Re: [Libquicktime-devel] qtinfo vs qtdump multichannel audio info

[Burkhard P. <bur...@ig...>]

Hi,

Am 04.03.2015 um 18:13 schrieb Steven Schultz:
> Hi —
>
> It appears that qtinfo’s multichannel audio handing is simplistic and isn’t looking in the correct place for information.
>
> A 4 channel audio file (produced with Quicktime Player)  produces this output from qtinfo:
>
[...]

> Ah ha - so the quicktime file is correct and qtinfo is wrong.

You are the first who reports about a wrongly labeled file more 8 years after I programmed that,
so it can't be that bad :)

> Seems that qtinfo uses the multichannel.c module rather than the quicktime_chan_dump logic of chan.c.

The problem is that chan.c defines an insane number of channel labels. So I decided to
simplify things a bit and define the LQT_CHANNEL_* enum, which is defined according to
real-life files and codecs like AC3, DTS or AAC.

> is there a reason multichannel.c is around instead of using the logic in chan.c?
The channel locations (as shown by qtinfo) are *always* taken from the chan atom (shown by qtinfo),
if that exists.

The mapping from chan atom channels to LQT channels happens in chan.c:

static struct
   {
   lqt_channel_t lqt_channel;
   channel_label_t channel_label;
   }
lqt_channels[] =
   {
     { LQT_CHANNEL_UNKNOWN,            CHANNEL_LABEL_Unknown },
     { LQT_CHANNEL_FRONT_LEFT,         CHANNEL_LABEL_Left },
     { LQT_CHANNEL_FRONT_RIGHT,        CHANNEL_LABEL_Right },
     { LQT_CHANNEL_FRONT_CENTER,       CHANNEL_LABEL_Center },
     { LQT_CHANNEL_FRONT_CENTER_LEFT,  CHANNEL_LABEL_LeftCenter },
     { LQT_CHANNEL_FRONT_CENTER_RIGHT, CHANNEL_LABEL_RightCenter },
     { LQT_CHANNEL_BACK_CENTER,        CHANNEL_LABEL_CenterSurround },
     { LQT_CHANNEL_BACK_LEFT,          CHANNEL_LABEL_LeftSurround },
     { LQT_CHANNEL_BACK_RIGHT,         CHANNEL_LABEL_RightSurround },
     { LQT_CHANNEL_SIDE_LEFT,          CHANNEL_LABEL_LeftSurroundDirect },
     { LQT_CHANNEL_SIDE_RIGHT,         CHANNEL_LABEL_RightSurroundDirect },
     { LQT_CHANNEL_LFE,                CHANNEL_LABEL_LFEScreen },
   };

Channels 3 and 4 of your file have the fields CHANNEL_LABEL_RearSurroundLeft
CHANNEL_LABEL_RearSurroundRight for which no LQT_* equivalent exists. So they
are labeled as "Unknown" by qtinfo.

Maybe the most pragmatic solution would be to map CHANNEL_LABEL_RearSurroundLeft and
CHANNEL_LABEL_RearSurroundRight to LQT_CHANNEL_BACK_LEFT and LQT_CHANNEL_BACK_RIGHT
respectively.

Another possibility would be to make the chan atom visible in the public API, but I
don't know it it worth the effort. The defitions are so messy and redundant that it's
better to keep them private.

Burkhard