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dev:instruments_and_devices [2018/02/07 17:07] (current)
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 +====Problem description====
 +
 +
 +We would like a way to insert MIDI program changes in the middle of tracks.
 +
 +This is a common feature in many sequencer programs, but it is not currently
 +supported by RG. Also, the "​import MIDI files" function can't be properly
 +fixed now to process MIDI files having PC events in the middle of the tracks.
 +
 +====Proposal summary====
 +
 +Put program changes at the beginning of the segments. Implement this in
 +Rosegarden associating the Instrument (which holds the bank/​program numbers)
 +to the Segment class instead of the Track. Change the MIDI file import
 +function to create a new segment whenever it finds a bank/​program change
 +events in the middle of a track.
 +
 +====Discussion====
 +
 +<​code>​
 +On Tuesday 20 Jun 2006 21:04, Pedro Lopez-Cabanillas wrote:
 +> On Monday, 19 June 2006 18:32, Chris Cannam wrote:
 +> > Note that none of this is an issue when importing MIDI files, because
 +> > each distinct program gets separated out to a distinct track
 +>
 +> This isn't the current behavior of the MIDI import function, and I don't
 +> remember it was as you say in the past.
 +
 +No, you're right. I was thinking of tracks that have channel events sent to
 +more than one channel, which we split out into more than one track in order
 +to assign each to a different instrument.
 +
 +> On the other hand, I find some features related to Instruments very poor or
 +> even wrong. For instance: why the MIDI channel is an attribute of the
 +> Instrument? why the Instruments are attributes of the tracks instead of the
 +> segments? And as a consequence:​ there shouldn'​t be a limitation of 16
 +> instruments per device.
 +
 +16 instruments per device is an arbitrary number that just coincidentally
 +happens to be the same as the number of MIDI channels. If you change the 16
 +at AlsaDriver.cpp line 1005 to something else, you'll get a different number.
 +It works just as well with 24 or 32, or 200 -- try it!
 +
 +But I fear and mistrust the instrument/​device handling code, and particularly
 +the logic that creates the instruments in the first place and tries to keep
 +them in sync between GUI and sequencer.
 +
 +For example, look at what the .rg file loader (rosexmlhandler.cpp:​1868 and
 +thereabouts) does when reading the instrument elements within each device
 +element. It looks up the instrument according to its ID, which is a global
 +numbering from 2000 upwards for MIDI instruments,​ and applies the given bank
 +and program to that instrument if it exists, dropping them entirely if it
 +doesn'​t.
 +
 +The significant points are (a) this code never actually creates instruments or
 +assigns instrument IDs to devices -- that's done when the devices are
 +created, which is originated by the sequencer if the devices are created to
 +match existing available MIDI connections -- and (b) the instrument ID is not
 +local to the device, but global. Put together, this means this code has no
 +way of dealing with (or really knowing about) the situation where an
 +instrument ID is found on the wrong device. For example, if you save a file
 +with 16 instruments per device (so that instrument IDs 2000-2015 are on
 +device 0, 2016-2031 on device 1 etc), then increase the number of
 +auto-generated instruments per device to 24 and reload the file, the
 +instruments 2016-2023 will have moved from device 1 to device 0 and tracks
 +set to those instruments will suddenly play through a different MIDI port.
 +
 +The root cause of this is that instruments are being created in the wrong
 +place -- they'​re created on the whim of the sequencer, and then propagated
 +back to the GUI. This happens because we want to make sure there are
 +candidate instruments with likely connections available for the MIDI ports
 +that the sequencer finds on startup. As a result the code that really should
 +have the ultimate control over the device and instrument setup (the code that
 +reads in the saved devices and instruments from the .rg file) doesn'​t have
 +any real control at all, because the instruments are already there. It can
 +ask for the sequencer to create a new device, but it's still at the mercy of
 +the sequencer'​s built-in count of instruments for that device. In fact, all
 +the instruments and devices should be created at the GUI, or at least the GUI
 +should have control over when and what is created -- only the available
 +connections should be managed on the sequencer side.
 +
 +> My proposal is to associate Instruments to segments instead of tracks, and
 +> dissociate the MIDI channels from instruments moving them to the track
 +> level.
 +
 +I'm not sure that this latter bit is necessary, if there can in fact be more
 +or less than 16 instruments per device -- is it? Would it make much
 +difference either way?
 +
 +Associating instruments with segments seems at first glance like an excellent
 +idea. I wonder if there are any big disadvantages?​ It sounds equally useful
 +for audio instruments,​ to allow you to apply different effects plugins to
 +different segments on the same track.
 +
 +
 +[ later ]
 +
 +
 +> Yes, but the extra instruments don't bring any extra advantage.
 +
 +Not in terms of the overall capability, but that's a limitation of MIDI (as
 +there only are 16 channels and you can't have two different programs on the
 +same one at the same time). I was just making the aside that the number of
 +instruments is not technically fixed to the number of MIDI channels. (It
 +could be an advantage to have more, simply from a management point of view --
 +assign separate instruments to all of your tracks and then worry about which
 +channels to play them through afterwards. That's not very relevant though.)
 +
 +But you're right, if you make the instruments per-segment and channels
 +per-track, the problem goes away. Nice.
 +
 +> I don't understand why the methods generateInstruments() and
 +> addInstrumentsForDevice() are members of the class AlsaDriver.
 +
 +I sort-of answered this in the previous email -- the AlsaDriver creates a
 +default set of instruments to match up with the available output ports it
 +finds on startup. That's really the only reason this logic is there. The
 +original idea was probably to have the set of instruments completely fixed by
 +the sequencer -- you'd have to look at the aRts driver to see what the
 +initial design was, it's probably quite different (and simpler).
 +
 +> The sequencer doesn'​t need to care about the Instruments at all.
 +
 +Not true -- it needs to know about instruments because it also sequences to
 +things other than MIDI devices (e.g. synth plugins -- and note that DSSI
 +plugins don't have channels). Receiving events associated with particular
 +instruments,​ and then doing the mapping from instrument to output type based
 +on a separate relationship between the two, is probably the right thing.
 +It's just a question of who creates and manages that relationship.
 +
 +> It is not strictly necessary, but may be a nightmare otherwise. Imagine
 +> this scenario: track one has two segments, both assigned to channel#1.
 +> Second track has an instrument assigned to channel#2. If you change the
 +> instrument for the second segment on the first track, and assign an
 +> instrument having channel #2, you are changing in fact the program for the
 +> second track.
 +
 +Yes, with this and the earlier example I'm convinced.
 +
 +> Can it be useful for DSSI instruments too? I think so, but I don't know too
 +> much the internals.
 +
 +I would expect so.
 +
 +Quite a bit of work though, all in all.
 +
 +</​code>​
  
 
 
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