38. MIDI Sequencers
A MIDI sequencer is a hardware device programmed specifically to record, edit, and play back MIDI data, or a software program running in a computer that accomplishes these same tasks. The type of MIDI data that a sequencer records includes (but is not limited to) Note On and Note Off messages, Aftertouch, Pitch Bend, and Control Change messages. The musical document that the MIDI sequencer records is referred to as a sequence.
A MIDI sequencer works very much like the analog cassette recorder. You insert a tape, depress the record button, and then input the signal you want to record. A magnetic imprint on the tape is recorded analogously to the signal as the tape passes the record head of the tape recorder.
Diagram 38-1: A sequence.
A MIDI sequencer, as sophisticated as it is, works in a similar fashion.
You also depress "record" (either literally on a hardware device, or via its equivalent in a software program), and then input the MIDI data you want to record.
While the user knows by looking at the magnetic imprint when and in what order sounds were recorded on the tape, a MIDI sequencer needs a different method to document when each of the MIDI events occurred in time.
In order then to document each of the MIDI events in time, a MIDI sequencer must have its own clock, which permits every MIDI event (like "play middle C") to have its own time coordinate. The MIDI sequencer can record that, indeed, middle C has been played, and that it was played in measure 4, and on beat 2. In order to allow the MIDI sequencer to capture all the nuances of a performance on, say, a keyboard controller, the internal clock divides a quarter note into very small but equally spaced time-coordinates called clocks.
There is a tremendous variance among MIDI sequencers in their ability to divide musical time into clocks per quarter note. Depending upon the sequencer, there may be as few as 24 clocks per quarter note or as many as 960. This number, by the way, seems to be ever increasing. Obviously, the more divisions of the quarter note, the more accurately a performance can be documented. A time coordinate for "play middle C" might very well be "measure 4, beat 2, clock 120". And, if there were 240 clocks per quarter note, then middle C would occur on "the and of beat two".
To enable better management of the recorded data, a MIDI sequencer usually permits the user to divide MIDI data into tracks, each of which has its own stream of MIDI events. This means that in a MIDI-prepared arrangement for piano and flute, the flute part could record on one track, the right hand of the piano on a second, and the left hand of the piano on a third.
One MIDI sequencer may have as few as eight tracks; yet the number of tracks permitted by another sequencer may exceed a thousand. With MIDI technology it is entirely possible that in the creation of an orchestral score sequence, the music of each instrument in the orchestra could be placed on its own individual track.
Once the sequence has been recorded by a MIDI sequencer the user has the opportunity to change and edit the sequence. Editing the MIDI sequence is not unlike editing a text document in a word processor program. The writer makes changes by selecting anything from a single letter to the whole document, then decides how the selected text will be changed, making additions, corrections, and deletions as desired.
Editing in a MIDI sequencer is essentially the same process. Events are identified, and then changed. The types of changes possible in a MIDI sequencer vary tremendously, but almost universally a sequencer permits the user to change the MIDI channel of any MIDI event or events, choose the duration of any note or notes, alter the key velocity of note or notes, and transpose any note or notes. These are all crucial elements in shaping any musical performance.
Another feature common to MIDI sequencers and word processing programs is "Cut and Paste." In a MIDI sequencer a region of MIDI events (the music) may be selected, copied, and then inserted into the MIDI score at the desired location in time. This feature is especially important to a musician since most music contains repetition to one extent or the other.
Once a sequence has been recorded and then edited the sequencer can be directed to play back the sequence, presumably shaped to musical perfection.
