Digital Sound & Music: Concepts, Applications, & Science, Chapter 6, last updated 6/25/2013
Figure 6.12 Three types of tracks in Logic
A channel is a communication path. According to the MIDI standard, a single MIDI
cable can transmit up to 16 channels. Without knowing the details of how this is engineered, you
can just think of it abstractly as 16 separate lines of communication.
There are both input and output channels. An incoming message can tell what channel it
is to be transmitted on, and this can route the message to a particular device. In the sequencer
pictured in Figure 6.13, track 1 is listening on all channels, indicated by the word OMNI. Track
2 is listening only on Channel 2.
The output channels are pointed out in the figure, also. When the message is sent out to
the synthesizer, different channels can correspond to different instrument sounds. The parameter
setting marked with a patch cord icon is the patch. A patch is a message to the synthesizer – just
a number that indicates how the synthesizer is to operate as it creates sounds. The synthesizer
can be programmed in different ways to respond to the patch setting. Sometimes the patch refers
to a setting you’ve stored in the synthesizer that tells it what kinds of waveforms to combine or
what kind of special effects to apply. In the example shown in Figure 6.13, however, the patch is
simply interpreted as the choice of instrument that the user has chosen for the track. Track 1 is
outputting on Channel 1 with the patch set to Acoustic Grand Piano. Track 2 is outputting on
Channel 2 with the patch set to Cello.