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

Mark Knecht markknecht at gmail.com
Tue Nov 23 06:36:27 PST 2004

On Tue, 23 Nov 2004 08:38:38 -0500, Paul Davis
<paul at linuxaudiosystems.com> 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.

Hi Paul,
   Don't despair. I believe that you'll find an answer that works OK,
but I'll also say here that my focus is still 99% on stereo and mono
tracks only.

   Two weeks ago, on the 11th of November, I wrote the following email
to Ardour-Users. I received no answers:


That in itself says that so far either no one knows much about this,
or no one is using plugins, or no one cares. (I do not think it's the

   I'm on vacation right now and don't any Pro Tools hardware with me
so I cannot run Pro Tools and double check any of this until later in
the week. I'll be happy to do that when I get home should we need to.
In the meantime let me try and address my view of how I think Pro
Tools works.

1) Pro Tools architecture is far more constrained than Ardour. There
are only mono and stereo tracks.

For mono tracks:
a) You can add a 1-in/1-out plugin. The track stays mono following the plugin.

b) You can add a 1-in/2-out plugin. The track (from that point on)
becomes stereo.

For stereo tracks:

c) You can add a 2-in/2-out plugin. The track remains stereo.

d) You can add 'multi-mono' plugin. The track remains stereo. (Note:
Controls for this plugin are ganged. You see one set of controls but
the effects happen to both instances on both sides below the hood.)

(NOTE: If you want a mono signal from a stereo track just set the
panning of both sides to center.

I do not think it's true that Pro Tools restricts mono tracks to mono
plugins. What Pro Tools does is allows me to insert certain 1-in/2-out
plugins, like an auto-panner, into a mono-track and create a stereo
image from a mono signal. However, from that point on the track is
stereo. If I insert a mono plugin at that point I get 'multi-mono'

With respect to mono/stereo tracks only I do not agree with Steve's
comment: "If plugin[N] has more outputs than plugin[N+1] has inputs
then you throw the later (numerically higher indexed) outputs away." I
don;t want to lose information. If I place a mono plug after a stereo
output then I expect multi-mono operation. The track is still stereo.
Nothing is lost. ( A pretty common example would be a mono limiter at
the end of the chain. I expect two mono limiters.)

I agree with Steve that the rules need to be easily understandable and

I think all of this can be extended into Ardour's N-width track
structure. A J-in/N-out plugin creates a N-out track. From there on I
can only use plugins that are:

1) N-in/M-output plugins. The track is now M-output. (where M is any
number - greater, smaller or equal.)


2) 1-in/1-out plugins. The track remains N-out. Ardour does multi-mono.

This does restrict what a user can place on a track. There's no simple
answer to having a 5-out plugin drive a 3-in plugin. Don;t try placing
two 3-in plugins into the system and only using 5 of the 6 inputs.
Just don't go there. Support equal widths and multi-mono and it's
pretty clean.

- Mark

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