It seem that the current implementation , it seem that the application can rewind the application pointer to hardware pointer , rewrite the next sample.for all drivers
static snd_pcm_sframes_t snd_pcm_hw_rewindable(snd_pcm_t *pcm) { return snd_pcm_mmap_hw_avail(pcm); }
However this is almost impossible for any sound cards since the hardware register just report the number of samples processed. this mean that the sound card is already processing the next sample.
2009/9/24 Anuj Aggarwal anuj.aggarwal@gmail.com
Yeah, I am looking for some sample application to test forward/rewind functionality.
Regards, Anuj
On Thu, Sep 24, 2009 at 1:27 PM, Raymond Yau superquad.vortex2@gmail.com wrote:
Do you mean that there is no test program to verify the implementation is correct ?
Some sound card 's hardware has FIFO buffer.
For example , au88x0
according to patent 6167465
Additionally, a first-in-first-out (FIFO) buffer is established for the
DMA
channel. Then a DMA request is generated in order to begin filling the
FIFO
buffer with data from the first data segment 150(0) using the information provided by CPU 210.
The FIFO buffer may be any suitable size capable of handling bus
latencies
while small enough such that the memory requirement is not overly burdensome. In one embodiment of the present invention, the FIFO buffer
is
32 samples or words in depth, which would withstand a maximum bus latency
of
about 4 microseconds. By way of example, if 48 DMA channels trigger simultaneously, with a trigger level set at half the buffer size (16 samples), each DMA channel can wait 16 samples (20 microseconds per
sample),
or approximately 320 microseconds before running out of data.
It seem that alsa-lib should not allow application to rewind the
application
pointer to hardware pointer since you cannot write the next sample
because
of FIFO buffer even using mmap read/write
or bug in *snd_pcm_rewindable() ?*
2009/9/24 Takashi Iwai tiwai@suse.de
At Thu, 24 Sep 2009 13:04:24 +0530, Anuj Aggarwal wrote:
Hi,
I want to try the forward/rewind APIs supported by the ALSA library. Can someone point to an ALSA application which exercises these APIs?
Pulseaudio is using snd_pcm_rewind(). About snd_pcm_forward(), I don't know of...
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-- Best Regards, Anuj Aggarwal