nforce ac97 plays too fast

nforce ac97 plays too fast

[Stas Sergeev]

Hello.

Trying to get a software amplification to
work with NForce CK804 AC'97, I've found
out that whatever the asound.conf I write,
the sound starts playing much faster than
it should.
After some debugging, I've found out the
following:
- The card (or the driver) only supports
48000KHz.
- The program (any, because libao and artsd
suffer from that) is trying to set some
rate with snd_pcm_hw_params_set_rate_near(),
and that succeeds, but the returned rate
is 48000, while the requested is, say, 22050.
No wonder it then plays with the wrong speed.
Of course the program can check the returned
rate, but it sets the "direction" parameter
to 0, which should mean "take exactly that
value".
I've looked into an alsa-tutorial, and the
examples there do not check the returned
rate too - dir==0 should be enough.

Looking into the sources, it seems the culprit
is the snd_interval_refine_min/max(), which do
not care about the (i->min > min && !openmin)
case at all.

The question is: is setting dir=0 (or even
NULL, since it is a pointer) enough to set
the exact rate?
And if so - how snd_interval_refine_min()
should handle the (i->min > min && !openmin)
case?
Or if not - what does the dir==0 mean
and why it is not adjusted to the proper
value on return, when the rate is changed?
(i.e. the rate parameter changes from
22050 to 48000, but the dir parameter remains
0, doesn't change)

With the default configuration the rate is
proper, but I find it annoying that altering
asound.conf in pretty much any way gets the
wrong results.

Re: nforce ac97 plays too fast

[Lee Revell]

On 9/29/07, Stas Sergeev <stsp@aknet.ru> wrote:
> Hello.
>
> Trying to get a software amplification to
> work with NForce CK804 AC'97, I've found
> out that whatever the asound.conf I write,
> the sound starts playing much faster than
> it should.
> After some debugging, I've found out the
> following:
> - The card (or the driver) only supports
> 48000KHz.
> - The program (any, because libao and artsd
> suffer from that) is trying to set some
> rate with snd_pcm_hw_params_set_rate_near(),
> and that succeeds, but the returned rate
> is 48000, while the requested is, say, 22050.
> No wonder it then plays with the wrong speed.
> Of course the program can check the returned
> rate, but it sets the "direction" parameter
> to 0, which should mean "take exactly that
> value".
> I've looked into an alsa-tutorial, and the
> examples there do not check the returned
> rate too - dir==0 should be enough.

What ALSA tutorial are you looking at?  All the ones I am familiar
with state that you must check the returned rate.  Direction is just a
hint.

http://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m___h_w___params.html#ga3602c3c8912acf66c4db29f96608021

For example:

http://www.alsa-project.org/alsa-doc/alsa-lib/_2test_2pcm_8c-example.html#a18
http://www.alsa-project.org/alsa-doc/alsa-lib/_2test_2latency_8c-example.html#a12
http://www.suse.de/~mana/alsa090_howto.html

Lee

Re: nforce ac97 plays too fast

[Stas Sergeev]

Hello.

Lee Revell wrote:
> What ALSA tutorial are you looking at?
This one:
http://equalarea.com/paul/alsa-audio.html
But now I noticed that it passes the rate
as an integer, not as a pointer, so it might
be somehow outdated...
Nevertheless, libao doesn't check, which
is a problem.

> Direction is just a hint.
> http://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m___h_w___params.html#ga3602c3c8912acf66c4db29f96608021
It says:
---
target/chosen exact value is <,=,> val following dir (-1,0,1)
---
Cannot parse, what does this mean?
Does this mean that dir is only set by
the call, and its initial value does not
matter? But it doesn't seem to be the
case either. No matter what value I set
the dir to, the rate is being set to 48000,
and the dir is set to 0.
Is this really an expected result?
If the initial value of dir is not counted
and the new value is always 0, then why
this parameter is needed at all?

> For example:
Yes, they seem to check the rate. But then
I don't understand what does this "dir" stand
for... And I guess the libao developers had
the same question. :)