Midi messages  (Page 2/4)

Making a sound: note-on and note-off events

As mentioned earlier, most MIDI messages are three bytes in length: a status byte followed by two data bytes. The status byte for the Note-On event is 1001nnnn (0x 9n in hexadecimal), where nnnn indicates the channel number . Since a four-bit value selects channel, there are a total of 16 channels available. The channels are called “Channel 1” (nnnn=0000) to “Channel 16” (nnnn=1111).

The first data byte indicates the note number to begin sounding, and the second data byte indicates the velocity at which the key was pressed. Since the MSB (most-significant bit) of a data byte is zero by definition, note number and velocity are each 7-bit values in the range 0 to 127.

Hexadecimal notation is commonly used to describe MIDI messages. For example, the three-byte message 0x93 0x5C 0x42 indicates a Note On event on Channel 4 for note number 92 with velocity 66.

The Note-Off event is similar; its status byte is 1000nnnn (0x 8n ). For example, the three-byte message 0x83 0x5C 0x35 indicates that note number 92 on Channel 4 should cease sounding, and that the key release velocity was 53. Synthesizers and sound modules generally equate keypress velocity with amplitude; however, they are less likely to use the release velocity. A “Note On” event with zero velocity is equivalent to a “Note Off” event.

The following screencast video shows MIDI Note On and Note Off messages produced by the Roland XP-10 synthesizer and visualized using the MIDI OX software application . MIDI OX is a free MIDI utility that serves as a MIDI protocol analyzer. The video also shows how MIDI OX can play standard MIDI files such as instruments.mid , a short multi-instrument composition that illustrates the concept of MIDI channels:

If you plan to start using MIDI OX right away, take a look at the following screencast that explains how to set up MIDI OX to work with a MIDI device such as a synthesizer, and how to set up your soundcard so that you can visualize the MIDI messages produced by a MIDI player such as Winamp or Windows Media Player. You will also need to install the support application called MIDI Yoke , which serves as a virtual MIDI patch bay to interconnect all of the MIDI-related devices on your computer.

Selecting a voice: program change

The Program Change message selects which of 128 possible voices (also called sounds, tones, or patches) to associate with a particular channel. The status byte for the Program Change message is 1100nnnn (0x Cn ), where nnnn indicates the channel number. Only one data byte called the program number follows the status byte for this message type. For example, the two-byte message 0xC7 0x5F directs the synthesizer to use program number 95 for all subsequent Note-On and Note-Off messages directed to Channel 8.

The original MIDI standard did not constrain equipment manufacturers in what sounds or voices to associate with program numbers. However, as it soon became apparent that defining standard voices for the 128 program numbers would make it easier for composers to distribute multi-voice (or multitimbral ) compositions with the expectation of the listener hearing what the composer had intended. The General MIDI (GM) standard defines 128 basic voices to be associated with the 128 program numbers. Moreover, Channel 10 is always defined as a percussion instrument under the General MIDI standard. See the GM Level 1 Sound Set for a table of voices and percussion key map. Note that the seven-bit program number ranges from 0 to 127; the associated GM voice will range from 1 to 128. The following screencast video gives you a quick tour of the 128 General MIDI voices as they sound on my Roland XP-10 synthesizer.