Re: [ecasound] Problems changing transport position with JACK

Hi,

[ btw, hello everyone, I've been away on holiday and then struggling with
a flu (or probably a few different flu variants at least)). I'll try to
have time in near future to look at the couple of 2.5.2 bug reports I've
got. At least the loop device saving problems seems high-priority.. ]

On Mon, 5 Jan 2009, Joel Roth wrote:

> I'm reporting what I think is a bug when using Ecasound
> 2.5.2 with JACK.
[...]
> What I notice is many seconds of delay from issuing a
> playback-head repositioning command like "fw 40" until
> playback resumes.

this can be in fact normal. JACK transport changes are zero-delay actions
in the sense that the system doesn't wait for clients to catch up with the
new position, but instead JACK just keeps running forward from the new
positions. An app like Ecasound, which reads samples from disk (and does
not cache all samples in memory), cannot manage these seeks in real-time.
In normal processing, meeting the real-time reqs is only possible as
ecasound can buffer ahead, but this obviously won't work with seeks.

For the user, the positive side is that you can do continous looping with
a JACK system across multiple apps, but for the developer, this makes the
implementation really hairy and may suprise some users as well.

What Ecasound does is to implement is a "catch JACK's current position"
algorithm that issues new read-ahead requests to all file objects until
they eventually catch up. Depending on your disk i/o capacity and the number
of files in your setup, this may take long (even multiple seconds).

If you want to avoid this and make sure all samples from all clients are
played from the new position, the JACK transport position needs to be
changed with a different JACK API -> jack_transport_reposition() instead
of .._position(). The former will wait until all "slow clients" have
managed to seek to the new position before starting processing.
Unfortunately ecasound does not support the reposition() variant, and in
practise you need to stop processing, seek to a new position, and start
again to get the same effect. Hmm, it seems that 'jack_transport' also
does not support the reposition variant.

Hmm, it might make sense to add jack-reposition support to interactive
mode, though...

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Received on Tue Jan 13 04:15:01 2009