Digital Sound & Music: Concepts, Applications, & Science, Chapter 6, last updated 6/25/2013
Another significant difference between digital audio and MIDI is the way in which you
edit them. You can edit uncompressed digital audio down to the sample level, changing the
values of individual samples if you like. You can requantize or resample the values, or process
them with mathematical operations. But
always they are values representing changing
air pressure amplitude over time. With MIDI,
you have no access to individual samples,
because that’s not what MIDI files contain.
Instead, MIDI files contain symbolic
representations of notes, key signatures, durations of notes, tempo, instruments, and so forth,
making it possible for you to edit these features with a simple menu selection or an editing tool.
For example, if you play a piece of music and hit a few extra notes, you can get rid of them later
with an eraser tool. If your timing is a little off, you can move notes over or shorten them with
the selection tool in the piano roll view. If you change your mind about the instrument sound
you want or the key you’d like the piece played in, you can change these with a click of the
mouse. Because the sound has not actually been synthesized yet, it’s possible to edit its
properties at this high level of abstraction.
MIDI and digital audio are simply two different ways of recording and editing sound –
with an actual real-time recording or with a symbolic notation. They serve different purposes.
You can actually work with both digital audio and MIDI in the same context, if both are
supported by the software. Let’s look more closely now at how this all happens.
Channels, Tracks, and Patches in MIDI Sequencers 6.1.4
A software MIDI sequencer serves as an interface between the input and output. A track is an
editable area on your sequencer interface. You can have dozens or even hundreds of tracks in
your sequencer. Tracks are associated with areas in memory where data is stored. In a
multitrack editor, you can edit multiple tracks separately, indicating input, output, amplitude,
special effects, and so forth separately for each. Some sequencers accommodate three types of
tracks: audio, MIDI, and instrument. An audio track is a place to record and edit digital audio.
A MIDI track stores MIDI data and is output to a synthesizer, either a software device or to an
external hardware synth through the MIDI output ports. An instrument track is essentially a
MIDI track combined with an internal soft synth, which in turn sends its output to the sound
card. It may seem like there isn't much difference between a MIDI track and an instrument track.
The main difference is that the MIDI track has to be more explicitly linked to a hardware or
software synthesizer that produces its sound, whereas an instrument track has the synth, in a
sense, "embedded" in it. Figure 6.12 shows each of these types of tracks.
Aside: The word sample has different
meanings in digital audio and MIDI. In MIDI,
a sample is a small sound file representing a
single instance of sound made by some
instrument, like a note played on a flute.