I have some sound files which were playing at the wrong sample rate (the pitch
was about a step high and the speed was elevated). The sample rate of the
files is 88.2 kHz. I believe that they were playing at 96 kHz, judging from
the pitch error. My output device is HDA Intel PCH.
I found that I could fix the problem by augmenting my /etc/asound.conf file
with
a rate specification:
pcm.!default {
type hw
card 0
rate 192000
}
ctl.!default {
type hw
card 0
}
However, I do not understand what this change actually does. When I play one
of the sound files now and then do
cat /proc/asound/card0/pcm0p/sub0/hw_params
I see that rate is 192000. In fact, I see that same value for rate regardless
of the sample rate of the sound file. This observation is true whether I play
using
gst-launch-0.10 playbin2 uri=file:///home/lubuntu/09.ogg
or play (sox). The change that I made to asound.conf must have specified that
the sample rate be converted to 192000. However, I understood that "type hw"
meant that ALSA would send a file directly to the sound device without any
processing, so why is the conversion happening and where?
I tested asound.conf with other values for rate. 44100, 48000, 96000, and
192000 all work, and the value in hw_params for sample rate always
corresponds. When I specify 88200, the play command fails with "invalid
argument".
I also tried speaker-test. When I run the command
speaker-test -D hw:CARD=PCH,DEV=0 -c2 -r 48000
the sample rate that I read in hw_params corresponds to the value I specify in
the command -- except when I specify 88200, which produces an "invalid
argument" error. Why is the sample-rate conversion not happening for this
command although it is happening for the play commands? That is, when I
specify 192000 in asound.conf, I get 192000 regardless of the sample rate of
the sound file I play, but with speaker-test I get the value that I specify on
the command line regardless of what I put in asound.conf. And am I supposed
to believe that HDA Intel PCH is not capable of handling a sample rate of 88.2
kHz?
The system is running Ubuntu 12.04 or 13.10 with GStreamer version 0.10 or
1.2.0.
I would like for the DAC to run at the sample rate specified by the sound file
so that the samples it receives are exactly the samples in the sound file.
------------
Jeffrey Barish
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