[Ardour-Dev] ardour as audio backend

[Evan Laforge]

Hi!  Sorry in advance about the huge first message.  I could stop by the irc
channel but I think something with this level of detail would just swamp
people.

For the last couple of years I've been working on a music sequencer.  It's
only concerned with writing music, and so it has no ability to deal with sound
at all.  It simply transforms a user written high level score into a lower
level format, which currently means MIDI.

Not supporting audio simplifies things, but it means I'm missing some features
I'd like to have.  Since my program is purely focused on writing music and MIDI
and ardour is mostly focused on recording and editing audio, they seem nicely
complementary.  Cooperation with ardour seems like the most practical way to
get these audio oriented features implemented.  I think ardour's existing OSC
and MTC support can get me part of the way, but it may require some additional
features to get the integration nice, hence this email.  Of course I'm willing
to do the work myself, but it would be nice to have someone who understood the
code say something like "this seems practical, here's the code you should be
looking at, come back when you have more specific questions or some patches for
review" or maybe "this would be a lot of work and we're iffy about accepting
giant patches that do things we're not too interested in" or "you're confused,
here's what you should really be doing" or whatever.  I know that features to
turn ardour into the audio backend for an as-yet unreleased sequencer are
probably not a core design goal for ardour (but... in my personal opinion I
think it's probably a better idea than trying to make ardour into a sequencer
by putting in a whole bunch of MIDI features, though I guess I can understand
why people would want that).

Anyway, what I'd like to achieve can be broken into three features:

The first and most complicated is a software synthesizer host.  I currently use
plogue bidule for this, which is not convenient because once I release this
thing as free software it's not very nice to tell people "oh but if you want to
use software synths you have to buy this commercial program or come up with
some solution on your own, have fun!".  And bidule is not convenient because as
far as I know it can't be controlled from another program.  For this I would
need the ability to list plugins, load and unload them, configure them, and
configure their position in the audio graph.  And I'd need to get MIDI to them
somehow, which probably means configuring a MIDI graph as well, though that can
be as simple as "make an OS-level virtual MIDI port or 3 for every plugin".

>From poking around a bit, ardour2 doesn't seem to support synth plugins, but it
looks like ardour3 does.  Presumably ardour already has an internal notion of
an audio graph, so ideally I'd just need to make sure there are (say) OSC
commands to manipulate it in the ways I want.  I don't think OSC really
supports bidirectional communication so maybe I really want a real RPC
mechanism... or perhaps ardour can just open up an OSC port coming back.  Or if
necessary all these commands could be made unidirectioral by just displaying
output on the ardour UI.  Or write it to a log file, whatever is simplest.  So
something like:

    plugins - list available plugins
    load "name" - instantiate a new plugin, return a plugin ID
    unload ID - unload plugin, disconnecting everything from it
    ls audio/{in,out} midi/{in,out} - list port IDs for global ports
    make_midi_input "name" - make a new midi input port, return port ID
    ls plugin/ - list loaded plugin IDs
    ls ID/audio/{in,out} ID/midi/{in,out} - list port IDs for relevant input or
        output available on a given plugin
    connect port_id port_id - connect two audio ports or two midi ports
    disconnect port_id port_id
    get_state ID - return state of plugin with ID, as an opaque chunk of bytes
    set_sate ID state - restore state, given previous chunk of bytes
    edit ID - bring up editor window for plugin

        I gather plugins have additional control inputs that might be nicer
        than crummy old low res midi control messages:
    ls ID/auto - list automation targets on some loaded plugin
    automate ID/auto/auto_id signal - feed automation signal to automation
        parameter.  I suppose this would be best sent as chunks of (Time, Val)
        pairs.


The second is just the ability to record audio and do the normal DAW things.
The main feature for this is synchronizing play position as well as play and
stop times.  I think MTC and existing OSC support may already get me there.
I just need to set the cursor to n seconds into the session, possibly receive
a message saying ardour wants to set the cursor at X, and send a message saying
to start plaing at some time and n seconds into the session.  Then if both
ardour and my sequencer are using the same underlying time source (presumably
the one used by the OS's midi scheduler), I can tell ardour "start at xyz"
and send midi data to the OS tagged starting at xyz and they should start in
sync.  And as long as the clock ardour uses for audio and the clock the OS uses
to schedule MIDI don't wander with respect to each other, they should stay in
sync.  I'm kind of a dummy about synchronization though so it might not be so
simple?  I know jack also deals with synchronization, but I gather it's
concerned with synchronizing the creation of audio streams, which is not really
the case here.  My program doesn't understand audio, it doesn't even have a
scheduler.


The third is just a combination of the first two, I'd like to be able to bounce
tracks from synthesizer output (either a plugin synth or external hardware
one), aka "track freezing".  I think I just need to be able to tell ardour "arm
the track associated with this synth and overwrite anything already there".
The former is presumably part of the audio graph, and the latter is presumably
what ardour calls "tape mode".


I inteded to try out ardour3 to see what the current situation is wrt synth
plugins, but it seems like the OS X building process is even more tricky than
usual because the GTK patch doesn't apply cleanly to current libgtk 2.20.1 (not
surprising if it's really 2 years old), and that it still hasn't been applied
upstream (!? really?).  I might be able to muddle through but it looks
different enough to be somewhat painful and maybe there's an updated patch, or
maybe it's no longer necessary?  Or maybe the OS X build process has bitrotted
for ardour3?  If a working build is a prerequisite for doing any mac hacking
then I suppose I could try to help out with that too.  I've been poking around
in libardour's ardour/plugin.h and whatnot trying to figure out what ardour's
concept of an audio graph is like and how it's configured, but it's always
easier to actually have a running binary to prod and inspect.


thanks for making it this far!