[Ffmpeg-devel] dts decoding broken?

[Ian Caulfield]

On 16/02/07, Andreas ?man <andreas at olebyn.nu> wrote:
> Uoti Urpala wrote:
> Here are some ideas from the top of my head.
>
> * avctx->channels should be replaced with an enum much like PixelFormat
> for video, or we collide with formats having the same amount of channels
> (such DTS_3F1R and DTS_2F2R). These enumerations implicitly specifies
> the number of channels, the channel locations (front, back, side,
> center, etc) and channel order (interleave offset in output data).
> Call it 'ChannelFormat' / 'ChannelLayout' or somthing.

Instead of an enum, I'd suggest having a bitmask of channel positions
- the latest dolby codecs such as DD+ and TrueHD can supply an
arbitrary combination of something like 20 different speaker positions
(although everything is still 5.1 at the moment). SMPTE RP226 has a
fairly comprehensive list of possible channels.

> * For presentation purposes we should always keep the original
> channellayout stored in the codeccontext.
>
> * It might also be worth pondering a way of letting the application say
> 'hey, i can handle these formats, give it your best downmix'. One way of
> doing this is by letting the application specify a bitmask of the
> soundformats it is willing to accept. This of course limits us to 32 (or
> 64 maybe) different channelformats, which might be a bad thing :/

Possibly the application supplies a bitmask of the channels it can
output, and then there's some code which can downmix or discard
various channels.

> I'm willing to give the channel-layout-code a try (assuming people
> think it's a wise thing to do)

I think it's wise - I currently have problems using MythTV to output
analogue 5.1 surround, as the avctx->channels variable is used to
indicate both the number of channels in the stream and the number of
channels to decode - Myth will set it to 6 to indicate it can decode 6
channels, but if an AC3 stream is stereo it will get set to 2, and if
the stream then changes to 5.1 it will be downmixed to stereo...

Ian