[ardour-dev] issues with plugin in/out counts

[Steve Harris]

On Tue, Nov 23, 2004 at 08:38:38AM -0500, Paul Davis wrote:
> in the last few days, i've been staring down some of the problems
> people have reported when using plugins that change the number of
> active streams in a track (e.g. a 1in/2out reverb).
> 
> my conclusion after all this work is that ardour's noble attempt to
> allow users to use plugins in the most convenient way is doomed to
> failure. for a long time, ardour has allowed a user to use a mono
> plugin in a track with stereo inputs, for example. it does this by
> replicating the mono plugin, and making the plugin GUI control both
> instances. 
> 
> for the simple case, this works very very well and i think is a
> welcome feature.
> 
> the problem is that it rapidly leads to untenable situations. consider
> what happens if you add a mono plugin to a track with 1 input. The
> plugin is not replicated - there is no need to. so far, so good.
> 
> now the user wants to add a 1in/2out plugin, for example most
> reverbs. no problem still, as long as they are ordered in the same
> order as they were added.
> 
> but now the user moves the 1in/2out plugin before the 1in/1out
> plugin. all of a sudden, 1 of the outputs of the reverb is being
> thrown away, and in addition, it becomes unclear how many active
> streams the track has: does it have 2 (the number coming out of the
> reverb) or 1 (the number coming out of the gate)? this number is
> important because it determines how many panners there are for the
> track. 

My 2p's worth of guess is, that algorithm is correct:

Only the last plugin in the chain contributes outputs to the panner 
(anything else would be confusing).

If plugin[N] has more outputs than plugin[N+1] has inputs then you throw 
the later (numerically higher indexed) outputs away.

If plugin[N] has less ouputs than plugin[N+1] has inputs then you zero the
later inputs.

That's deterministic, and unsupprising. Sounds good to me.

In the ideal world you could rely on there being mono, stero, 5.1, ...
versions of all the LADSPA plugins where it mattered, and some sort of
assertion or heuristic telling you that they were differently channeled
versions of the same effect, but I suspect plugin authors are too lazy ;)

- Steve