[FFmpeg-devel] avcodec: add a WavPack DSD decoder

[Paul B Mahol]

On 7/23/19, David Bryant <david at wavpack.com> wrote:
> On 7/21/19 11:23 PM, Paul B Mahol wrote:
>> On 7/22/19, David Bryant <david at wavpack.com> wrote:
>>> Hi,
>>>
>>> As I promised late last year, here is a patch to add a WavPack DSD
>>> decoder.
>>>
>>> Thanks!
>>>
>>> -David Bryant
>>>
>>>
>> Please correct me if I'm wrong, but why this uses new codec id?
>> Apparently is also copies some logic from other files.
>
> Yes, I created a new codec ID for WavPack DSD. It would be possible to just
> piggyback on the existing WavPack codec by silently
> decimating the DSD to PCM, but that seemed weird and wrong to me. For one
> thing, the user would have no idea that the file was
> actually DSD and not high sample-rate PCM.
>
> Also, since regular WavPack has threading enabled but WavPack DSD can't
> (because of the dsd2pcm conversion) I would have to turn
> off threading for all WavPack (unless there's some way of making that
> conditional at runtime). It would also mean that regular
> WavPack would be dependent on the dsd2pcm component even if DSD was not
> required (not everyone needs DSD). And of course I was
> looking closely at the only other DSD codec in FFmpeg (DST) which has its
> own codec ID.
>
> Because regular WavPack PCM and DSD share the same block format and metadata
> structure, there is a little code that is shared
> between the two codecs (although they are no longer identical because of the
> threading difference). Is this a problem? I could
> combine the two codecs into one file and sprinkle in a few conditionals, but
> I don't think it would be as clean or clear (but
> might save a little duplicate code).
>
> That's my thinking, but obviously I am not as familiar with the FFmpeg
> philosophy as you guys.

Looking carefully at dsd2pcm code in your patch, it appears it would
work only with 1 or 2 channels.
Is this correct?

About multi-threading, as dsd2pcm can not be made multi-threaded then
just do locking when doing dsd2pcm to make it single threaded and
update other code as necessary.

I still see no valid reason to add separate codec id. Add another
wavpack profile if you wish to help users know the difference between
the two.