A status byte has an msb of 1 (most significant bit) a data byte has an msb of 0. There are only 7 status messages (note on, off, polyphonic pressure, channel pressure, program change, controller change, pitch bend). These 7 are identified in the first nibble. Status bytes must also identify the midi channel in the 2nd nibble e.g. The note on status byte starts with 1001, then 4 more bits id the midi channel, so 1001 0000 would be a note on for channel 1 (0000 = 1, 1111 = 16). The status byte is followed by up to 2 data bytes. in the case of note on the first data byte ids the actually pitch and the second the velocity. So, a pitch of 60 (middle c) would be 00111100 and a velocity of 127 (max) 01111111. There can only be 128 different values (0-127) because only 7 bits can be used as the msb must be 0. Therefore the full note on message for a maximum velocity middle c on channel one would be 10010000 00111100 01111111. see its simple.
