Audio formats questions

[LeHobbit]

I have a couple of questions about audio formats applied to Blu-ray :

1) Is the data rate of Dolby Digital Plus and Dolby TrueHD constant or variable ?
2) Can a Dolby TrueHD be downmixed to another format like Dolby Digital ?
3) What is the maximum data rate for LPCM ? Is the data rate constant or variable ?
4) How a 7.1 LPCM soundtrack is downmixed to 5.1 or 2.0 ?

Thanks for your answers.

[masob98]

I think I can awser 2 and 4.
If your using an optical cable, the TrueHD is downmixed to Dolby Digital 5.1 and 7.1 LPCM is downmixed to 2.0.

[LeHobbit]

As far as I know Dolby TrueHD stream contains also the stream for a 5.1 and 2.0 representation. But is the representation in Dolby Digital or in PCM ?

[un4gvn94538]

the core of true hd is dobly digital 5.1. you can do what ever you want to the audio like before. true hd is only pcm when the player decodes the signal and send it. i think what your asking if its any better quality and i would say no. its still lossy 5.1.

[toothless_farmer]

My answers to question 1 - as far as I understood from reading various sources:
1) Dolby Digital Plus data rate is constant.
2) The data rate in Dolby True HD is variable.

LPCM data stream rate is always constant.

[LeHobbit]

Thanks for your reply. It was exactly what I understood. But what is the data rate of the LPCM and how it is downmixed to a lower number of channels ?

[mikenike]

The two extra back channels (left back and right back) of a 7.1 track is downmixed into the surround channels (left surround and right surround) of a 5.1 setup. The volume of those extra channels is slightly lowered to simulate the farther distance of those channels.

The data rate for PCM is constant, as you know, but at what constant depends on the sampling rate and bit-depth. In other words, sampling rate and bit-depth affects data rate. For a 5.1 movie whose bit-depth is at 16-bit and sampling rate of 48kHz per channel will have a data rate of 4.6Mbps. Boost the bit-depth to 24-bit, and you get 6.9 Mbps. Maxed out at 7.1 channels with the usual 24-bit/48kHz, the bitrate is about 9.2Mbps. Doubling the sampling rate to 96kHz (which would compromise quality by upconversion because most movie audio are mastered at 24-bit/48kHz) would give a data rate of 18.4Mbps, which theoretically isn't possible if video is maxed at an average of 40Mbps (BD data rate overall is 54Mbps, which leaves 14Mbps for audio). But mind you, this is just theoretical; video can be compromised for better audio.

[LeHobbit]

Thank you so much for your detailled answer. But, I have stil a question about Dolby TrueHD. I don't think that the stream is the same as DD+. So if a DD decoder process a Dolby TrueHD, it cannot just process the DD signal. As far as I have read, Dolby TrueHD stream already contains data for the 2.0 and 5.1 mix. Is that right ?

[Stephan.klose]

Originally Posted by LeHobbit

Thank you so much for your detailled answer. But, I have stil a question about Dolby TrueHD. I don't think that the stream is the same as DD+. So if a DD decoder process a Dolby TrueHD, it cannot just process the DD signal. As far as I have read, Dolby TrueHD stream already contains data for the 2.0 and 5.1 mix. Is that right ?

Nope. At True HD Sountrack always contains the core mix too.

[Kilian]

Read Roger Dressler's Dolby Audio Whitepaper (here). It explains all the Dolby formats in some detail. It is always preferable to read the best official reliable source.

The core of DD+ is DD (p.4, 6).

Dolby TrueHD uses MLP for all the substreams. The core 5.1 in 7.1 presentation is still Dolby TrueHD lossless. Dolby TrueHD can carry a two- and 5.1-channel presentation independently if desired to avoid downmixing (p.11). The DD lossy track is a separate companion track (p.6).

[mikenike]

Originally Posted by Stephan.klose

Nope. At True HD Sountrack always contains the core mix too.

The TrueHD soundtrack doesn't contain the core. The "core" is a separate track. Kilian's link to the whitepaper sheds light on this. The paper even states that the compression efficiency allows for both the lossless track and lossy track to be present on the media at the same time.

I love how on page 11, Dressler subtlely talks about DTS with the following: "[Dolby TrueHD and DD+ are efficient] compared to other audio coding systems which are required to encode the lossy core and lossless extension as a single bitstream."

[LeHobbit]

Originally Posted by Kilian

Read Roger Dressler's Dolby Audio Whitepaper (here). It explains all the Dolby formats in some detail. It is always preferable to read the best official reliable source.

The core of DD+ is DD (p.4, 6).

Dolby TrueHD uses MLP for all the substreams. The core 5.1 in 7.1 presentation is still Dolby TrueHD lossless. Dolby TrueHD can carry a two- and 5.1-channel presentation independently if desired to avoid downmixing (p.11). The DD lossy track is a separate companion track (p.6).

I have already read that but some points were not clear. Correct me if I am wrong : the Dolby TrueHD stream can contain 3 substreams for a 2.0, 5.1 and 7.1 presentations. All these presentations are in Dolby TrueHD. But to avoid downmixing, all Blu-ray discs must have a DD soundtrack if a Dolby TruHD soundtrack is present.

[Kilian]

The term "downmixing" (p.11 in the whitepaper) in the usual usage refers to a reduction in the number of channels, say from 5.1 to 2.0 (still remaining lossless in this context), rather than "transcoding" from lossless to lossy (but retaining the same number of channels). The companion DD (5.1) track is to "ensure playback compatibility" (p.6 ).

So if you desire a 2.0 Dolby TrueHD lossless output for some reason, it is possible for the disc to have independent two- and 5.1-channel mixes done by the studio ("due to artistic reasons", p. 11) and the player won't need to do the downmixing from 5.1 to 2.0 on the fly.

Roger Dressler sometimes posts in AVS: you can ask him there.

[LeHobbit]

Originally Posted by Kilian

The term "downmixing" (p.11 in the whitepaper) in the usual usage refers to a reduction in the number of channels, say from 5.1 to 2.0 (still remaining lossless in this context), rather than "transcoding" from lossless to lossy (but retaining the same number of channels). The companion DD (5.1) track is to "ensure playback compatibility" (p.6 ).

So if you desire a 2.0 Dolby TrueHD lossless output for some reason, it is possible for the disc to have independent two- and 5.1-channel mixes done by the studio ("due to artistic reasons", p. 11) and the player won't need to do the downmixing from 5.1 to 2.0 on the fly.

Roger Dressler sometimes posts in AVS: you can ask him there.

In fact, if your a/v receiver only outputs 6 channels, the dolby TrueHD soundtrack will be "downmixed" to Dolby TrueHD 5.1 with rear channels being "mixed" and again the same process for 2.0. Is that right ? But I also understound that the 2.0, 5.1 and 7.1 Dolby TrueHD soundtrack are in the same stream. Is that also right ?

[Kilian]

The downmixing occurs during the encoding stage, into "preconfigured subsets" of two-channel, 3.1-channel and and two-channel extension B, as explained on p.3-4 of the whitepaper. The decoder uses these substreams (think of them as building blocks) to rematrix the desired 5.1 and 7.1 mixes. The 5.1 and 7.1 contains the same 2.0-ch and 3.1-ch substreams with no duplication.

[LeHobbit]

Originally Posted by Kilian

The downmixing occurs during the encoding stage, into "preconfigured subsets" of two-channel, 3.1-channel and and two-channel extension B, as explained on p.3-4 of the whitepaper. The decoder uses these substreams (think of them as building blocks) to rematrix the desired 5.1 and 7.1 mixes. The 5.1 and 7.1 contains the same 2.0-ch and 3.1-ch substreams with no duplication.

Correct me if I am wrong, the Dolby TrueHD stream is made of 3 substreams which contains a 2.0 stream, 3.1 stream and 2.0 stream for the rear channels of the 7.1. Is that right ?
I would also thank for your responses. You are great!

[Kilian]

Thanks, you're welcome.

That is right.

From the original post:

Originally Posted by LeHobbit

3) What is the maximum data rate for LPCM ? Is the data rate constant or variable ?

The max. bitrate for LPCM is 27.748 Mbps, CBR.

See:
BD-ROM AV Primary Audio Stream

[LeHobbit]

Originally Posted by Kilian

Thanks, you're welcome.

That is right.

From the original post:



The max. bitrate for LPCM is 27.748 Mbps, CBR.

See:
BD-ROM AV Primary Audio Stream

Great. Thanks for all Killian.
But in the Blu-ray specs I saw a bitrate of 27.648 Mpbs for LPCM

[Big Daddy]

For DVD-A, LPCM supports up to two channels of 192kHz/24bit or up to six channels of 96kHz/24bit.

Theoretically, for Blu-ray disc, LPCM and HDMI cable can support up to multiple channels of 192kHz/24bit audio. However, no studio is taking advantage of this. The best we can hope for is 8 channels of 96kHz/20bit or 96kHz/24bit.

8(channels) x 192,000(samples per second) x 24(bits per sample) = 36.9Mbps
8(channels) x 192,000(samples per second) x 20(bits per sample) = 30.7Mbps
8(channels) x 96,000(samples per second) x 24(bits per sample) = 18.4Mbps
8(channels) x 96,000(samples per second) x 20(bits per sample) = 15.4Mbps

6(channels) x 192,000(samples per second) x 24(bits per sample) = 27.7Mbps
6(channels) x 192,000(samples per second) x 20(bits per sample) = 23.0Mbps
6(channels) x 96,000(samples per second) x 24(bits per sample) = 13.8Mbps
6(channels) x 96,000(samples per second) x 20(bits per sample) = 11.5Mbps

BDA format specifications limit the audio to 6 channels of 192kHz/24bit. See the the BDA white paper.

[Kilian]

Originally Posted by Big Daddy

Theoretically, for Blu-ray disc, LPCM and HDMI cable can support up to 8 channels of 192kHz/24bit audio. However, no studio is taking advantage of this.

The Blu-ray spec puts a limit on 6 channels of 24-bit 192 kHz, according to the BDA whitepaper 2.B Audio Visual Application Format Specifications for BD-ROM, March 2005, section 3.4, unless they have since revised it.

Originally Posted by LeHobbit

But in the Blu-ray specs I saw a bitrate of 27.648 Mpbs for LPCM

I must have read the table half-way down that page and that says 27.748. From calculations 27.648 Mbps is correct.