Re: Question Regarding Simple ALSA Stereo Output

[James Courtier-Dutton <James@superbug.demon.co.uk>]

Bill Kapralos wrote:
> Hello
> 
> I am failry new to Linux programming and recently started using the ALSA
> API for some software I am writing involving the output of sound to a apir
> of headphones. Using (and slightly modifying) a simple example found on
> the ALSA web site (Matthias Nagorni, "ALSA 0.9.0 Howto") I am able to
> open the auduio port and send data to it however,  I am having some
> difficulty with the output of stereo data.  In particular, I am
> initializing the device for interleaved output (e.g.
> SND_PCM_ACCESS_RW_INTERLEAVED) and specifying two channels (at 16 bit
> little endian format).  I then have some stereo data (which I "create"
> which is in interleaved format (e.g. first word (2-byte sample) is for the
> left channel, second for the right channel, third for the left etc... but
> when i do send this data to the audio device (using the "writei" function,
> I do not get the desired result.  It appears as if it is simply outputting
> the data assuming one channel e.g. the same signal is heard in both the
> left and right ear even when I have deliberately made the signals
> different (e.g. introduced a considerable delay to one channel etc.).
> Furthermore, when I explicitly set all left channel samples to "0" and the
> right channel samples to "non-zero", no sound is heard at all - it seems
> as if only the left channel data is being output which is of course all
> zero.  In contrast, if i set all the right channel samples to zero and the
> left channel samples to non-zero, the sound is heard.
> 
> I am really confused with this problem! It appears as if I cannot control
> what is sent to the left/right channel! Am i completely overlooking
> something here? I am really puzzled by what I initially thought of as a
> trivial matter!!!
> 
> Any help would be greatly appreciated! Thank You in advance!
> 
> Please reply to: billkATcs.yorku.ca (of course replace "AT" with "@".
> 
> Regards,
> 
> 

Please post your code to a url so we can look at it.
Alternatively, look at the sample code in
./alsa-lib/test/pcm.c
or
./alsa-utils/speaker-test/speaker-test.c

Cheers
James


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