[alsa-devel] Noise Problem with rate convert plugins

[Takashi Iwai]

At Wed, 11 Jul 2007 17:41:21 +0200,
Andreas Rumpler wrote:
> 
> Hello,
> 
> I'm new to this list.I hope someone could help me with a problem 
> concerning the samplerate converter plugins of the alsa driver.
> 
> Firstly some facts about my application.
> 
> - Envy24 (VT1270) pci audio chip on a custom mainboard
> - playback of sounds/music with different sample rates from 8kHz to 
> 48kHz (sample rate of the files)
> - the hardware (Envy24 chip) must always run with 44.1kHz or 48kHz 
> (selectable by user), because the audio is outputed by a DAC and SPDIF 
> and the SPDIF clock must not be changed according to the sample rate of 
> the files.
> - Kernel is 2.6.22 and alsa is 1.0.14
> 
> So I'm using the samplerate (libsample rate based) plugin from the 
> alsa-plugin package. Generaly it works very good.
> 
> My problem is:
> Every time a playback is started a short plop noise is heard. The noise 
> is on analog and SPDIF output. If I don't use the samplerate converter 
> the noise is not heard at all. So it comes definitely from the rate 
> converter plugin, I think. Changing  the quality level of the plugin 
> don't solve the problem.

Could you try the patch below for alsa-plugins?


> I've also switched to the new Speex rate 
> converter plugin. With this plugin there is no noise at the start of 
> playback. But here I have trouble with awful noise at some rate 
> conversions (6kHz(source) -> 48kHz(output); 11.025kHz -> 44.1kHz; 
> 22.05kHz -> 44.1kHz). So it's even worse than using the libsamplerate 
> plugin. Finaly I've tried the libavcodec plugin, which is the worst 
> according to noise.

I can confirm the noise (like flanger effect) at 11025 -> 44100
conversion, too, but no at others.
Since 11024 -> 44100 works fine, it appears specific to quater or so.


Takashi

diff -r 83b528a8ca2e rate/rate_samplerate.c
--- a/rate/rate_samplerate.c	Mon Jun 04 15:23:44 2007 +0200
+++ b/rate/rate_samplerate.c	Thu Jul 12 15:36:38 2007 +0200
@@ -116,6 +116,7 @@ static void pcm_src_convert_s16(void *ob
 				const int16_t *src, unsigned int src_frames)
 {
 	struct rate_src *rate = obj;
+	unsigned int ofs;
 
 	rate->data.input_frames = src_frames;
 	rate->data.output_frames = dst_frames;
@@ -123,7 +124,12 @@ static void pcm_src_convert_s16(void *ob
 	
 	src_short_to_float_array(src, rate->src_buf, src_frames * rate->channels);
 	src_process(rate->state, &rate->data);
-	src_float_to_short_array(rate->dst_buf, dst, dst_frames * rate->channels);
+	if (rate->data.output_frames_gen < dst_frames)
+		ofs = dst_frames - rate->data.output_frames_gen;
+	else
+		ofs = 0;
+	src_float_to_short_array(rate->dst_buf, dst + ofs * rate->channels,
+				 rate->data.output_frames_gen * rate->channels);
 }
 
 static void pcm_src_close(void *obj)