[FFmpeg-devel] [PATCH] activate ac3 decoder

Justin Ruggles justinruggles
Tue Aug 7 12:14:59 CEST 2007

Guillaume POIRIER wrote:
> Hi,
> On 8/7/07, Justin Ruggles <justinruggles at bellsouth.net> wrote:
>> Diego Biurrun wrote:
>>> On Sat, Aug 04, 2007 at 09:36:05PM -0400, Justin Ruggles wrote:
>>>> --- configure        (revision 9911)
>>>> +++ configure        (working copy)
>>>> @@ -1594,6 +1595,9 @@
>>>> +# disable the native AC-3 decoder if liba52 is enabled
>>>> +enabled liba52 && disable ac3_decoder
>>> Possibly we should remove liba52 support, same as we did for
>>> libdca/libdts.
>> Right now it's a bit slower.  For straight channel-for-channel decoding
>> it's about 3%-per-channel slower on my machine.  With downmixing 5.1 to
>> stereo it's closer to 30% slower.  I'm working on better downmixing
>> though.  Basically I need to work out how to downmix before the IMDCT,
>> not after.  I've got papers on the subject, but I need to delve into the
>> math to implement it properly.
> Does it take advantage of existing DSP functions of FFmpeg?
> Does it take advantage of SIMD-optimized code?

yes, and yes.

> What's the slowest part of AC3 decoding currently?

I haven't profiled it yet, but my best guess is the 512-point IMDCT.


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