[vlc-devel] [PATCH] opus: apply soft clipping when decoding

[Tristan Matthews]

This patch uses Opus' soft clipping API to avoid clipping when decoding.
This is preferable to hard clipping which will introduce undesired
harmonics.

---
 modules/codec/opus.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/modules/codec/opus.c b/modules/codec/opus.c
index a5953bc..10645a9 100644
--- a/modules/codec/opus.c
+++ b/modules/codec/opus.c
@@ -89,6 +89,7 @@ struct decoder_sys_t
      */
     OpusHeader header;
     OpusMSDecoder *p_st;
+    float *p_softclip_state;
 
     /*
      * Common properties
@@ -190,6 +191,7 @@ static int OpenDecoder( vlc_object_t *p_this )
     p_dec->pf_flush     = Flush;
 
     p_sys->p_st = NULL;
+    p_sys->p_softclip_state = NULL;
 
     return VLC_SUCCESS;
 }
@@ -343,6 +345,12 @@ static int ProcessInitialHeader( decoder_t *p_dec, ogg_packet *p_oggpacket )
     p_dec->fmt_out.audio.i_channels = p_header->channels;
     p_dec->fmt_out.audio.i_rate = 48000;
 
+    /* State memory for the soft clipping process, 1 float/channel */
+    p_sys->p_softclip_state = calloc( p_header->channels, sizeof(float) );
+    if( !p_sys->p_softclip_state ) {
+        return VLC_ENOMEM;
+    }
+
     if( p_header->channels>2 )
     {
         static const uint32_t *pi_ch[6] = { pi_3channels_in, pi_4channels_in,
@@ -492,6 +500,8 @@ static block_t *DecodePacket( decoder_t *p_dec, ogg_packet *p_oggpacket,
             buf[i] *= gain;
     }
 #endif
+    opus_pcm_soft_clip((float *)p_aout_buffer->p_buffer, i_nb_samples,
+        p_sys->header.channels, p_sys->p_softclip_state);
     p_aout_buffer->i_nb_samples = i_nb_samples;
     p_aout_buffer->i_pts = date_Get( &p_sys->end_date );
     p_aout_buffer->i_length = date_Increment( &p_sys->end_date, i_nb_samples )
@@ -507,6 +517,7 @@ static void CloseDecoder( vlc_object_t *p_this )
     decoder_t * p_dec = (decoder_t *)p_this;
     decoder_sys_t *p_sys = p_dec->p_sys;
 
+    free( p_sys->p_softclip_state );
     if( p_sys->p_st ) opus_multistream_decoder_destroy(p_sys->p_st);
 
     free( p_sys );
-- 
2.7.4