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alsa: determine signal precision

Use snd_pcm_hw_params_get_sbits() to ask the driver for the number of
significant bits, which may differ from the encoding size.

--- a/src/alsa.c

+++ b/src/alsa.c

@@ -102,6 +102,12 @@

   _(snd_pcm_hw_params_set_channels_near, (p->pcm, params, &n));

   ft->signal.channels = n;

+  /* Get number of significant bits: */

+  if ((err = snd_pcm_hw_params_get_sbits(params)) > 0)

+    ft->signal.precision = min(err, SOX_SAMPLE_PRECISION);

+  else lsx_debug("snd_pcm_hw_params_get_sbits can't tell precision: %s",

+           snd_strerror(err));

+

   /* Set buf_len > > sox_globals.bufsiz for no underrun: */

   p->buf_len = sox_globals.bufsiz * 8 / NBYTES / ft->signal.channels;

   _(snd_pcm_hw_params_get_buffer_size_min, (params, &min));