Re: [proaudio] Real-Time for audio playback?

[Mark Knecht <markknecht@xxxxxxxxx>]

On Mon, Jan 18, 2010 at 8:16 AM, Grant <emailgrant@xxxxxxxxx> wrote:
<SNIP>
>
> But in the end, there's really no way to know, right?  From what I
> gather, jack can add another buffer and report on it, but it's the
> sound card buffer that determines whether there are problems or not,
> right?

No, I think Jack does know.

In non-Jack apps the application pumps out data. If the buffers
overflow or run empty it's just a 'system problem' and the system has
failed.

In Jack apps all audio is moved by Jack. All Jack apps are callback
based. Jack itself issues a demand for data from the application, then
if the application supplies it then everything works correctly. If the
app doesn't supply the data then we know where the problem is and we
can fix it.

Note that the above description says nothing about real-time. It works
whether we are doing real-time audio or not. Without a real-time
kernel we just increase our latency to cover the extra latency in the
non-rt kernel, drivers and other apps. None the less it works and if
it fails then we still know where the problem lies.

>
> I suppose using jack and real-time gives you extra assurance that the
> sound card buffer is being served as necessary.  Using jack without
> real-time wouldn't provide any more assurance than a setup without
> jack, would it?

No more assurance (as I understand it) but much more knowledge about
where the problem lies...

You may not feel a need to use Jack but I wonder if you've had a
chance to look over the Jack website as it has better info than I can
provide. Paul's original slides are a good place to start:

http://jackaudio.org/documentation

You may not have considered this yet but using a non-rt kernel and
doing things like context switches, dropping out of and coming back
into Xorg, working with Wine, and other things, can cause huge 1-time
latencies. In the old days these were huge. Today they are not so bad,
and our machines are faster, so you don't see as many problems, but
technically they still exist.

In case there's any question there are more than enough Jack
applications to satisfy most any need:

http://jackaudio.org/applications

And one last point - Jack won't save you from a total system hardware
failure. My main AMD64 Gentoo machine that has run rt-kernels and
sub-1mS latency for over 5 years went up in smoke last week. I wish
Jack could have told me that problem was coming... ;-)

- Mark