Re: [Sound-open-firmware] Mixer doubts

17 Jul 2018


      On Tue, 2018-07-17 at 08:45 +0200, slawomir.blauciak@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 1 will equal 1 once again, no longer
valid, because it will yield the same result as division by 2, instead of 3.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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Re: [Sound-open-firmware] Mixer doubts

Liam Girdwood