the movie, then pause it, then chance audio track, then unpause!

I expect this has been discussed before?

Many thanks

[mike admin]

I don't understand the question. MakeMKV CAN preserve TrueHD audio, its AC3 core or both - you just have to select what you want before making MKV file. In any case AC# core will be a separate track since TrueHD has no core by definition - unlike DTS it's always two separate tracks.

[liquidskin76]

I mean having the option to keep the ac3 core within the truehd track, not separating them. So when you play the movie on a computer that cannot bitstream truehd, and is connected with spdif, you get the ac3 core automatically (without having to select a second track manually).

Many thanks

[mike admin]

Unlike DTS TrueHD has no core. It's always two separate tracks on a blu-ray disc - one TrueHD and one AC3.

[liquidskin76]

TrueHD does have a AC3 core, it's just that it doesn't rely on it like DTS-HD does. Doesn't MakeMKV gives you the option to use the ac3 core instead of TrueHD? I've created mkv's using MakeMKV from movie only backups (created with DVDFab) that only have the TrueHD soundtrack. All other tracks have been removed. MakeMKV shows the option to use ac3 instead of TrueHD... it has to be using the core ac3 file, right?!

The are loads of tools to extract the ac3 core... eac3to, tsmuxer, etc.

You're right in that blu-ray discs sometimes have a seperate ac3 copy of the sound track, however the main TrueHD track will always have a core ac3 file. When you play a blu-ray disc in a player, if you are connected to a receiver that doesn't bitstream truehd, you get the ac3 core. You don't have to select a different audio track.

Thanks!

[trondmm]

You're right in that blu-ray discs sometimes have a seperate ac3 copy of the sound track, however the main TrueHD track will always have a core ac3 file. When you play a blu-ray disc in a player, if you are connected to a receiver that doesn't bitstream truehd, you get the ac3 core. You don't have to select a different audio track.

Are you sure about this? As far as I know, on Blu-ray, TrueHD does not have a core track. There's a TrueHD and a separate, often hidden, AC3 track. If you select TrueHD, and your player detects that it's connected to a receiver that doesn't support it, it'll secretly switch to the AC3 track instead.

On HD DVD, TrueHD tracks would always include a stereo core.

[liquidskin76]

Actually, i've since found out we're all kinda right!

TrueHD doesn't have to have a core, and sometimes it doesn't, however i've yet to come across that one doesn't!

Would it be possible to have an option to copy audio as is? Then if theres a ac3 core its kept, if there isn't then so be it, you just get TrueHD?

[skittle]

TrueHD does not have a core like dtshd, they are seperate tracks.

[liquidskin76]

Make a backup of a disc. Then run it through tsmuxer removing everything but the TrueHD track, and demux. Then open the demuxed TrueHD track in tsmuxer. Right, you can now demux using the option to downconvert to ac3. How can you do that if there is no ac3 core!?

If you have movie that has been backed up as a movie only m2ts file, containing only the vid and TrueHD track. Play that movie in say mpc-hc, using ffdshow as your audio decoder. Within ffdshow, disable TrueHD decoding. You'll get ac3. Again, how can you do that if there is no ac3 core!!?

Hell, here's the simplest way of telling!! Open a backup in tsmuxer... you'll see in the TrueHD track info, it says 'AC3 core+TRUEHD. Peak bitrate...'!!!

I've done those 3 tests on loads of backups. What i'm saying is that, yes TrueHD doesn't 'need' a core like DTS-HD, however that doesn't mean TrueHD doesn't have a core. 99.9% of the time it does (i've never seen one without). There are countless tools to extract the ac3 core... eac3to, tsmuxer, etc.

Will people stop telling me TrueHD doesn't have an ac3 core!!!!!

Hi Mike,

What do the MakeMKV team think about when keeping a TrueHD track, just muxing the track 'as is'? Then if there's an ac3 core its kept, if there isn't then so be it you just get TrueHD with no core?

Many thanks!

[liquidskin76]

Where's SamuriHL, he'll back me up on this one!!

[skittle]

Because it is down converting the file. All truehd are able to down convert, but it is an extra step, makemkv cant just extract the core data! It is different than the core dts... where it is dts core+residual coding to make DTSHD. Its a bit like saying you can get an mp3 from FLAC... or it will have a seperate sound track.

[mike admin]

Will people stop telling me TrueHD doesn't have an ac3 core!!!!!

If that is your wish, yes, but that doesn't change a fact that TrueHD tracks have no core.

What do the MakeMKV team think about when keeping a TrueHD track, just muxing the track 'as is'?

That's what it does.

Overall, you are confusing concept of a "track". The logical "track" on blu-ray is a MPEG TS program, that can in theory have hundreds of tracks. Blu-ray specification requires two tracks for TrueHD - one MLP and one AC3 and a blu-ray player must choose only one during playback. This is different from DTS-HD where there is only one backward-compatible track with core and XLL packets.

[SamuriHL]

Where's SamuriHL, he'll back me up on this one!!

On vacation, but, I get now what they're saying. How it's stored inside the m2ts container is different than what we were thinking when we discussed this. With DTS-HD MA, there's literally an "extension" to the DTS core that allows for a higher bitrate and bit depth. But the core is still there in the same "file". (I'm gonna go with that for this explanation as it'll be easier than calling it a track which is where I think the confusion comes in). With TrueHD, what they're saying is that it's different. TrueHD isn't an extension of AC3 within the same "file". It's a separate "file". Some programs (eac3to, tsMuxer, etc) show it as being the same as having a core but it's not physically stored the same. So I see where the confusion is. And this was educational for me, as well, as I didn't know that it was stored differently. Also remember, that AC3 core is not required on the blu-ray standard, but, generally with all the ones I've seen it's usually there.

Now for the part that confuses me. Tools like eac3to and tsMuxer can extract AC3 audio from the logical "track". But what I think liquidskin76 is saying is that after he makes an MKV with MakeMKV, the tools no longer are able to "see" the AC3 audio track. So, I'm not sure what's happening there, but, I think this should explain both sides of what's going on and maybe help to bridge the gap a little bit. Maybe?

[liquidskin76]

Hey guys,

Check this out over at doom9...

http://forum.doom9.org/showthread.php?t=155499

The general consensus is that TrueHD does have a separate AC3 core file within the TrueHD file. This can be extracted natively without re-encoding.

There's new muxer posted at Doom9 (http://forum.doom9.org/showthread.php?t=154055) that just muxes the TrueHD audio as is, and therefore includes the core AC3 file if it exists (which is always!). MKVMerge doesn't mux as is and removes the ac3 core if it exists. I've discussed this with Mosu, the author of MKVMerge.

I did another test last night. I made a movie only BD (using tsmuxer) with 'ONLY' the TrueHD track in it. I burn't that to a BD disc and played it in my Sony BD player. Using HDMI i get TrueHD. Using old fashion optical i get AC3. How can it do that if there no AC3 core. It's not re-encoding to AC3 on the fly!

There's not a lot more i can say! All i'm after is a way to get a TrueHD file in an mkv container, the extact same way as a TrueHD file sits in a m2ts container!

Hey Mike,

You said MakeMKV just copies the TrueHD track as is. I'll run another test using a MakeMKV mkv containing ONLY the TrueHD track and see if i get TrueHD over HDMI and AC3 over optical. It maybe that i'm mistaken, and the TrueHD track produced by MakeMKV does keep the AC3 core.

I'm sure i've done this test before however i think i'm starting to think i'm losing my mind!

Thanks

[liquidskin76]

Hi Mike,

When you load up a TrueHD movie with MakeMKV, branching off from TrueHD is the option use AC3. I'm not refering to other AC3 audio on the disc, just the AC3 audio that is seen branching off the TrueHD audio.

How is MakeMKV getting that AC3 audio? Surely that's the AC3 core?

Thanks