When I pass audio_samplerate = 32000 to
ioctl(afd, SNDCTL_DSP_SPEED, &audio_samplerate)
it changes my audio_sample rate from 32000 to 31627.
Now, this is causing problems with my downstream apps so I did some
investigating. Looking into i810_set_adc_rate() in i810_audio.c,
there is some wierd math going on..
1) first, the original rate (32000) is converted to a new rate ..
rate = (rate * clocking)/48000
where clocking, in my case, is 48566.
2) this new value is fed into newrate=ac97_set_adc_rate() which returns
the orignal sampling rate value (32000). This number is then modified
by the ratio
dmabuf->rate = newrate * 48000 / clocking.
3) This results in the value 31627 that I'm seeing. Now, is this the
actual sample rate, or some internal value used to generate a 32000kbps
sample rate? If so, is it perhaps a bug that dmabuf->rate is being
returned rather than newrate?
Thanks,
-Reza
On Mer, 2003-09-17 at 11:42, Reza Naima wrote:
> 1) first, the original rate (32000) is converted to a new rate ..
>
> rate = (rate * clocking)/48000
>
> where clocking, in my case, is 48566.
We measure the bitrate of the codec, because a lot of them are clocked
at odd frequencies not the official 48000 (I gather it saves parts). We
then adjust the rate to get what should be the right one.
> 3) This results in the value 31627 that I'm seeing. Now, is this the
> actual sample rate, or some internal value used to generate a 32000kbps
> sample rate? If so, is it perhaps a bug that dmabuf->rate is being
> returned rather than newrate?
We take the rate you want, shift it for the 48Khz clock offset then ask
the codec to do that rate. ac97_set_dac_rate returns the value the codec
actually used (which may be quite different), and we then bend that back
the other way for the clocking.
So in your case it is going
I want 32000
I need to ask the codec for an adjusted value
Write codec
Read codec
Codec replies 32000 (ie it doesnt do a full range of clocking)
We realise it means you get 31627
Btw - from my experience with video playback audio clocks are +/- 5%
some of the time even when you ask for a rate it can do so software
needs to be fairly adaptive to such things.