Re: [ecasound] Double-buffering and audio

Hi,

On Sat, 12 May 2012, S. Massy wrote:

> When double-buffering is enabled, if one issues stop and start, there is an
> audible "gap" in the audio, i.e some samples are never played back. Is
> that a bug or a feature?

with JACK+transport enabled, you may miss some samples at start as JACK
doesn't wait for clients. This shouldn't happen though if ecasound is the
transport master (sending the start/stop commands).

Without JACK (or transport disabled), there should be no missed samples.
If position is changed, the disk i/o buffers are refilled with the new
position. Without position change, buffers should not be touched so
position should not change.

> Which leads me to ask... When I issue getpos,
> what does the number reflect? Is it more or less what I'm hearing, give
> or take a jack period or an internal buffer, or is it where the read
> pointer is at in the audio files? Does double-buffeering affect this?

That's the main engine position. E.g. at next engine iteration (or next
JACK process() callback), samples [getpos,getpos+buffersize] will be
processed. The output latency depends on the output device latency alone
(e.g. JACK or ALSA buffering), and a typical (low) value would be one jack
period (depends on how you are running jackd).

The double-buffering does not affect getpos and should be totally
transparent. On code level, the buffering is implemented as a wrapper
class that can proxy any file-type audio object. To the engine, it doesn't
really see any differences, but under the hood, the samples are first read
to big memory buffers, and when engine calls read_buffer(), it never needs
to wait for disk i/o.

But like any abstractrion layer, it is usually, to some degree, also a new
layer of bugs, so in practise not 100% transparent. :P

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Received on Mon May 14 04:15:03 2012