about bt878/879

about bt878/879

[Joe Hsu]

    I have a sound device which only offer bt878 digital audio at sample
rate 8000 HZ.  I modify the 'btaudio' module source( an OSS driver) and
then it works. But when I want to capture data from bt878 and play it 
back on AC97 codec in real time, underrun condition happens.(AC97)

    I've tried to modify the bt87x driver(alsa) to work for me but 
I apparently do not have the ability to make it work. Then I write 
a simple driver for bt878 and it works as follows:
    1. Allocate a fixed-size audio buffer
    2. Write audio raw data to that audio buffer in a circular way.
       (This also includes a write pointer to indicates the current 
       location in which the bt878 RISC program is writing data.)
    3. AP can mmap that buffer and read the value of the write pointer.

    I've tried that simple driver with storing raw audio data in the 
harddisk and then play it back later on AC97 codec(ALSA driver) with no
problem. But here comes the problem again when I try to capture data
and play it back in real time( still buffer underrun). I've used
interrupt-driven model in my program for playback. At first, the 
frames I captured and the available space(snd_pcm_avail_update) 
for playback is almost the same( or at a fixed difference). But after
few seconds, the available space for playback is larger and larger.
(The increasing rate is 1 frame every 1/125 second. One frame is 4
bytes long.) And I am quite sure I did not miss any capture data.

    Any one here can give some hints about this? I know full duplex
for same audio codec is already possible for ALSA. But when it comes
to two different audio codecs(apparently have different crystal clocks),
how can we record and playback in real time???
    Plz help. thanks.

    

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Re: about bt878/879

[Peter Zubaj]

Hi,

Just blind shot: Maybe diffrence in crystal frequence. 8000 Hz on
bt878 is not 8000 Hz on AC97. 

Peter Zubaj

____________________________________
Vsetko o SuperStar
http://superstar.atlas.sk



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Re: about bt878/879

[Joe Hsu]

Well.....I believe the AC97 alsa driver can operate at 8000 HZ in
software level.


On Wed, 30 Mar 2005 12:56:27 +0200, Peter Zubaj <pzad@pobox.sk> wrote:
> Hi,
> 
> Just blind shot: Maybe diffrence in crystal frequence. 8000 Hz on
> bt878 is not 8000 Hz on AC97.
> 
> Peter Zubaj
> 
> ____________________________________
> Vsetko o SuperStar
> http://superstar.atlas.sk
> 
> 


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Re: about bt878/879

[Peter Zubaj]

Yes, they can. But 8000 Hz on bt878 or AC97 is not exactly 8000 HZ. It 
can be for example 8010 Hz for AC97 and 7990 for bt878. Only high end 
sound card can be synchronized together. For your case, you should 
handle this situation for example by discarding or inserting some samples.

Peter Zubaj

Joe Hsu wrote:

>Well.....I believe the AC97 alsa driver can operate at 8000 HZ in
>software level.
>
>
>On Wed, 30 Mar 2005 12:56:27 +0200, Peter Zubaj <pzad@pobox.sk> wrote:
>  
>
>>Hi,
>>
>>Just blind shot: Maybe diffrence in crystal frequence. 8000 Hz on
>>bt878 is not 8000 Hz on AC97.
>>
>>Peter Zubaj
>>


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Re: about bt878/879

[Joe Hsu]

Thanks for your reply....I have tried inserted some samples. And the
buffer underrun
for playback is gone. However, the sound quality is not right, ie.
having some noise.
 I think I may do something wrong. For example, I capture 64 frames
once at a time
and check 'snd_pcm_avail_update', and then decide to insert one extra frames.
This extra frame is an average of 32th and 33th frame. The numbers of
extra frames
are not fixed every time and the biggest num is 8.  And the number of
frames I capture
is raging from 64 to 256.(ie., 64, 128, 192, 256)

Should I measure HZs both in AC97 and Bt878 at first to get an overview
for up/down sampling?

On Wed, 30 Mar 2005 19:31:09 +0200, Peter Zubaj <pzad@pobox.sk> wrote:
> Yes, they can. But 8000 Hz on bt878 or AC97 is not exactly 8000 HZ. It
> can be for example 8010 Hz for AC97 and 7990 for bt878. Only high end
> sound card can be synchronized together. For your case, you should
> handle this situation for example by discarding or inserting some samples.
> 
> Peter Zubaj
> 
> Joe Hsu wrote:
> 
> >Well.....I believe the AC97 alsa driver can operate at 8000 HZ in
> >software level.
> >
> >
> >On Wed, 30 Mar 2005 12:56:27 +0200, Peter Zubaj <pzad@pobox.sk> wrote:
> >
> >
> >>Hi,
> >>
> >>Just blind shot: Maybe diffrence in crystal frequence. 8000 Hz on
> >>bt878 is not 8000 Hz on AC97.
> >>
> >>Peter Zubaj
> >>
> 


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