[SoX-users] alsa: repeated fragments

[Tim M. <or...@ac...>]

When using the "play" utility of sox-14.0.1, I sometimes notice a short 
repeat at the end of playback of a small fragment of the previously 
played audio material.  My test files are 8-bit .wav and 8-bit .au 
files, both sampled at 8k and both around 40k long.  The problem doesn't 
occur with "aplay" (alsa-1.0.14a).

What I have also noticed is that 14.0.1/src/alsa.c contains code to pad 
the alsa buffer with frames of silence.  This code was apparently 
introduced in 2007-05-17 to fix bug #1720683 but has now been disabled 
(line 652):

   while (0 && frames_of_silence > 0) {

Re-enabling the code stops the repeats, but brings up another minor 
problem at line 650

   memset(alsa->buf, 0, frames_of_silence * ft->signal.size * 
ft->signal.channels);

Zero does not represent silence for unsigned formats, so stuffing the 
buffer with zeros can result in a noticeable "ker-chunk" at the end of 
playback.  I found that for these files, inserting 128's instead of 
zeros solves the problem.  At least this seems to work for low-quality 
.wav files that use an unsigned 8-bit format.  (I don't know if unsigned 
16-bit formats will ever be an issue.)

This is what I have done to make my own installation work:

--- src/alsa.c    2007-11-04 08:28:29.000000000 -0800
+++ src/alsa.c    2008-02-23 17:25:11.000000000 -0800
@@ -647,9 +647,12 @@
      * whole periods to the hardware */
     snd_pcm_uframes_t frames_of_silence = alsa->period_size - 
alsa->frames_this_period;
 
-    memset(alsa->buf, 0, frames_of_silence * ft->signal.size * 
ft->signal.channels);
+    if (ft->signal.encoding == SOX_ENCODING_UNSIGNED)
+      memset(alsa->buf, 128, frames_of_silence * ft->signal.size * 
ft->signal.channels);
+    else
+      memset(alsa->buf, 0, frames_of_silence * ft->signal.size * 
ft->signal.channels);
 
-    while (0 && frames_of_silence > 0) {
+    while (frames_of_silence > 0) {
       int err;
       err = snd_pcm_writei(alsa->pcm_handle,
                            alsa->buf,

Hope this helps someone.

Tim