[FFmpeg-devel] [PATCH] Port biquads filters from SoX

[Paul B Mahol]

On 1/31/13, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Wed, Jan 30, 2013 at 08:17:40PM +0000, Paul B Mahol wrote:
>> Adds allpass, bandpass, bandreject, bass, biquad,
>> equalizer, highpass, lowpass and treble filter.
> [...]
>> +#define BIQUAD_FILTER(name, type, min, max)
>>     \
>> +static void biquad_## name (const void *input, void *output, int len,
>>     \
>> +                            double *i1, double *i2, double *o1, double
>> *o2,   \
>> +                            double b0, double b1, double b2,
>>     \
>> +                            double a1, double a2)
>>     \
>> +{
>>     \
>> +    const type *ibuf = input;
>>     \
>> +    type *obuf = output;
>>     \
>> +    int i;
>>     \
>> +
>>     \
>> +    for (i = 0; i < len; i++) {
>>     \
>> +        double o0 = ibuf[i] * b0 + *i1 * b1 + *i2 * b2 - *o1 * a1 - *o2 *
>> a2; \
>> +        *i2 = *i1;
>>     \
>> +        *i1 = ibuf[i];
>>     \
>> +        *o2 = *o1;
>>     \
>> +        *o1 = o0;
>>     \
>> +        if (o0 < min) {
>>     \
>> +            av_log(NULL, AV_LOG_WARNING, "clipping\n");
>>     \
>> +            obuf[i] = min;
>>     \
>> +        } else if (o0 > max) {
>>     \
>> +            av_log(NULL, AV_LOG_WARNING, "clipping\n");
>>     \
>> +            obuf[i] = max;
>>     \
>> +        } else {
>>     \
>> +            obuf[i] = o0;
>>     \
>> +        }
>>     \
>> +    }
>>     \
>
> i suspect the code would be faster if it uses local variables instead
> of all the pointer dereferences (unless gcc manages to optimize that
> out)
>
> the assignments between i*/o* variables can also be removed by
> unrolling the loop
>
> also this looks like it can be done in SIMD
> the FIR & cliping part is trivial. the IIR part can be shifted to
> make dependancies more distant unless i miss something
>
> o[i  ] = I[i  ] + a*o[i-1] + b*o[i-2]
> o[i-1] = I[i-1] + a*o[i-2] + b*o[i-3]
>
> o[i  ] = I[i  ] + a*(I[i-1] + a*o[i-2] + b*o[i-3]) + b*o[i-2]
> o[i  ] = I[i  ] + a*I[i-1] + (a*a+b)*o[i-2] + a*b*o[i-3]

I'm really not in shape to do any kind of such optimizations myself so I
will leave that to someone else.

>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> Observe your enemies, for they first find out your faults. -- Antisthenes
>