[FFmpeg-devel] [PATCH] Add a parser for DNET (byte-swapped AC3).

Justin Ruggles justin.ruggles
Wed Mar 2 23:43:32 CET 2011

On 03/02/2011 05:18 PM, M?ns Rullg?rd wrote:

> Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:
>> On Wed, Mar 02, 2011 at 09:10:21PM +0000, M?ns Rullg?rd wrote:
>>> Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:
>>>>> Then how did you intend to detect it as ac3 in the first place?
>>>> By the fact that the RealMedia header says "DNET"?
>>> AC3 exists outside of realmedia file, you know...
>> So what? Has anyone seen the byte-swapped variant outside a container?
>> Either way libavformat has a better infrastructure to do this kind
>> of format detection when it's hard to decide, a parser is not really
>> such a great place for it.
> The parser can reasonably assume it is being fed AC3 data in some form
> or other, so the difficulty of detection is irrelevant there.
>> Which, as said, is in addition to the fact whether there is a point
>> at all of supporting byte-swapped raw AC3. It obviously exists
>> in other containers, but already for that there seem to be no samples
>> available, DNET is the only one actually common (and even that seems
>> relative).
> AC3 in WAV is fairly common.  I'm sure we have a few samples of that.
> All else aside, calling it DNET is ridiculous.  It is called AC3
> whatever container you wrap it in.  We don't have a CODEC_ID_ for every
> tag used in some obscure container, we have one per codec.  Please keep
> it that way.

I agree. We can handle this in a different way.

For example, I think it would be fine for the AC3 parser to detect byte
order and then not accept switching byte order during parsing.  This
would mitigate the false positive probability increase for all but the
first frame, which is ok with me.

On a related note, it would be quite trivial to add byte-swapped
decoding support to the AC3 decoder.  We already copy the input buffer
to an internal buffer by default.  Making it always copy for
byte-swapped streams, then swapping the buffer before decoding seems
like a reasonable solution.


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