[LAD] the role of lv2 extensions

[David Robillard]

On Tue, 2009-08-11 at 09:57 +1200, Jeff McClintock wrote:
> > David Robillard -
> 
> > Anyway, ...replication in basic form is pretty straightforward: the host
> > needs a way to pass a 'replication factor' to the plugin.  This raises
> > a
> > few obvious questions:
> > 
> > - Should changing this at run-time be possible?  This is more useful
> > with polyphony than with replication for multi-channel purposes.  This
> > raises nasty realtime issues, but since buffer allocation is the host's
> > problem and a clever host could possibly do it, I think the spec should
> > make this possible.  Maybe there are problems with internal plugin data
> > that needs to replicate as well but in a non-realtime way though?
> 
> I have written plugins that support port replication. In my case they
> *appear* to do so in realtime, but don't actually.
>   The host re-instantiates a replacement (with the extra ports), and the
> same parameter settings, then inserts the replacement in the graph 'swapping
> out' the old instance (and destroying it).  This happens pretty darn fast -
> milliseconds.

This is generally how stuff like this has to happen.  Allocate and set
up in some other thread, then replace in the process thread (usually
just some pointer swapping).

However, instantiation of a plugin isn't necessary a fast operation
here.  In some cases it could take much, much longer than a few
milliseconds  (plugins are free to do I/O or whatever they please at
instantiation time).  It is probably best to keep replication separate
from instantiation for this reason.

>   The advantage is a minimal, simple API - when a plugin in created it
> queries it's 'replication factor' and adjusts itself accordingly. I don't
> need to support changes 'on the fly' while the plugin is processing audio
> (which could be difficult to perform within the constraints of realtime
> operation (without memory allocation etc).
>  So you can avoid the nasty realtime issues, yet easily support run-time
> flexibility.

Sure, I don't think having new voices allocated in hard realtime is
really necessary or feasible.  Having the number of voices easy to
increase while the plugin rolls without dropouts would be very useful
though (e.g. the user increasing the maximum polyphony).

Cheers,

-dr