Notes About MIDI
MIDI files, just like sheet music, cannot represent the full range of expression that exists in live music performed by humans. This is the reason why several people can perform the same piece of sheet music, yet the resulting sound is different. Even a MIDI file played by one computer might sound different than another because the sound generation hardware may be different. Wave table sound cards utilize actual recordings of real instruments, whereas FM (frequency modulation) sound cards create synthesized sounds. Additionally, many musicians know that the music emanating from a computer playing a MIDI file can sound flat because it lacks the small errors introduced by human performers. A wave file (as well as AIFF file) is a digital recording similar to that on an audio CD, and captures all the nuances in a performance. Similarly, an MP3 file, WMA file, or AAC file is a compressed form of a wave file. A MIDI file, by contrast, represents musical information symbolically as a series of notes, volume, duration, etc. Therefore, a certain amount of information is lost when converting audio files to MIDI files. This section describes how information is mapped between the two formats.
Each note is tracked from when it begins sounding until when it ends. The volume of the note as when it rises above the MIDI Threshold is considered the "note on velocity". Similarly, the volume of the note when it subsequently falls below the MIDI Threshold is recorded as the "note off velocity". These two events may coincide with the attack and release of the note if the MIDI Threshold is low enough.
The volume and/or brightness may fluctuate while the note is sounding, and these changes are recorded using "continuous controllers". If the pitch of the note fluctuates, and these changes are recorded using "pitch bend" events. The exception to this rule is when the pitch change is extreme enough to be considered a different note. In this case, the first note is considered to end and the new note begins. The note off velocity of the first note and the note on velocity of the new note are recorded as the volume of the note when the pitch change to the new note occurred. Due to the limitations of the MIDI file format, pitch, volume, and brightness fluctuations apply to an entire channel of MIDI rather than individual notes. Accordingly, although intelliScore can detect these fluctuations separately for each note, it writes continuous controller and pitch bend events only to MIDI channels containing one note at a time.
Because note on velocity, note off velocity, and continuous controllers can be programmed on synthesizers to effect sounds in different ways beside volume, the resulting sound may not be as expected. Differing envelopes of musical instruments also affect the velocity values. An envelope consists of the attack, decay, sustain, and release volumes. When a note is struck, the volume increases from zero during the attack stage then decreases partially during the decay stage. The volume remains steady or continues to fall at a slower rate while the note is sustained. When the note is released, the volume returns to zero. The sustain phase of a piano's envelope, for example, decreases constantly. In this case a note with a short duration (sooner release) on a piano would have a higher note off velocity than the same note with a longer duration. However, the same note on a flute would result in a higher note off velocity than a piano, since the volume during sustain phase of a flute is constant.
You should also know that the MIDI standard enforces certain elements of modern Western music theory. Music is represented using a twelve-tone scale, for example. Only major and minor scales are recognized, for another. This limitation of MIDI may also affect how accurately audio files can be represented as MIDI files.