[vlc-devel] [PATCH] audio_filters: add sample converter in pipeline

[Thomas Guillem]

On Fri, Oct 30, 2015, at 11:30, Thomas Guillem wrote:
> When creating a new chain a filter, the sample rate "audio converter" is
> not
> added in the chain since a "audio resampler" is already added at the end
> of the
> chain.  This last filter is used to convert the sample rate, but also
> used to
> fix audio/video delay via a variable-rate resampling.
> 
> Unfortunately, we don't have any audio_filter modules that is capable of
> doing
> a variable-rate resampling with good quality and good performances.
> Indeed,
> when fixing audio/video delay, the input rate is changed for every block
> processed and the setup of this new rate can take a long time.
> 
> Because of this bad performance, the ugly audio_filter is used to convert
> the
> sample rate and to fix this delay, but it's quite ugly (hence the name).
> 
> This patch will allow to add back in the chain a good quality/good
> performance
> resampler with a fixed rate. The ugly resampler will still be added at
> the end
> of the chain but will only be used when there is a delay to fix.
> 
> For information:
> 
>  - SoXR with a fixed rate: the "single-precision-SIMD" engine is used, it
>  can
>    use av_rdft from avcodec to process DFT via sse/neon: performance and
>    quality are very good.
>  - SoXR with a variable rate: "the single-precision variable-rate" engine
>  is
>    used. This engine doesn't have any SIMD or ARCH specific
>    accelerations:
>    quality is good but performances are bad.
> 
>  - SpeexDSP with a fixed rate: good quality and performances.
>  - SpeexDSP with a variable rate: speex_resampler_set_rate can take a
>  very long
>    time, therefore performances are very very bad (it can take more time
>    to
>    process than the length of the block).
> ---
>  src/audio_output/filters.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/src/audio_output/filters.c b/src/audio_output/filters.c
> index 74c5a6a..d90e6da 100644
> --- a/src/audio_output/filters.c
> +++ b/src/audio_output/filters.c
> @@ -449,7 +449,6 @@ aout_filters_t *aout_FiltersNew (vlc_object_t *obj,
>      }
>  
>      /* convert to the output format (minus resampling) if necessary */

- /* convert to the output format (minus resampling) if necessary */
+ /* convert to the output format if necessary */

> -    output_format.i_rate = input_format.i_rate;
>      if (aout_FiltersPipelineCreate (obj, filters->tab, &filters->count,
>                                AOUT_MAX_FILTERS, &input_format,
>                                &output_format))
>      {
> -- 
> 2.1.4
>