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From the Mad Dog/Allison music collection...
What's Missing In MIDI?
By Chris Muir and Keith McMillen
While MIDI has been a major boon to the keyboard player for the last
few years, it is just now becoming a major force in the guitar world. Midi was
designed to be a simple way to connect a remote keyboard to a synthesizer. It
has grown (or mutated, if you prefer) into a fairly complete interface for
keyboard-based synths, but it has severe limitations when applied to
non-keyboard instruments, such as guitar or bass. The limits as we see them
are:
Pitch changes are tied to note changes. The inability to change the
pitch of a note without causing a new attack can be very limiting. This is best
illustrated by having, say, a percussive Hammond B-3 organ sound on your synth,
picking a note, and then sliding up the neck. On a guitar, you hear one attack
when you pick the note, and then the pitch smoothly rising in semitones. The
synth, on the other hand, makes a new percussive attack every time your finger
passes over a fret.
Midi "congestion". This problem occurs when the amount of information
is greater than the time available to transmit it. When presented with six
decaying string envelopes and/or six channels of pitch-bend information, there
may not be time to transmit all the data before a new update is needed. This
can noticeably slow down the response of the synth to new notes and other MIDI
information. It also affects the smoothness of a bend and amplitude changes,
making a continuous effect sound choppy.
These problems stem mainly from the fact that a guitar can do things a
keyboard can't, such as:
Slur notes. With MIDI it is only possible to turn a note off, and then
re-attack on a new note. A guitar string can be picked once, and then many
additional pitches can be generated by hammering-on, sliding, slurring, and so
forth, but it is not always desirable to give each pitch change a new attack.
Bend individual notes in a chord. A given MIDI channel only supports
one pitch-bending channel, the pitch-bend wheel. A guitar has individual
control of pitch bending for each note. When you play a chord on a keyboard and
tweak the bend wheel, each note in the chord is affected equally. Guitarists
can, and do, bend just one note in a chord. This is not possible over one MIDI
channel.
Change amplitude. A note played on a guitar has its own natural
envelope that can be applied to the synthesizer, instead of (or in addition to)
a peak velocity such as that generated by a keyboard. This is important
because guitarists do things other than pluck a string and let it decay to
silence. Techniques such as muting or palming strings provide unique sounds on
the guitar, but it's not easy to transmit them over MIDI. If bassists slap and
pop their strings, shouldn't the synth do its best to slap and pop in response?
There is no standard way to make these amplitude changes over MIDI.
Currently, there are a only few synths that deal effectively with
advanced guitar controllers, including the Oberheim Xpander (as well as
Oberheim's Matrix 12, and to a limited extent the Matrix 6) and the Yamaha TX
816. Such synths can "listen" to multiple MIDI channels in a Mono Mode
(hereafter called Multiple-Mono). The Xpander has an envelope mode that allows
playing in a legato fashion.
To facilitate the use of guitar controllers with existing MIDI
synthesizers, we propose the following software changes to the interpretation
of MIDI protocol:
Multiple-Mono Mode. Even a standard polyphonic synth could listen to
several (six, for example) MIDI channels in a multiple-mono mode. All the notes
could still sound alike, but there would be the individual articulation
necessary for a guitar. This would allow bending of individual notes in a
chord, etc. As this would probably entail only a software change, it wouldn't
impact the cost of synths too much, and it would be a major advance for guitar
synth players everywhere.
Legato Interpretation. If a note is turned on in a given channel before
the previous note is turned off, it is interpreted as pitch change only, not a
new attack. This is currently available in the Oberheim Xpander as one of many
envelope modes (single trigger).
Velocity Update. A standard way to update the velocity of a note (how
hard it's hit) without a new attack would be nice. Currently, velocity can only
be transmitted when a note is turned on. Since velocity is almost always used
to control loudness and loudness-related timbre changes, changing it with
string envelopes would be ideal. The alternative method would be to send string
envelope information over one of MIDI's Continuous Controller channels.
Unfortunately, different manufacturers use these for a wide variety of things
(pedals, footswitches, modulation wheels, etc.) with little or no
standardization among them. Perhaps this could be made easier by some
standardization of Breath Controller numbers.
The bandwidth problem is a little more complex. To get more technical
for a moment: the current crop of MIDI synths generally can't operate at full
MIDI bandwidth, 31.25 kilo-baud. The baud rate is roughly equivalent to the
rate that bits are transmitted in series; at 10 transmitted bits for every
byte, it takes .3125 milliseconds to transmit a byte at 31.25 kbaud. Most MIDI
commands are three bytes long. This works out to just under a millisecond for
each midi command at full bandwidth. So at this rate, it takes about one
thousandth of a second to turn a note on. The trouble is, while each individual
command is sent at this rate, some time is needed between commands, often as
much as two thousandths of a second. This might not seem too slow, but it only
lets about 330 commands per second through the MIDI bottleneck, which can cause
some slowing of response, especially when multiple notes are bent, for example.
The amount of time needed between commands varies widely from manufacturer to
manufacturer. Just speeding up the rate at which a synth can receive MIDI data
to full bandwidth would be a big improvement. Even with this speedup, the
real-time continuous signals a guitar can generate can completely fill all
available MIDI time and more.
As a side issue, it should be noted that sampling synths, as a class,
are gettubg a few MIDI System Exclusive commands for general use. Maybe there
should be a System Exclusive command or two reserved for guitar controller use.
If a guitar controller could issue a standard System Exclusive command telling
receiving sythesizers to "go into guitar mode" this would free guitarists from
having to set up many somewhat arcane MIDI modes on their synths.
The changes we are suggesting are simply software changes and
standardizations that would not affect the price of the synth. However, they
would greatly enhance its usefulness for guitar and bass players.
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