[FFmpeg-devel] [PATCH] DCA floating point output

[FB2000]

2010/4/26 M?ns Rullg?rd <mans at mansr.com>

>
> Trust me, it will.  It will spend 5x more time in the conversion than
> in the decoder itself.  I've seen it.
>
>
I am not sure about how efficient the DCA decoding part works. At least when
downmixing is involved, I see a lot of floating point multiplications. Take
DCA_3F2R to DCA_STEREO downmixing for example, it's done as:

case DCA_3F2R:
    DOWNMIX_TO_STEREO(MIX_FRONT3(samples, coef),
                      MIX_REAR2(samples, i + 768, i + 1024, 3, coef));

And MIXFRONT3 and MIX_REAR2 is defined as:

#define MIX_REAR2(samples, si1, si2, rs, coef) \
     samples[i]     += samples[si1] * coef[rs][0] + samples[si2] *
coef[rs+1][0]; \
     samples[i+256] += samples[si1] * coef[rs][1] + samples[si2] *
coef[rs+1][1];

#define MIX_FRONT3(samples, coef) \
    t = samples[i]; \
    samples[i]     = t * coef[0][0] + samples[i+256] * coef[1][0] +
samples[i+512] * coef[2][0]; \
    samples[i+256] = t * coef[0][1] + samples[i+256] * coef[1][1] +
samples[i+512] * coef[2][1];

So you see, for each sample, there will be 5 multiplications. I guess adding
another floating point division at output won't hurt that much.

I assume downmixing 5.1 channel to stereo is the scenario most people used
when it comes to DTS, especial for those who is still on old hardware.