Amiga Plus 2000 #4/Amiga Plus CD - 2000 - No. 4.iso/Vollversion/CamD/development/docs/midibasics.doc

Text File | 2000-05-15 | 6KB | 112 lines

Parsing MIDI
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    First off, I suggest that you send for a copy of the MIDI specification
from the International MIDI Association (5316 W. 57th St., Los Angeles, CA
90056, U.S.A). The cost for the basic specification is $40.00 for non-members.

    In the mean time, however, here is a quick summary of some of the most
important aspects of MIDI:

    MIDI is primarily a _gestural_ language, in other words it does not
represent sounds or even music, but rather a series of actions by a performer,
such as hitting a key, pressing a pedal, etc. The instrument can interpret
the messages however it chooses (for example, many instruments have a feature
which allows the musical keyboard to be remapped to a set of non-standard
musical frequencies, allowing non-western musical styles).

    MIDI data comes in a stream of bytes. Bytes which have the high bit (bit 7)
set are _status bytes_. Bytes which have the high bit cleared are _data bytes_.

    Each midi message consists of a status byte followed by zero or more
data bytes. The number of data bytes depends on the status byte.

    Midi messages are organized into the following general classes:

    Channel Messages (status bytes $80-$EF)
    System Common Messages (status bytes $F0-$F7)
    System Real-Time Messages (status bytes $F8-$FF)

    Channel messages are messages which are sent to specific MIDI "channels".
Each channel message has a 4-bit channel number which is stored in bits 0-3
of the status byte for that message. MIDI instruments generally listen to one
or more channels, and any messages which have a channel number which the
instrument is not listening to is ignored. In addition, a MIDI instrument
may assign instruments or other state information on a channel-by-channel
basis, for example channel #1 might be a trumpet sound, channel #2 an organ,
etc.

    System Common messages and System Real-time messages generally cause
global changes to the instrument. The difference between the two is that
the latter are generally very short and tend to come rapidly.

    The include file "mididefs.h" supplied with CAMD lists all the types
of status bytes (the status bytes for each type of channel message is defined
without the channel number -- you must OR in the channel number into the
status byte. So for example, the "Note On" messages is listed as 0x90, but
can actually be any value between 0x90 and 0x9F, depending on the channel).

    Running Status: In order to increase the limited bandwidth of MIDI
devices, a form of simple compression called running status is used.
Basically, what this means is that you can omit a status byte if the
previous status byte was exactly the same. So for example, if you wanted
to turn on two notes on the same channel, instead of sending the
byte stream:

        0x92 0x41 0x12
        0x92 0x52 0x44

    You could send:

        0x90 0x41 0x12 0x52 0x44

    Note that only Channel messages may use running status. This is because
channel messages are all fixed in length, while many types of system messages
are variable length (They continue until the next status byte).

    Fortunately, if you're writing an application that uses the CAMD library,
you don't need to worry about running status or the number of data bytes
because CAMD automatically handles running status on both input and output,
and knows how long each message should be. But there may be times when you
need to deal with MIDI streams directly.

    The most important MIDI messages to support are "note on" and "note off".
All other messages can simply be ignored for a minimalist implementation.
A "note on" message consists of three bytes:

        status byte (0x90 plus the channel number)
        pitch       (in other words, the keyboard key that was hit, 0-127,
                     where 60 is middle C)
        velocity    (How hard you hit the key, 1-127. 0 is a special case.
                    Velocity usually affects how loudly the note is played)

    A "Note Off" message has exactly the same format, except that the
status byte is (0x80 | channel number) instead of (0x90 | channel number).

    Most instruments support _polyphony_, which means that multiple note-on
messages can be sent to turn on several notes at once, and then later
note-off messages can be sent to turn them off again. Each note-off will
turn off one note of the given pitch. The velocity field for note-offs is
usually ignored in most instruments, but can be used for special effects
such as decay rate.

    There are two additional ways to turn off a note.

    The first of these is to send a note-on with a velocity of zero. This
is exactly equivalent to sending a note-off with a velocity of 64. The
reason for this is to take greater advantage of running-status compression.
This allows many note-on's and note-off's to be sent without changing the
status byte.

    The second is the "All notes off" command. This is a special command
supported by some instruments, and is detailed in the MIDI specification.
You don't need to support it for a first cut implementation. The reason
for this command is simple: Since the MIDI note-on and note-off messages
are seperate messages, it is possible for notes to become "stuck" if the
MIDI cable is unplugged (or other transmission problem occurs, such as
crashing the computer) between the note-on and the note-off command.
Since the note-off command never arrived, the note will continue to play
(annoying everyone in the room) even if the cable is plugged back in.
(Note that unlike serial cables, it is electically safe to abuse MIDI cables
this way). "All Notes Off" is a conveniant way to silence everything on a
channel.