What's the deal with Dolby TrueHD/AC3 Core?

[Starburst]

As I understand it both of them are codecs but people are telling me that both of them are in the same track with AC3 at the core, what does that mean and how is that possible? Isn't only one codec used for each track and isn't AC3 a lossy codec?

[hello_hello]

The way I understand it, TrueHD and AC3 are separate audio streams, but in the case of Bluray players at least, they're seen by the player as a single audio track, and the player can decode either just the AC3, which is lossy, or the TrueHD, which isn't..... assuming it supports TrueHD (it's optional for Bluray players). TrueHD doesn't have an AC3 "core" as such. They're two independent streams. The lossless TrueHD can be decoded on it's own without needing data from the AC3 stream.

The "core" concept probably comes from the way DTS works. In the case of DTS, backwards compatibility was achieved in a different way. DTS-HD has a lossy "core" as well as a stream containing the data required to "make up the difference" between the lossy encoding and the original lossless audio. In that respect they're tied together. A player can decode just the lossy core, or if it supports DTS-HD it can decode the core along with the additional data required for lossless.

[Cornucopia]

That is my understanding of both of those codec families as well.

Scott

[Starburst]

The way I understand it, TrueHD and AC3 are separate audio streams, but in the case of Bluray players at least, they're seen by the player as a single audio track, and the player can decode either just the AC3, which is lossy, or the TrueHD, which isn't..... assuming it supports TrueHD (it's optional for Bluray players). TrueHD doesn't have an AC3 "core" as such. They're two independent streams. The lossless TrueHD can be decoded on it's own without needing data from the AC3 stream.

The "core" concept probably comes from the way DTS works. In the case of DTS, backwards compatibility was achieved in a different way. DTS-HD has a lossy "core" as well as a stream containing the data required to "make up the difference" between the lossy encoding and the original lossless audio. In that respect they're tied together. A player can decode just the lossy core, or if it supports DTS-HD it can decode the core along with the additional data required for lossless.

That is my understanding of both of those codec families as well.

Scott

TY, it cleared things up for DTS-HD, but for Dolby there's still one thing that I don't understand, if the TrueHD track can be decoded and played on it's own then why put the AC3 stream in there in the first place?

[pandy]

TY, it cleared things up for DTS-HD, but for Dolby there's still one thing that I don't understand, if the TrueHD track can be decoded and played on it's own then why put the AC3 stream in there in the first place?

Legacy heritage? AC3 is de facto widely accepted standard and as such in theory it is royalty free...

[Starburst]

TY, it cleared things up for DTS-HD, but for Dolby there's still one thing that I don't understand, if the TrueHD track can be decoded and played on it's own then why put the AC3 stream in there in the first place?

Legacy heritage? AC3 is de facto widely accepted standard and as such in theory it is royalty free...

I see ty

[hello_hello]

Plus for Bluray there's three compulsory formats all players must be able to decode. AC3, DTS and PCM. BD-ROM titles must use one of those types for the primary soundtrack. The newer formats, Dolby Digital Plus and DTS-HD High Resolution (lossy) and Dolby TrueHD and DTS-HD Master Audio (lossless) are all optional. So if a Bluray title was to contain only Dolby audio, it'd have to contain AC3 as it's one of the compulsory formats.

Chances are some of the budget model Bluray players only supported the compulsory formats, at least in the early days.

I haven't worked with much TrueHD audio but if you open a Bluray title using MeGUI's HD Streams Extractor (a GUI for eac3to). It shows a single TrueHD stream you can extract with a thd+ac3 extension, with a thd extension or with an ac3 extension, and that gives you both thd+ac3 together, just the TrueHD or just the AC3 respectively. If you open the thd+ac3 extracted file with MKVMergeGUI it shows two individual streams, so it's kind of like the thd+ac3 are together in a container which is in turn inside the m2ts file on the disc.

[_Al_]

What's the difference between encoding TrueHD into 640kbps 5.1 or extracting that core 640kbps, 5.1 using eac3to? What advantages are there between those two approaches just to get AC3 out of TrueHD audio on Blu-Ray, did anybody try to hear the differences, if there are any?

[anana]

AC3/DD5.1 -> core of Dolby TrueHD and Atmos
DTS -> core of DTS-HD

Here's some info from my Bluray:

TrueHD
Dolby TrueHD Audio English 1540 kbps 5.1 / 48 kHz / 1540 kbps / 16-bit (AC3 Core: 5.1 / 48 kHz / 640 kbps)

Atmos
Dolby TrueHD Audio / 7.1 / 48 kHz / 4512 kbps / 24-bit (AC3 Embedded: 5.1-EX / 48 kHz / 640 kbps)

DTS-HD
DTS-HD Master Audio English 4229 kbps 7.1 / 48 kHz / 4229 kbps / 24-bit (DTS Core: 5.1 / 48 kHz / 1509 kbps / 24-bit)

[anana]

What's the difference between encoding TrueHD into 640kbps 5.1 or extracting that core 640kbps, 5.1 using eac3to? What advantages are there between those two approaches just to get AC3 out of TrueHD audio on Blu-Ray, did anybody try to hear the differences, if there are any?

In my experience, if I encode TrueHD / Atmos using FFmpeg, the ac3 sound louder than the extracted ac3

[hello_hello]

In my experience, if I encode TrueHD / Atmos using FFmpeg, the ac3 sound louder than the extracted ac3

If I'm re-encoding multichannl audio as multichannel audio I tend not to change the volume in the process. Does ffmpeg normalise? That'd tend to increase the volume a little.

[Starburst]

Some great info here, much appreciated

[AllisOne]

I came across this thread: http://www.makemkv.com/forum2/viewtopic.php?f=1&t=7168&start=15 . In general it's about confusing bits in makeMKV GUI and possible fixes, but the posts there can give some helpful hints.

[anana]

If I'm re-encoding multichannl audio as multichannel audio I tend not to change the volume in the process. Does ffmpeg normalise? That'd tend to increase the volume a little.

Here's the info:
TrueHD/AC3 (Atmos), 7.1 channels, 48kHz (embedded: AC3, 5.1 channels, 448kbps, 48kHz, dialnorm: -27dB)

Extraction settings:
eac3to "C:\Users\User\Desktop\00466.m2ts" "C:\Users\User\Desktop\out.thd+ac3"

eac3to "C:\Users\User\Desktop\out.thd+ac3" "C:\Users\User\Desktop\core.ac3"

encoding settings:
ffmpeg -y -threads 12 -loglevel fatal -i out.thd+ac3 -ac 23 -ar 48000 -f sox - | sox --multi-threaded --ignore-length -S -t sox - -t sox - | ffmpeg -y -threads 12 -f sox -i - -b:a 448k -ac 6 -ar 48000 encode.ac3
or
ffmpeg -i out.thd+ac3 -c:a ac3 -ac 6 -b:a 448k -r:a 48000 encode_2.ac3

without normalise input command

Here's the comparison
http://i.cubeupload.com/9DQcUc.png

The ac3 encode sounds louder & more like dts (in crash scene) than the ac3 core


-> The fact, not all TrueHD / Atmos contain AC3/DD5.1 core, it's because:

1. Remux to mkv using mkvmerge.
Mkvmerge can't mux the TrueHD / Atmos that contain AC3/DD5.1 core, it just can mux the TrueHD / Atmos stream and the AC3/DD5.1 core stream separately. It's really annoying

2. Using this command
eac3to "C:\Users\User\Desktop\00466.m2ts" "C:\Users\User\Desktop\out.thd"
proper:
eac3to "C:\Users\User\Desktop\00466.m2ts" "C:\Users\User\Desktop\out.thd+ac3"

3. The source itself

[pandy]

If I'm re-encoding multichannl audio as multichannel audio I tend not to change the volume in the process. Does ffmpeg normalise? That'd tend to increase the volume a little.

Probably metadata are different (dialogue normalization, RF vs Line mode and few other things). Itself ffmpeg should not modify any level (you need use audio filter explicitly to do this).

[anana]

Itself ffmpeg should not modify any level (you need use audio filter explicitly to do this).

I agree

[Cornucopia]

Unlike DTS, True HD doesn't have a "core" of AC3, but rather uses AC3 as an optional fallback.

Scott

[_Al_]

Extraction settings:
eac3to "C:\Users\User\Desktop\00466.m2ts" "C:\Users\User\Desktop\out.thd+ac3"

eac3to "C:\Users\User\Desktop\out.thd+ac3" "C:\Users\User\Desktop\core.ac3"

Those are encoding cmd lines, not demuxing, is it not? To extract stream you'd need something like:

eac3to.exe  "H:\BDMV" 1)   3: "Z:\destination\extract.THD+AC3"

or

eac3to.exe  "00001.m2ts"  3: "Z:\destination\extract.THD+AC3"

where 3: is stream track number and 1) is title number

[anana]

Unlike DTS, True HD doesn't have a "core" of AC3, but rather uses AC3 as an optional fallback.

Scott

Yes, you're right
ac3/dd5.1 isn't TrueHD / Atmos core, but TrueHD / Atmos spec required to embed the ac3/dd5.1 core (for optional fallback)

Extraction settings:
eac3to "C:\Users\User\Desktop\00466.m2ts" "C:\Users\User\Desktop\out.thd+ac3"

eac3to "C:\Users\User\Desktop\out.thd+ac3" "C:\Users\User\Desktop\core.ac3"

Those are encoding cmd lines, not demuxing, is it not? To extract stream you'd need something like:

eac3to.exe  "H:\BDMV" 1)   3: "Z:\destination\extract.THD+AC3"

or

eac3to.exe  "00001.m2ts"  3: "Z:\destination\extract.THD+AC3"

where 3: is stream track number and 1) is title number

No, it isn't. My commad and yours produce the same TrueHD file (just check the hash files)
Your commands just more specific

[_Al_]

ok, I try it when there is a time, but further, concerning extracting core from already demuxed stream:

eac3to  "C:\Users\User\Desktop\out.thd+ac3" "C:\Users\User\Desktop\core.ac3" -core

suppose to be the same as your cmd line as well? What specifies that eac3to extracts that core, that looks like set up for default encoding or something, your line:

eac3to "C:\Users\User\Desktop\out.thd+ac3" "C:\Users\User\Desktop\core.ac3"

[anana]

ok, I try it when there is a time, but further, concerning extracting core from already demuxed stream:

eac3to  "C:\Users\User\Desktop\out.thd+ac3" "C:\Users\User\Desktop\core.ac3" -core

suppose to be the same as your cmd line as well? What specifies that eac3to extracts that core, that looks like set up for default encoding or something, your line:

eac3to "C:\Users\User\Desktop\out.thd+ac3" "C:\Users\User\Desktop\core.ac3"

Ok, I already checked it. Once again, your command and mine produce the identical ac3 core file (you can check the hash again)

[_Al_]

So eac3to does not extract core at all, rather it encodes it anyway? Not sure about True-HD now, but the same happens even if trying to get core out from DTS MA. What is the purpose of -core flag anyway then?

[El Heggunte]

So eac3to does not extract core at all, rather it encodes it anyway?

By default, eac3to does not reencode, it only demuxes, OR/AND changes some metadata in the frame headers
(decoding bitdepth for DTS, dialnorm for AC3).

[_Al_]

ok, if there is a switch -384 for example, it encodes to 384 kbps, if no switch is present, default is 640kbps, but in bot cases, there is some encoding involved even if -core switch is present only. Log file with just -core switch looks like this:

DTS Master Audio, 5.1 channels, 24 bits, 48kHz
(core: DTS, 5.1 channels, 24 bits, 1509kbps, 48kHz)
Extracting DTS core...
Decoding with ArcSoft DTS Decoder...
Remapping channels...
Encoding AC3 <640kbps> with libAften...
Creating file "Z:\destinatin\core.AC3"...
The original audio track has a constant bit depth of 24 bits.
eac3to processing took 4 minutes, 55 seconds.
Done.

I do not have TrueHD handy right now, only DTSMA but that might be the same. What is that encoding line in log file for then? 24bit to 16bit or something?

[anana]

So eac3to does not extract core at all, rather it encodes it anyway?

By default, eac3to does not reencode, it only demuxes, OR/AND changes some metadata in the frame headers
(decoding bitdepth for DTS, dialnorm for AC3).

Yeah, it just demux the embedded ac3 core from truehd (truehd doesn't have any core)
no encode anyway

[_Al_]

I got a hint from the other thread concerning DTS MA, target must be *.DTS with -core switch in eac3to cmd line, then just core is extracted. If the target is *.AC3, then that core is encoded to 640kbps AC3. Do not want to bring confusion here, this is a thread about True HD+AC3, not DTSMA though.

[anana]

I got a hint from the other thread concerning DTS MA, target must be *.DTS with -core switch in eac3to cmd line, then just core is extracted. If the target is *.AC3, then that core is encoded to 640kbps AC3. Do not want to bring confusion here, this is a thread about True HD+AC3, not DTSMA though.

Here's my new test result:

1.

eac3to "C:\Users\User\Desktop\a.mkv" "C:\Users\User\Desktop\dtshd.dts"

just demux the dts-hd, not the dts core

eac3to "C:\Users\User\Desktop\a.mkv" "C:\Users\User\Desktop\core.dts" -core

demux the dts core

eac3to "C:\Users\User\Desktop\dtshd.dts" "C:\Users\User\Desktop\dtshd_2.dts"

just copy the file, no demux / encode


2.

eac3to "C:\Users\User\Desktop\00466.m2ts" "C:\Users\User\Desktop\core.ac3"

demux the ac3 core (remember, it's just embedded ac3 core, not TrueHD's core)

eac3to "C:\Users\User\Desktop\TrueHD.thd+ac3" "C:\Users\User\Desktop\core_2.ac3"

demux the ac3 core (same hash, as shown above -> C:\Users\User\Desktop\core.ac3)

eac3to "C:\Users\User\Desktop\TrueHD.thd" "C:\Users\User\Desktop\encode.ac3"

encode to 640k

eac3to "C:\Users\User\Desktop\core_2.ac3" "C:\Users\User\Desktop\core_3.ac3"

just copy the file, no encode

[_Al_]

Great, thanks for making it clear!