[RDD] Fixing ALSA playback on weird cards

Lorne Tyndale ltyndale at tyndaleweb.com
Mon Jan 6 10:02:08 EST 2014


Just out of interest, what sound card is causing this difficulty?  Any
chance you could try a different sound card?

Also, one thing I've found is that for some cards I have to specify the
rate in asound.conf for them to work properly (particularly intel
chipset based cards).  Putting a rate 44100 or rate 48000 in asound.conf
generally fixes playback issues that I've run into.

> I sent this earlier to rivendell-prog but never got a reply. Perhaps it was
> the holidays, or maybe that mailing list is deprecated?
> ---------- Forwarded message ----------
> From: Karl Koscher <supersat at uwave.fm>
> Date: Wed, Dec 25, 2013 at 2:11 AM
> Subject: Fixing ALSA playback on weird cards
> To: rivendell-prog at lists.rivendellaudio.org
> For whatever reason, one of our ALSA playback cards reports a sampling rate
> of 44099 instead of the usual 44100. Any playback on this device quickly
> gets distorted, and though some debugging, I found that a byte was being
> dropped. The culprit is the code in FillAlsaOutputStream that adjusts the
> amount of data it reads in if the card's playback rate is not the same as
> the stream's. The amount of data to read is divided by the ratio between
> the card's playback rate and the stream's rate. If the playback ringbuffer
> is empty, this increases the amount of data to read beyond the capacity of
> the ringbuffer. In that case, the ringbuffer code takes the first 2^18-1
> bytes, which causes the samples to become misaligned.
> I'm trying to figure out the purpose of this code. FillAlsaOutputStream
> will always fill the ringbuffer, so the rate that it's filled doesn't need
> to be adjusted. It could also be to do a form of poor resampling by
> dropping samples periodically, but this only works if the playback rate is
> lower than the stream's sample rate, and it doesn't do this resampling when
> the ringbuffer is initially filled. Is there a reason for this code, or can
> it be pulled out?<hr>_______________________________________________
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