[Matroska-devel] DTS-MA: number of channels
slhomme at matroska.org
Sun Jun 17 15:19:50 CEST 2012
I agree that the minimum support required should be put in the
existing field. The other field could be called MaxChannels rather
than TotalChannels. You never know if there are going to be codecs
with 8 channels but only 5 are usable at the same time. Or maybe in
that case it's better to tell the total and 3 would be in the existing
The amount of channels available is useful when you have multiple
track and the player wants to pick the best one. That means if we have
a codec that can do 8/6/2 channels and another that does 5/2 and the
player can handle 6, it should pick the version with 6. That means 8
and 6 should be put in the file. So I would make TotalChannels as non
unique so there can be various versions. And the default should be the
value of Channels.
On Thu, Jun 7, 2012 at 9:17 AM, Moritz Bunkus <moritz at bunkus.org> wrote:
> due to a recent bug report for mkvmerge I stumbled across this...
> undefined part in our specs. DTS-MA (Master Audio) can actually have
> two different number of encoded channels. For those that don't know
> how the -MA part works: DTS knows a core and optional extensions. Any
> decoder not capable of decoding those extensions can simply drop them
> (as can any muxer) and just work on the core. Therefore DTS-MA is a
> real extension of DTS and not just a modification.
> Now the problem is that it is possible that the core part only
> contains six channels while extensions carry two additional surround
> channels (often enough they only carry the information required to
> decode the existing channels in the core losslessly). This leaves us
> in the situation that there are actually two different numbers of
> channels. Which of those do we put into the track headers?
> MediaInfo's GUI reports both values (something like "6 core channels,
> 8 total"). ffmpeg's ffprobe only reports 5.1.
> The closest thing we have at the moment is the AAC SBR situation. For
> those we kept the nominal sampling rate (e.g. 24000) in the
> SamplingFrequency track header field and put the output sampling
> frequency (e.g. 48000) into a newly introduced element called
> OutputSamplingFrequency. We could do something similar for the number
> of channels, e.g. introduce "TotalChannels" or something similar that
> doesn't sound too DTS-centric.
> However, that still leaves the question which value to put into the
> existing "Channels" element. I think the number of core channels would
> be the correct value because those are the minimum number of channels
> that will be output by each and every decoder out there, whether or
> not they support the DTS extensions.
> Kind regards,
> Matroska-devel mailing list
> Matroska-devel at lists.matroska.org
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