[alsa-devel] trouble with alsa, wolfson, and TI OMAP35xx McBSP

Alan Horstmann gineera at aspect135.co.uk
Thu Apr 23 13:04:05 CEST 2009

On Thursday 23 April 2009 10:20, Peter Ujfalusi wrote:
> On Wednesday 22 April 2009 20:58:01 ext twebb wrote:
> > Problem 2:
> > Test tone is being presented by the user application, providing a 1Khz
> > tone sampled at 44.1Khz. The data are S16_LE, right channel only. Left
> > channel is quiet. The data seems to slip back and forth from left to
> > right channel. This is reproducable and verified with a scope trace.
> >
> > Anybody have any ideas what might be going wrong here?  Traces for
> > codec reg dump and mcbsp are attached.
> As Jarkko already asked: are you seeing the channel switch while running or
> is it happens on playback start?
> AFAIK this phenomena can be also seen on Beagle board.
> I have done some investigation regarding to these, but so far I can not say
> that I know what causes it.
> BTW: these are separate issues (startup switch and runtime switch).

I just want to declare an interest in this investigation.  Many months back I 
was working on a SAM9260 board with wm8731 codec; the project is rather 
'background' at present.  One of the outstanding unresolved (not really 
investigated much either) issues was of channel swapping at start of capture 
or playback.  It never changed during operation, but each start time the 
channels would be randomly correct or swapped.

Does this indicate that the implementation/drivers of i2s has a fundamental 
ambiguity?  Is it not rigidly defined which channel should follow a rising 
edge of L/R clock?


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