[alsa-devel] ca0106 Digital Output at 44.1kHz

Ben Stanley Ben.Stanley at exemail.com.au
Tue Mar 25 12:48:41 CET 2008


Thanks for taking the time to read through my long and perhaps slightly
prickly email.

On Sat, 2008-03-22 at 10:31 +0000, James Courtier-Dutton wrote:
> Ben Stanley wrote:
> > Dear List,
> >
> > Hello. I'm new here. This is my first attempt to hack an alsa driver.
> >
> > I recently purchased a "Sound Blaster Audigy Value!" card for use in a
> > MythTV box. As I have a digital receiver, I was mainly concerned with
> > getting a working S/PDIF output (coax or optical) and not too much more.
> >
> > Summary: I can make 44.1kHz digital output work, but I think that the
> > driver model of the card having 3 or 4 digital output channels is
> > incorrect. I believe the card only has 1 digital output channel.
> >   
> The ca0106 chip has 3 or 4 digital outputs. These outputs may or may not 
> be present on the output connectors of the card.
> I know that one of the SB cards does in fact work with 3 digital outputs 
> using the 4 pin jacks because I have tested it myself.
> I GPIO is programmed to switch it between 3 digital outputs and 1 in, 1 out.

So, could we then change the number of channels available depending upon
which card is detected? It doesn't seem to make sense to make things
available to the user via the driver that they can't plug something into
on their particular model card. I guess that to do this I would have to
know how many digital output channels are supported by each card. I
downloaded the user manuals for a few of the cards; I only got the
impression that each card supported one digital output. Do you know
which card you tested for multiple outputs?

(I appreciate that you still have very low version numbers on this
driver, and that therefore it remains a work in progress.)

Now, the Creative Labs user documentation gives me the impression that I
can use the analog output channels at the same time as the digital
output channel. That also doesn't appear to agree with how the driver is
written. I am currently under the impression that the driver is written
to allow 4 stereo analog channels or 4 digital channels. At least that
is how things look when configuring things using alsamixer. Perhaps I am
confused and should go and look at the GPIO that you mentioned above.

> > Details:
> >
> > The card I have is characterised by the following:
> > http://www.soundblaster.com/products/product.asp?category=1&subcategory=205&product=14189
> > Model: SB0570
> > serial: 100a1102
> > Also known as: Sound Blaster Audigy SE
> >
> > During initial testing, I noticed that 44.1kHz playback was not
> > implemented. Subsequent testing shows that speaker-test works fine with
> > 48kHz, 96kHz and 192kHz. (I only tested 16bit output so far.)
> >
> > I noticed in the source code that 44.1k was explicitly disabled. I added
> > code in snd_ca0106_pcm_prepare_playback to set up this rate for S/PDIF
> > output as per the comments in ca0106.h . Initial tests using hw:0,0 at
> > 44.1kHz produced recognisable signals with some noisy corruption. Later
> > I accidentally discovered that serially opening hw:0,2 , hw:0,1 and
> > hw:0,0  at 44.1kHz then produces perfectly good 44.1kHz sampled digital
> > audio output. Removing hw:0,1 or hw:0,2 from this sequence causes noisy
> > corruption. It seems that channels 0-2 in reg40 must all be set to the
> > same sampling frequency for S/PDIF to work where 44.1kHz is concerned.
> > Conversely, to sucessfully output 48kHz again, I have to open hw:0,2 ,
> > hw:0,1 and hw:0,0 at 48kHz to restore proper output. I do not have such
> > troubles with 96kHz and 192kHz, for which it suffices to just open
> > hw:0,0 at the relevant sampling rate.
> >   
> The ca0106 can do 44.1kHz for digital output ONLY.
> The ca0106 cannot output 44.1kHz to the DACs so it will only work in 
> Digital mode.
> It is a hardware restriction. You are correct, all the inputs and 
> outputs have to be at the same rate.

So far I haven't considered 'inputs', although I do now have the digital
I/O module so that I can get digital signals into the card. I suspect I
can only generate 44.1kHz and 48kHz sampled input signals to test with.

So for my card where only one output channel appears to be available
(can you tell from the doc if this is true?), then I would just slave
the other hardware channels to hw:0,0 settings. However, on other cards
where the other channels are in fact available, how do you enforce the
restriction that all the channels must have the same sampling frequency
within the ALSA model?

I'm just trying to plan how I would fix this properly within the driver.

> > Anyway, this is speculation. I'd love to have the docs. I haven't tried
> > to get them. Is it likely/unlikely that I would get them?
> >
> >   
> You can sign an open-source NDA and get the datasheets.
> It lets me write drivers like the current ca0106 and E-Mu drivers.
> If you are interested in a NDA, priv-email me.

I have applied to Creative's 'Partnership' program and faxed off the
forms last week. I've written user-space drivers for custom wire-wrapped
logic + PAL glued hardware before, but not for modern chips, or coded
for kernel space. Anyway, there's a first time for everything :-)

> Kind Regards
> James

Thanks for your help,

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