TrueHD/AC3 Core???

[liquidskin76]

Hi,

Can someone clear up a query i have regarding TrueHD/AC3Core?

Does a TrueHD soundtrack have a separate core AC3 file? When you downconvert to AC3 using tsmuxer for example, is it extracting a separate AC3 file, or is it converting it to AC3 (re-encoding)?

In tsmuxer, in the TrueHD track info, it reports 'TrueHD+ AC3 Core, Peak Bitrate...'. I take that as there being a separate AC3 file sitting within the TrueHD file.

Thanks!

[rica]

Hi,

Can someone clear up a query i have regarding TrueHD/AC3Core?

Does a TrueHD soundtrack have a separate core AC3 file? When you downconvert to AC3 using tsmuxer for example, is it extracting a separate AC3 file, or is it converting it to AC3 (re-encoding)?

In tsmuxer, in the TrueHD track info, it reports 'TrueHD+ AC3 Core, Peak Bitrate...'. I take that as there being a separate AC3 file sitting within the TrueHD file.

Thanks!

Yes, thd+ac3 means they are seperate files and for blurays ac3 core has been put for backward compatibility.
While dts core is a must and a part of dts-hd.(core+extension)
So it means without a dtscore, dtshd can not play while thd works seperately without an ac3 core.
So extracting ac3 is not a downconversion of thd. Just an extraction of the core, not an re-encode.

[Snowknight26]

Just an extraction of the core, not an re-encode.
Not always.

[Blue_MiSfit]

Yeah, rica almost had it right :)

THD doesn't always contain an AC3 core
DTS-HD DOES always contain a DTS core

So, if your THD does contain an AC3 core, you can extract this natively without any transcoding.

If your THD does NOT contain an AC3 core, you must decode the THD and re-encode to AC3 (presuming you want AC3 output, of course!)

Derek

[shon3i]

Not always.
When you need to rencode then? AC3 is embedded to THD (is not part of stream like DTS is), and can be always extracted without any transcoding/encoding.

THD doesn't always contain an AC3 coreWell THD on Blu-Rays must contain AC3 core, that is mandatory by specification. Only HDDVD's THD can arrive without core or with DD+ core. But it is already forgotten story

[liquidskin76]

Well THD on Blu-Rays must contain AC3 core, that is mandatory by specification. Only HDDVD's THD can arrive without core or with DD+ core. But it is already forgotten story

That's what i thought... TrueHD always has an AC3 core.

[rica]

Yeah, rica almost had it right :)

THD doesn't always contain an AC3 core


Oh sorry, i've almost forgotten HD-DVDs and some mkv files created by mkvmerge gui :rolleyes:

[liquidskin76]

I've been reading some of the official tech spec for TrueHD (plus mrbass over at the MakeMKV forum has also said). It looks like TrueHD doesn't contain an actual AC3 file, just metadata that tells how to downconvert to ac3.

Still not 100% sure on that one?

[liquidskin76]

In the end, what i'm trying to figure out is why (using ffdshow for instance) i can bitstream AC3 from a TrueHD file within an m2ts container, but can't bitstream AC3 from a TrueHD file within an mkv container (created with either MakeMKV or MKVMerge)?

I know mkv's created with the new SMM muxer can retain the same behaviour as an m2ts file, so why can't MakeMKV and MKVMerge? I'm guessing they discard some metadata?

[liquidskin76]

Going on the metadata idea, if the blu-ray truehd track contains only metadata, then how does tsmuxer, eac3to, etc create a thd+ac3, which is actually a truehd and ac3 file interweaved together?

I'm rapidly getting lost!

Can madshi explain with regards to eac3to?

[rica]

It might help?

http://forum.doom9.org/showpost.php?p=1183559&postcount=1



_ _ _ _

[liquidskin76]

Cheers rica for the info.

I see what happens now... so when demuxing with tools like tsmuxer and eac3to, they take the TrueHD file, create (by downconverting) an AC3 track from it, then interweave them together to create a thd+ac3 file.

So there is no separate AC3 core within TrueHD. Just metadata containing info on how to downconvert.

Well, i stand corrected... i always thought there was a separate file!!

[madshi]

There are 2 different TrueHD streams:

(1) TrueHD+AC3. This is a mixed format. There are both (alternating) TrueHD and AC3 frames in the stream.

(2) TrueHD. This stream only contains TrueHD frames and no AC3 frames.

All Blu-Ray TrueHD streams are (1). All HD DVD TrueHD streams are (2). eac3to can convert between both stream types. Converting from (1) to (2) is very easy: The AC3 frames are simply removed. Converting from (2) to (1) is more difficult: The TrueHD stream must be decoded and reencoded to AC3 and then the AC3 frames must be injected into the TrueHD stream in a specific way.

When muxing to MKV, the AC3 frames are usually removed, so MKV TrueHD streams are usually type (2).

There's no special metadata which tells anyone how to convert TrueHD to AC3, AFAIK.

[liquidskin76]

Bingo, cheers madshi, that's explains it perfectly. So there is a bloody ac3 core within blu-ray truehd!

Mosu has explained to me (thanks for the info mosu!) why mkv's created with mkvmerge only contain type (2) truehd. MKV spec says only one audio codec can be used per audio track. I guess this must just be a guideline as the new smm mkv muxer muxes as type (1).

I expect makemkv follows the same guideline as mkvmerge.

[SeeMoreDigital]

Mosu has explained to me (thanks for the info mosu!) why mkv's created with mkvmerge only contain type (2) truehd. MKV spec says only one audio codec can be used per audio track. I guess this must just be a guideline as the new smm mkv muxer muxes as type (1).Hmmm...

I've been testing the following .M2TS sources with my XtreamerPro hardware player (using beta firmware): -

5.1Ch TrueHD with 5.1Ch AC3 core
7.1Ch TrueHD with 5.1Ch AC3 core

5.1Ch DTS-HD HR with 5.1Ch DTS core
5.1Ch DTS-HD MA with 5.1Ch DTS core

7.1Ch DTS-HD HR with 5.1Ch DTS core
7.1Ch DTS-HD MA with 5.1Ch DTS core

On all occasions the player detects the HD audio segment... So on that basis, it is not advantageous to mux to the Matroska container :eek:

EDIT: I'm not looking forward to explaining this MKV container "HD audio issue" to Xtreamer hardware player owner users...

[mrbass]

I've been reading some of the official tech spec for TrueHD (plus mrbass over at the MakeMKV forum has also said). It looks like TrueHD doesn't contain an actual AC3 file, just metadata that tells how to downconvert to ac3.

Still not 100% sure on that one?

I am also over here but yes I believe from what I have read is it takes the 3 channels and 2 channels and by remixing them creates the 5.1 AC3 for you. This is more efficient way and saves amount of data to be decoded when unable to play truehd stream.

Specifically this section from the official truehd spec. Rematrixing not remixing my bad

If a listener selects a 5.1 presentation, the decoder reconstructs it from the
two-channel downmix plus the 3.1-channel extension A substream by means of rematrixing.

[tebasuna51]

But don't mistake the downmix of TrueHd 7.1 to 5.1 make by the decoder with the AC3 track always max 5.1.
Then:
"it takes the 3 channels and 2 channels and by remixing them creates the 5.1 AC3 for you."
is not exact, if the receiver (only 5.1) accept TrueHD make the downmix and play them, but not create AC3.
If the receiver don't accept TrueHD the AC3 5.1 track is sended and played.

[mrbass]

What it is saying is if you have 8 speakers you can take full advantage. If only six speakers then it'll only be required to decode that. Same with 5.1 setup. It makes sense since some might buy a new truehd receiver but wont quite just yet upgrade to Kevlar 8 speaker setup.

Below is tidbits from the PDF of the official specs I quoted over on makemkv forum.

Channel Extensions, Downmixing, and Dolby TrueHD
One channel extension technique is the method by which MLP Lossless, Dolby TrueHD, and MPEG-2 LII deliver compatible downmixes for soundtracks with expanded channels. In these codecs, a 7.1-channel soundtrack is first downmixed to create a 5.1 mix...

If a listener selects a 5.1 presentation, the decoder reconstructs it from the two-channel downmix plus the 3.1-channel extension A substream by means of rematrixing.

An A/V receiver with Dolby Digital decoding that has only a coaxial or optical digital audio input needs only a Blu-ray Disc player with a Dolby Digital Compatible Output that provides a compatible Dolby Digital bitstream from its internal mixer.

Due to the substream structure of Dolby TrueHD, a single Dolby TrueHD program can be used to deliver a two-, six-, or eight-channel presentation, each with precise control over the presentation as defined by the content producer. This means that an HD player needs to decode only the number of channels it can output, thus enabling more economical DSP decoder designs.

It should be noted that it is also possible for the two- and 5.1-channel presentations to be carried independently if it is important to avoid downmixing due to artistic reasons. In this case, however, the bit rate will increase due to the carriage of additional channels.

[madshi]

I am also over here but yes I believe from what I have read is it takes the 3 channels and 2 channels and by remixing them creates the 5.1 AC3 for you. This is more efficient way and saves amount of data to be decoded when unable to play truehd stream.
No, that "rematrixing" the spec is talking about has nothing to do with AC3. The TrueHD compressed audio frames consist of a number of substreams. With a 7.1 TrueHD track a decoder can choose to only decode 2.0, simply by ignoring the 5.1 and 7.1 subchannels. Or the decoder can choose to only decode 5.1, by ignoring the 7.1 subchannels. The Nero 7 TrueHD decoder does that, it never decodes 7.1. All this stuff about rematrixing and subchannels talks about TrueHD, only. This all has nothing to do with AC3. If you want to output AC3, there is no way to take the TrueHD audio frames and "rematrix" them into AC3. The only way to get AC3 from a TrueHD stream is either:

(1) If you have a Blu-Ray style interweaved TrueHD+AC3 track, simply use the AC3 frames and discard the TrueHD frames.

(2) If you have a TrueHD only track, the only way to get AC3 is to fully decode the TrueHD track and reencode it to AC3.

[jasonwc]

If you add a thd+ac3 track to an MKV, it will strip out the AC3 track. However, you can have eac3to demux the TrueHD and core AC3 track seperately, and merge both into your MKV. They'll just show up as separate tracks.

This isn't an issue for DTS-HD MA as the core is embedded in the track, and the lossless portion is essentially a remainder over and above the lossy core, necessary to decode losslessly. In contrast, a TrueHD track is independent of the AC3 core, and can be played without it.

[ddeell72]

How to make pure THD out of ".thd+ac3" ???

[tebasuna51]

Please delete this thread (double post), your answer is here: http://forum.doom9.org/showthread.php?p=1425368#post1425368