Re: [ecasound] Chain operator bypass, or other solutions

On Sun, Aug 30, 2009 at 02:04:40PM +1000, Ashley J Gittins wrote:
> Hi all,
> Over the last while I have been working on putting together a purely software
> system to use for jamming, on-site multitracking and potentially, performance.
> After trying out lots of different ladspa hosts and other engines, I think
> ecasound is the closest to meeting my needs by a fairly long margin.
>
> One thing I have come up against though is not being able to find an efficient
> way to bypass specific effects in a chain (like you do for an analogue board
> of effects pedals for instance). Ideally I would like to be able to bypass
> specific chainops via netECI so that I can (for example) switch in a gain
> boost for a solo, or a heavy phaser effect, or switch out a delay momentarily.
 
> Given that I couldn't find a cop-bypass function (which is something that
> seems to have been raised some years ago but might have been forgotten about
> since), I have been trying to think of alternative possibilities:
>
> a) use cop-add, cop-remove to insert and remove effects in realtime
> This seems to be how TkEca does it's bypass feature, but I don't much like
> the approach because it requires the controlling code to maintain all the
> state information for the given effects, meaning that the ecs file is no
> longer as useful as a master record of the setup, and also the add/remove
> steps seem fairly resource-wasteful, and I think causes breaks in processing
> which isn't ideal for use in realtime during performance

The breaks in the sound when adding and removing effects can
be annoying. One workaround is to fade the volume to zero
during the change over. I use this technique in Nama (an
Ecasound front-end.) However that may not be satisfactory in
situations where you want is a change in character of a
sustained sound. Also I have not implemented a bypass
function yet (it's on the TODO list.)
 
> b) put each chainop in it's own chain, link the chains together, and use the
> builtin chain-bypass feature
> This approach seems to have problems both with making the chainsetup
> horrifically complicated and messy and, as far as I know, the only way to do
> this is to use a lot of loop operators, which each introduce further latency
> equivalent to the buffersize (do I understand that correctly?). Again, this
> doesn't sound ideal for a live performance situation.

One chain per effect will add too much latency for live
performance. However for special cases, it might be worth
having two chains, one processed the other with an
equivalent latency, so you could fade smoothly between them.
 
> c) add a cop-bypass feature
> Given the above (and assuming I'm not missing something as far as other
> options go) this would appear the best solution, but I am open to suggestions.
> I would think this would have the advantage of not introducing any latency
> (indeed, might it reduce latency if some cops didn't participate in the
> process() loop?) and would allow for easy control via netECI or IAM.
>
> Has anyone already done or attempted this? I have so little experience with c
> (let alone c++) that I am not sure I'd be very effective trying to do this
> myself.

I expect there may be some technical limitations here.

Ardour was designed from the ground up to enable bypass.
I believe Paul Davis considered and rejected using Ecasound
as the audio processing engine for Ardour because of some
of these issues.
 
> As near as I can tell, it looks to me like the base class CHAIN_OPERATOR could
> get a new property, is_active or similar, a new method to toggle the property,
> then CHAIN::process() in eca_chain.cpp could add a check for chainops_rep[p]-
> >is_active before calling its process() method.
>
> The ECI/IAM code would then need glue to toggle the setting, via the engine
> queue, I would guess.
>
> Is that anywere near being on the right track? Or is there an existing
> solution to my problem that I haven't even considered? By the way, I am using
> mostly LADSPA plugins currently (I ported the effects setup from muddling
> around in jack-rack) but might change some to built-ins, if there are
> advantages to doing so.
>
> If anyone is able to take a stab at doing this I'd really appreciate it,
> otherwise just some direction on how I should approach it will definitely
> help.

I'm also interested in how we might approach this with
Ecasound.
 
> I am usually around on freenode (#jack and #lad) if anyone is interested in
> collaborating via IRC (when it comes to C/C++, I am going to need all the help
> I can get :-) ).
>
> --
> Regards,
> Ashley J Gittins
> web: http://www.purple.dropbear.id.au
> jabber: agittins@email-addr-hidden

-- 
Joel Roth
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