In my experience, only the most feature-rich computer-based MIDI synthesizers/sound-drivers will vary the timbre with the velocity. More expensive MIDI synthesizers will change the timbre of the played note as the velocity in the note-on message changes. Additionally, the timbre or instrument quality will change along with the velocity. The faster that one strikes a piano keys, the louder the note will sound. Velocity: The velocity specifies the volume or force, with which the note is played.That table assigns a specific instrument number for each MIDI key value. Again, General MIDI specifies a list of standard percussion instrument table. For percussion instruments the MIDI key value is used for specifying the instrument number. Before General MIDI, different mappings were supported by different manufacturers. The General MIDI specification assigns the number 69 to A440 (i.e., the international standard that assigns a pitch of 440 Hertz for Middle-A on the standard piano keyboard). Key Number: For pitched instruments (i.e., non-percussion instruments), the key-number specifies the piano-key number.All of the remaining channels (0 through 8, 10 through 15) are reserved for use with pitched or non-percussion instruments. The Timpani is not considered to be a General MIDI percusion instrument because you can specify a pitch. For instance, a Low Floor Tom is a percussion instrument in this context, while a Timpani is NOT a percussion instrument. In this context, a percussion instrument means an instrument that has no note-setting or pitch setting. MIDI Channel: In the General MIDI specificiation, MIDI channel 9 is reserved for the use with percussion instruments.The basic format of the MIDI note-on message is shown below:
The note-on message is a channel message and is consequently restricted to one of 16 pre-defined MIDI channels (i.e., numbers 0 through 15). The MIDI note-on message is used for turning on MIDI notes.