[Sound-open-firmware] Mixer doubts

[Liam Girdwood]

On Tue, 2018-07-17 at 08:45 +0200, slawomir.blauciak at linux.intel.com wrote:
> Hello, everyone.
> 
> I’ve been reading through the mixer code and there’s one part which leaves me
> somewhat doubtful:
> 
> /* mix N PCM source streams to one sink stream */
> static void mix_n(struct comp_dev *dev, struct comp_buffer *sink,
>        struct comp_buffer **sources, uint32_t num_sources, uint32_t frames)
> {
>        int32_t *src;
>        int32_t *dest = sink->w_ptr;
>        int32_t count;
>        int64_t val[2];
>        int i;
>        int j;
> 
>        count = frames * dev->params.channels;
> 
>        for (i = 0; i < count; i += 2) {
>               val[0] = 0;
>               val[1] = 0;
>               for (j = 0; j < num_sources; j++) {
>                      src = sources[j]->r_ptr;
> 
>                      /* TODO: clamp */
>                      val[0] += src[i];
>                      val[1] += src[i + 1];
>               }
> 
>               /* TODO: best place for attenuation ? */
>               dest[i] = (val[0] >> (num_sources >> 1));
>               dest[i + 1] = (val[1] >> (num_sources >> 1));
>        }
> }
> 
> 
> More specifically:
>               dest[i] = (val[0] >> (num_sources >> 1));
>               dest[i + 1] = (val[1] >> (num_sources >> 1));
> 
> Going back in the history of the file I found it was supposed to be an
> average.
> However, it will only be an average if num_sources is power of 2, because
> otherwise such bitwise operations will yield different results from what I
> take should be an optimized way for division.
> 
> For example, suppose we have 2 cases:
> • We have 2 sources, thus num_sources >> 1 will equal 1, valid right bitshift,
> same as division by the number of sources, which is 2
> • We have 3 sources, thus num_sources >> 1 will equal 1 once again, no longer
> valid, because it will yield the same result as division by 2, instead of 3.
> 
> There’s one more “problem”, by averaging the samples like that we will end up
> with quieter output for when we have 1 source completely silent. Is that
> intentional?
> 
> Whether it’s all deliberate or not, or simply whether I misunderstood the
> code, I hope you can help me clear this up.
> 

The intention is we have a generic optimised version that is easy to vectorize
for 2,4,8 channels (on any architecture). We will need to have a separate
generic mixer for 1,3,5,6 etc.

We should also have architecture specific mixing too like we have for xtensa
HiFi3 volume. 

Liam

> Thank you,
> Sławomir Błauciak
> 
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