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 ?

[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.