Introduction to Computer Music: Volume One

1. How does the MIDI system work? Page 12

Compositional Uses of MIDI continued

MIDI Sequencing Software

The most common type of software used to link MIDI instruments and computers is a large variety of MIDI sequencing programs. The term sequencer was carried over from voltage-controlled analog synthesizers, in which sequencing modules could be set to step through 8 or 16 stages of voltage-controlled notes. Famous analog sequencer-based instruments included the Buchla sythesizer series, made famous by Morton Subotnik's album-long compositions, Silver Apples of the Moon and The Wild Bull. The limited number of sequenced notes was increased when small microprocessors, capable of recording and playing back several hundred notes, were added to digitally-controlled analog synthesizers. Current sequencing software allows recording and playback of notes, patch changes, controller information, and even SysEx codes to the limits of the program's memory allocation, usually hundreds of thousand of events.

Sequencers normally organize their information along the lines of multitrack audio recorders. Information is recorded and edited on individual tracks which are assignable to one or more MIDI channels. Previously recorded tracks can be played back while new ones are recorded. Current software accommodates up to 200 tracks of information. Rhythm is usually organized along traditional musical divisions. Note events are assigned measures, beats, and beat subdivisions (down to resolutions of 480 per beat). Unlike audio tape, sequenced performances can be recorded at a slow tempo and sped up for playback with no change in pitch. The inaccuracies of real-time performance can be quantized to the nearest selected note values. Notes can be entered in a precise rhythmic fashion by step-recording them -- that is, selecting a rhythmic value, usually from a computer keyboard, then playing the pitch or chord on a MIDI keyboard and proceeding to the next note. While this facility has led to a plethora of unimaginative, "switched-on Bach"-style musical transcriptions, the computational facilities that usually accompany step-recording features can be of great service to serious composers in need of complex rhythmic precision.

Sequencer tracks can be edited in a wide variety of manners. For example, one may select a recorded area and transpose it, shorten its rhythmic durations, and have a smooth increment of velocity for a crescendo. Since MIDI's numerical structure makes rapid musical operations quite simple, many compositional techniques, such as inversion, retrograde, rhythmic diminution or augmentation, are often included as editing features. Tracks or portions of tracks also may be looped to create continuous ostinato patterns.

The capability to edit metronomic tempi is included in most sequencers. Composers can now specify smooth accelerandos or ritards from one measure to another. Sequencing software often provides composers with both musical time (measure, beat, and subdivision) and real time (minutes, seconds, hundredths of seconds) by computing all the metronomic changes in a particular sequence. The computational capability of the sequencer enables film composers, who previously used cues sheets and stop watches, to request the rate of acceleration from measure x to arrive at measure y by a particular SMPTE frame number.

Finally,programs called digital audio workstations or DAW's include the capability to record and edit both MIDI and digital audio. This includes current programs such as MOTU's Digital Performer, Digidesign's Pro Tools, Steinberg's Cubase and Apple's Logic.


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