[Speex-dev] Resampler saturation

[Stephane Lesage]

> Quoting Stephane Lesage <stephane.lesage at ateis-international.com>:
> > Is this a bug ? Is it possible to fix it ?
> > (I use version speex 1.2beta2, because newer versions just 
> don't work 
> > on my
> > platform)
> 
> This is probable the cause. 1.2beta2 was the first release to 
> include the resampler and it had many bugs. I suggest trying 
> to get 1.2rc1 (or latest git) working.

Hi,

I found the problem preventing to use version >= 1.2beta3.
After the patch for SSE, resampler_basic_direct_single() has been modified to use float instead of the generic spx_word16_t type...

Anyway, float implementation is perfect, because handling saturation when converting back to int.

After reconverting to spx_word16_t, the fixed point version is now good, except for saturation.

Using MULT16_16 for convolution, we have 1 guard bit, which seems to be enough in resampler_basic_direct_single(), so we can saturate before storing.

But in resampler_basic_interpolate_single(), we can also saturate in the cubic interpolation part.
So I propose shifting the 32 bits accumulators 1 bit right before this operation.

--> Implemented and tested in resample4.patch


About the 4 accumulators:
Simple DSPs have only 2 accumulators, so this is really not good on DSPs for 2 reasons:
- this inserts extra load/store operations, cutting performance in half
- we lose the accumulator register guard bits

However, for resampler_basic_interpolate_single(), this is actually better to keep only 1 loop, because of caching for the input samples.

--> See resample.patch

-- 
Stéphane LESAGE
ATEIS International

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