[FFmpeg-devel] [PATCH] updated LGPL AC-3 decoder

[Michael Niedermayer]

Hi

On Sun, May 06, 2007 at 06:49:00PM -0400, Justin Ruggles wrote:
[...]
> > 
> > [...]
> > 
> >>+    /* convert float to 16-bit integer */
> >>+    for(ch=0; ch<ctx->out_channels; ch++) {
> >>+        for(i=0; i<256; i++) {
> >>+            ctx->output[ch][i] = ctx->output[ch][i] * ctx->mul_bias +
> >>+                                 ctx->add_bias;
> >>+        }
> >>+        ctx->dsp.float_to_int16(ctx->int_output[ch], ctx->output[ch], 256);
> > 
> > 
> > this isnt exactly ideal
> > the bias should be applied in some more efficient way like multiplying some
> > scaling coefficients, allthough i dunno if thats possible in AC3 ...
> 
> I changed it so that the bias is multiplied in before the IMDCT along
> with the dialogue normalization and dynamic range compression.
> 
> On top of your suggestions, I also did several other changes.  I added
> dialogue normalization, put in some references to the specs in various
> places, fixed some bugs, simplified a few things, added more checks for
> error conditions, and removed some unused variables.
> 
> I still can't figure out why the 5.1 decoding is so much slower than
> liba52.  My guess is the downmixing, but it might be other things as
> well.  Stereo decoding is pretty much the same speed on my system as
> liba52, but 5.1 decoding takes twice as long...

you mean 5.1 with downmixing to stereo or true 5.1 ?

the downmixing should be done before the imdct, that way you dont need to
do the imdct on 6 channels but rather just on 2 (i didnt check where the
downmix is done exactly but its just IIRC ...)


[...]

> +    /* the rest of the bsi. read twice for dual mono mode. */
> +    for(i=0; i<=(ctx->acmod == AC3_ACMOD_DUALMONO); i++) {
> +        ctx->dialnorm[i] = dialnorm_tbl[get_bits(gb, 5)]; // dialogue normalization
> +        if(get_bits1(gb))
> +            skip_bits(gb, 8); // skip compression gain
> +        if(get_bits1(gb))
> +            skip_bits(gb, 8); // skip language code
> +        if(get_bits1(gb))
> +            skip_bits(gb, 7); // skip audio production information
> +    } while(i--);

hmm for(){}while(); ?

and patch looks ok

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

There will always be a question for which you do not know the correct awnser.
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