The OS/2 sequencer device plays a MIDI song by sending commands from a MIDI file to a synthesizer, where the commands are converted to the sounds of a specific instrument. Typically, a digital signal processor (DSP) is used to generate the sounds of the instrument, which results in an authentic reproduction of the original performance.
General MIDI (Musical Instrument Digital Interface) is a standard specification for playing back music from a series of commands, rather than actual audio data. The commands represent musical events, such as turning a note on and off ("Note On" and "Note Off"), as well as timing mechanisms for specifying the duration of the note sound. The sequencer uses the timing commands to sequence the playing of the music.
Following is a text example of the commands generated when someone depresses the "middle C" key of a synthesizer keyboard: a note-on command X'90' and two bytes of data:
X'90' - "Note-on" command to MIDI channel 0 X'3C' - Keyboard note (middle-C) X'40' - Velocity (X'00'-X'7F').
MIDI augments waveform audio as a means of producing sounds in the multimedia environment. MIDI data offers the advantage of requiring far less storage than waveform data. For example, suppose a three-note chord-middle-C, E and G- is held for one second. Following are the MIDI commands required to reproduce the chord on a synthesizer:
TIME = 0 sec. 90 3C 40 90 40 40 90 43 40
(C-on) (E-on) (G-on)
.
.
.
TIME = 1 sec. 90 3C 00 90 40 00 90 43 00
(C-off) (E-off) (G-off)
The storage required for the MIDI commands is 18 bytes. To store the same information as 16-bit, PCM, 44 kHz, stereo waveform audio data requires 176KB.
Another advantage of storing musical performances as a series of instructions is that the information can be edited, the same way words in a document can be edited by a word processor. The musical editing process can be used, for example, to correct mistakes in an artist's original interpretation, or to change certain points of style before playback or final recording. Playback of MIDI data using the sequencer media device can be used to reproduce the original performance or to print out musical scores.
Sequencer Device Specifics
The sequencer device sends MIDI messages and data to the audio adapter. Some audio adapters, such as the M-Audio adapter, perform FM synthesis to produce music. Other audio adapters, such as the Sound Blaster adapter, have the capability to send the MIDI data through a MIDI port to an external synthesizer device.
The OS/2 sequencer device does not currently support recording new MIDI information.