RE: [Alsa-devel] buffer_size and period_size

[Mark Knecht]

Paul,
   So with -p 64 -n 2 settings, what number of bytes of audio data is
transferred across the PCI bus between each interrupt?

   I guess I had mistakenly thought -p was setting the number of bytes. I no
longer think that is true.

   Also, does the number of bytes transferred change based on how many
channels are enabled? Or does my RME always transfer 26 channels of data
even if I am not using some channels?

   I am assuming that a card like the RME is a bus master, moves so many
bytes, and then interrupts to tell the system that the bytes are there. Is
this basically the case?

Thanks,
Mark

 -Original Message-
 From: [EMAIL PROTECTED]
 [mailto:[EMAIL PROTECTED]]On Behalf Of Paul Davis
 Sent: Monday, January 27, 2003 9:32 AM
 To: Jozef Kosoru
 Cc: [EMAIL PROTECTED]
 Subject: Re: [Alsa-devel] buffer_size and period_size


 I would like to fully understand the exact meaning of buffer_size and
 period_size and how can I compute the final latency in the full duplex
 processing from these variables.

 period_size = frames between interrupts from the hardware
 buffer size = total frames for the hardware buffer

 max output latency = buffer_size
 min output latency = buffer_size - period_size

 max input latency = period_size + interrupt overhead
 min input latency = 1 frame + interrupt overhead

 the latency numbers assume:

 a) the buffer is generally full
 b) you process data 1 period at a time
 c) your s/w keeps up with the h/w
 d) its an average, computed across a period's worth of data

 --p


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Re: [Alsa-devel] buffer_size and period_size

[Paul Davis]

Paul,
   So with -p 64 -n 2 settings, what number of bytes of audio data is
transferred across the PCI bus between each interrupt?

the period size is always (as with almost all ALSA metrics) in units
of audio frames (1 sample for each channel). so with JACK running at
-p 64, 64 frames of data are transferred. how many bytes that is
depends on the sample width and the number of channels.

   Also, does the number of bytes transferred change based on how many
channels are enabled? Or does my RME always transfer 26 channels of data
even if I am not using some channels?

its hardware dependent. for the RME, the same amount is always moved,
regardless of how many channels are enabled. in the case of the RME
Hammerfall series, its more accurate to say that you cannot change the
number of channels enabled. this is not true of all other hardware.

   I am assuming that a card like the RME is a bus master, moves so many
bytes, and then interrupts to tell the system that the bytes are there. Is
this basically the case?

yes. keep in mind that this only covers 1 direction: the interrupt
also tells the system that its possible to put data into the hardware
buffer. 

--p


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