How to know the peak bitrate of a TrueHD

[mp3dom]

Hi to all,
I need to know the peak bitrate of a Dolby TrueHD audio track. Is there a way/tool to know it? With tools like BDInfo it's possible to know the average bitrate but currently I need to know the peak. :thanks:

[tebasuna51]

To know exactly the peak bitrate you need analyze all the TrueHD frames, I don't know a tool to do this.

BTW, you can't expect bitrates greater than the double of the average bitrate.
For what need you this?

[mp3dom]

I need to encode a video for a BD where the TrueHD track was already made. If I know the peak I can set the max bitrate for the video accordingly.

[ramicio]

Try putting the TrueHD stream into an mkv and play the file with something that uses the haali splitter, and look at the properties of that filter. It reports bitrate for me, but I don't have an TrueHD streams to try this on.

[patmann03]

Take the average bit rate that is already on the file, double it and set that to max. It won't go over it.

[mp3dom]

Thanks I'll try the suggestion by ramicio.
Patmann03: the track is very *huge*, it's about 10 Mbps of TrueHD audio. If I double it to 20 Mbps to be on the safe side, I will cut too much for the video. This is why I need to know the peak... to cut only the necessary.

[Snowknight26]

You can have a max of 54Mbps of data and even then the time sample size isn't strictly defined so I'm sure you could get away with 35Mbps+ for the video.

[ramicio]

10 Mbps? Jeez what format is it. The easiest thing to do would be to figure out what the bitrate would be as PCM. Say 6 channel x 48 khz, 24 bit would be 6,912 Kbps. If I compressed that spec. PCM with a lossless codec it would be impossible for the bitrate to ever go over 6,912 Kbps.

[Snowknight26]

It's definitely possible with a poor compression implementation.

[patmann03]

You can try using the mediainfo tool to let you know what the bitrate of the video is and what the bitrate of the audio is.

[ramicio]

It's definitely possible with a poor compression implementation.

I'd like to hear this explaination. That's like saying a compressed image format can result in a bigger file than a bitmap, being the same color depth and mode.

[mp3dom]

The track is a 5.1, 24bit, 192KHz (the max specs allowed by BD). It's a music mix that I'm obliged to use 'as is' but since it was made in another studio, I currently don't know the bitrate used.

[ramicio]

27,648 kbps would be the maximum bitrate then.

[bigotti5]

Max. bitrate for TrueHD in BD is 18.64 Mbps (24.5 Mbps DTS-HD, 27.748 Mbps LPCM)

[ramicio]

Wow, what a format. If the file needs more bitrate than that at any given point, then what happens with the data? Does it just get thrown away to conform to Blu-ray? Personally I'd be downsampling it to 96 khz. 6 channels of 192 khz is pointless.

[Midzuki]

If the file needs more bitrate than that at any given point, then what happens with the data? Does it just get thrown away to conform to Blu-ray?

I suppose something similar to discWelder's "ReBit" would be used in such cases:

The ReBit process reduces the effective bit-depth of
selected channels by resetting the Least-Significant Bits (or
LSB - the 24th bit, the 23rd bit, etc.) to zero, either manually
or automatically, in 2-bit steps. This reduces the amount
of data compression necessary to encode the channel, without
reducing the actual number of bits (which is why this is
known as “effective bit-depth reduction”). The significance
of this is that if the DVD-A player has a “24-bit” indicator,
it will stay lit.
ReBit is a “lossy” process, because it reduces the actual bitdepth
resolution of one or more channels before encoding
(the MLP encoding process itself remains Lossless). If the
source material has failed the encode process, the original
soundfiles must be altered or be removed from the DVDAudio
disc. In these rare cases, ReBit is an effective solution.
Another consideration is that the ReBit process (in the
“Automatic” mode) affects the LFE channel first, and then
occasionally the Ls/Rs channels, and typically by only 2-4
bits. The LFE channel is often frequency-limited in 5.1 surround
playback to 120Hz low-pass, and the reduction of
effective bit-depth on this channel is very nearly inaudible.
Likewise, the Surround channels are frequently reproduced
by less-than-full-bandwidth speakers, and the reduction of
effective bit-depth on these channels is also nearly inaudible.


Personally I'd be downsampling it to 96 khz. 6 channels of 192 khz is pointless.

http://forum.videohelp.com/images/smilies/agree.gif

[ramicio]

Yes, that's kind of what I was referring to, and a codec that advertises as lossless being made slightly lossless to conform to standard bitrate is a lossy codec in that one instance. I doubt a 5.1 track at 24/192 would ever have an instance in a file to use that much bitrate. The LFE channel would be fairly low in bitrate, bass is easy to reproduce, and unless you are playing a different song from every other channel it should remain under the maximum bitrate. But still, I'd just go 24/96. I enjoy my 24/192 music in FLAC, but it's only stereo.

[mp3dom]

I agree too in the downsampling... but unfortunatly I'm forced to use it 'as is' without touching anything so I've no other choice :(
Anyway, thanks for all your help!

[ramicio]

I don't get why you CAN'T downsample it.

[Midzuki]

OTOH, couldn't you create an SD or 720p video stream :confused:

so the total bitrate would be less of an issue...

[shon3i]

You can have a max of 54Mbps of data and even then the time sample size isn't strictly defined so I'm sure you could get away with 35Mbps+ for the video.
By blu-ray specs in 48mbps and/or 28mbps is max allowed for sum of all streams in transport stream, depends of writting speed.

[kypec]

Or made them DVD-Audio instead...

[ramicio]

DVD itself is a maximum of 9,600 kbps, so he could only do 96 khz, but as stated before it becomes lossy when it has to constrain to stay below the maximum bitrate.

[mp3dom]

I don't get why you CAN'T downsample it.
Because the committer want that track.

OTOH, couldn't you create an SD or 720p video stream :confused:
so the total bitrate would be less of an issue...
It will be a commercial/replicated product, so a 720p video is not acceptable since the master is on 1080p.

[Audionut]

Use x264. Max bitrate of 28Mbps should be enough for everything but extreme scenes.

edit: And if they really want 24bit, 192khz, go to the effort of filtering any extreme scenes that block with max bitrate 28Mbps.

I seriously doubt that the audio track would hit anywhere near 20Mbps anyway. 5.1 24/192 PCM is about 25Mbps if i'm correct.

[Snowknight26]

By blu-ray specs in 48mbps and/or 28mbps is max allowed for sum of all streams in transport stream, depends of writting speed.

User data transfer rate: 53.948 Mbit/s (Movie application)
There has only been one non-3D disc that has reached anywhere near that and it only hit ~51.5Mbps, so no, neither 48Mbps nor 28Mbps are the max allowed rates.

[bigotti5]

48Mbps for the sum of all (raw) streams is correct. 53.948Mbps includes the muxing overhead.

[shon3i]

There has only been one non-3D disc that has reached anywhere near that and it only hit ~51.5Mbps, so no, neither 48Mbps nor 28Mbps are the max allowed rates.
That is maximum data rate, that has nothing with TS recording rate, which is defined in Blu-Ray A/V specification. There is two options, first if data rate is 54mbps then TS rate can be max 48mbps, and if data rate is 33mbps, maximum ts rate can be 28mbps.