[alsa-devel] SoC Atmel SSC stereo problem

Patrick Ziegler patrick.ziegler at fh-kl.de
Fri Oct 22 23:39:01 CEST 2010

>> I have some trouble with stereo audio through the atmal ssc interface
>> based on at91-ssc.c.
> (Merged into atmel_ssc_dai.c since about one year ago.)
>> Unfortunately left and right channels were swapped periodically.
>> In my environment the codec supplies the BLCK and LRC clocks and
>> the SSC is configured to transmit one sample on both edges of the LRC clock.
>> But I guess the audio frames will always start with a frame of the left channel
>> while the LRC clock is in an undefined state.
> Indeed.
>> How does normally the audio driver detect when a left and when a right channel
>> frame should be send and with which it should start ?
> The driver cannot do anything about this; the hardware must be told
> to start transmitting at the falling edge.
> If possible.
If there is only one channel to transfer, I set the start of 
transmission on the rising edge (left justified format). But if I do 
this for two channels, both channels will be transmit on the left 
channel, this is what I would expect. Is there way to tell the hardware 
(Atmel SSC on at91rm9200) that 2 Frames shall be send after a rising 
edge with an indication that the second frame shall be send after the 
falling edge ? This is the way it should work, but of course I don't 
know if it's even possible.
> Is there a reason why the codec supplies LRC?
The SSC is connected to a FPGA that assigns the bus to different devices 
depending on the application. And for all applications the FPGA 
generates the clocks. Maybe this is not the best solution but I will try 
to deal with this limitation first before I try to persuade other people 
to change it.

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