man MIDI::Track () - MIDI::Track -- functions and methods for MIDI tracks


MIDI::Track -- functions and methods for MIDI tracks


 use MIDI; # ...which "use"s MIDI::Track et al
 $taco_track = MIDI::Track->new;
  ['text_event', 0, "I like tacos!"],
  ['note_on',    0, 4, 50, 96 ],
  ['note_off', 300, 4, 50, 96 ],
 $opus = MIDI::Opus->new(
  {  'format' => 0,  'ticks' => 240,  'tracks' => [ $taco_track ] }


MIDI::Track provides a constructor and methods for objects representing a MIDI track. It is part of the MIDI suite.

MIDI tracks have, currently, three attributes: a type, events, and data. Almost all tracks you'll ever deal with are of type MTrk, and so this is the type by default. Events are what make up an MTrk track. If a track is not of type MTrk, or is an unparsed MTrk, then it has (or better have!) data.

When an MTrk track is encoded, if there is data defined for it, that's what's encoded (and encoding data means just passing it thru untouched). Note that this happens even if the data defined is "" (but it won't happen if the data is undef). However, if there's no data defined for the MTrk track (as is the general case), then the track's events are encoded, via a call to CWMIDI::Event::encode.

(If neither events not data are defined, it acts as a zero-length track.)

If a non-MTrk track is encoded, its data is encoded. If there's no data for it, it acts as a zero-length track.

In other words, 1) events are meaningful only in an MTrk track, 2) you probably don't want both data and events defined, and 3) 99.999% of the time, just worry about events in MTrk tracks, because that's all you ever want to deal with anyway.


MIDI::Track provides...

the constructor MIDI::Track->new({ ...options... })
This returns a new track object. By default, the track is of type MTrk, which is probably what you want. The options, which are optional, is an anonymous hash. There are four recognized options: CWdata, which sets the data of the new track to the string provided; CWtype, which sets the type of the new track to the string provided; CWevents, which sets the events of the new track to the contents of the list-reference provided (i.e., a reference to a LoL see perllol for the skinny on LoLs); and CWevents_r, which is an exact synonym of CWevents. This duplicates the contents of the given track, and returns the duplicate. If you are unclear on why you may need this function, consider:
          $funk  = MIDI::Opus->new({'from_file' => 'funk1.mid'});
          $samba = MIDI::Opus->new({'from_file' => 'samba1.mid'});
          $bass_track = ( $funk->tracks )[-1]; # last track
          push(@{ $samba->tracks_r }, $bass_track );
               # make it the last track
          &funk_it_up(  ( $funk->tracks )[-1]  );
               # modifies the last track of $funk
          &turn_it_out(  ( $samba->tracks )[-1]  );
               # modifies the last track of $samba
So you have your routines funk_it_up and turn_it_out, and they each modify the track they're applied to in some way. But the problem is that the above code probably does not do what you want because the last track-object of CW$funk and the last track-object of CW$samba are the same object. An object, you may be surprised to learn, can be in different opuses at the same time which is fine, except in cases like the above code. That's where you need to do copy the object. Change the above code to read:
          push(@{ $samba->tracks_r }, $bass_track->copy );
and what you want to happen, will. Incidentally, this potential need to copy also occurs with opuses (and in fact any reference-based data structure, altho opuses and tracks should cover almost all cases with MIDI stuff), which is why there's CW$opus->copy, for copying entire opuses. (If you happen to need to copy a single event, it's just CW$new = [@$old] ; and if you happen to need to copy an event structure (LoL) outside of a track for some reason, use MIDI::Event::copy_structure.) Returns the list of events in the track, possibly after having set it to CW@events, if specified and not empty. (If you happen to want to set the list of events to an empty list, for whatever reason, you have to use $track->events_r([]).) In other words: CW$track->events(@events) is how to set the list of events (assuming CW@events is not empty), and CW@events = CW$track->events is how to read the list of events. Returns a reference to the list of events in the track, possibly after having set it to CW$events_r, if specified. Actually, $events_r can be any listref to a LoL, whether it comes from a scalar as in CW$some_events_r, or from something like CW[@events], or just plain old CW\@events Originally CW$track->events was the only way to deal with events, but I added CW$track->events_r to make possible 1) setting the list of events to (), for whatever that's worth, and 2) so you can directly manipulate the track's events, without having to copy the list of events (which might be tens of thousands of elements long) back and forth. This way, you can say:
          $events_r = $track->events_r();
          @some_stuff = splice(@$events_r, 4, 6);
But if you don't know how to deal with listrefs outside of LoLs, that's OK, just use CW$track->events. Returns the type of CW$track, after having set it to 'MFoo', if provided. You probably won't ever need to use this method, other than in a context like:
          if( $track->type eq 'MTrk' ) { # The usual case
          } # Else just keep on walkin'!
Track types must be 4 bytes long; see MIDI::Filespec for details. Returns the data from CW$track, after having set it to CW$kooky_binary_data, if provided even if it's zero-length! You probably won't ever need to use this method. For your information, CW$track->data(undef) is how to undefine the data for a track. This adds the event ('event', ...parameters...) to the end of the event list for CW$track. It's just sugar for:
          push( @{$this_track->events_r}, [ 'event', ...params... ] )
If you want anything other than the equivalent of that, like some kinda splice(), then do it yourself with CW$track->events_r or CW$track->events. This dumps the track's contents for your inspection. The dump format is code that looks like Perlcode you'd use to recreate that track. This routine outputs with just CWprint, so you can use CWselect to change where that'll go. I intended this to be just an internal routine for use only by the method MIDI::Opus::dump, but I figure it might be useful to you, if you need te dump the code for just a given track. Read the source if you really need to know how this works.


Copyright (c) 1998-2002 Sean M. Burke. All rights reserved.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


Sean M. Burke