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MMD > Archives > March 2000 > 2000.03.19 > 17Prev  Next

Limitations of MIDI Systems
By Richard Brandle

The article written by Douglas Henderson is not describing the
limitations of MIDI but the design choices of the various elements that
make up a MIDI system.  A MIDI system consists of the MIDI protocol,
the transport and the sound generator.

The MIDI protocol is very robust and largely unchanged from its
original design.  Its primary limitation is a maximum of 16 sound

MIDI is usually transported via a 31,250 BPS [bits per second] serial
link which limits the speed the events can be delivered to the sound
module to about one per millisecond.  When introduced (early 1980's
I believe) it was blindingly fast, but by today's standards (and the
demands of today's MIDI systems) it is pathetically slow.  A few MIDI
systems using USB are available.  These operate at about 12,000,000 BPS
and do not exhibit the delays and "smearing" commonly associated with
low speed serial link MIDI.  However, there is an entire MIDI based
industry which must adapt to a new proposed standard.

Any MIDI sound module, either electronic or acoustic, has a limitation
on the number of simultaneous notes it can generate.  Early systems
were designed to be played by people, not computers.  Hence, polyphony
of 8 or 16 was sufficient.  Today's electronic units typically provide
polyphony of 32 or 64 notes and a few provide 128 note polyphony.

Real acoustic units become limited by the power requirements to drive
the solenoids or whatever is actually generating the sound and any
associated mechanical delays.  Again these are design choices imposed
by the manufacturer of the equipment to meet a specific price/profit
point at the retail level.

Regarding Robbie's comment on American in Paris in the "Gershwin Plays
Gershwin" CD, it was a 4-hand arrangement and the polyphony of that
roll required 25 simultaneous sounding (and sustained) notes.  This
exceeded the 16 simultaneous note capability of the Disklavier used
when the recording was made.  Hence, a second Disklavier was voiced to
sound similar to the first and the MIDI stream was divided to play both
instruments within their limitations.  It was not the producers choice:
it was required by the polyphony limitations imposed by the equipment
at that time.

The serial MIDI link speed was a concern with the PowerRoll.  Laurent
Corey and I designed special MIDI "system exclusive" messages that
allowed all 128 note changes to take place in 22 bytes.  The special
message can be transmitted in about 8 milliseconds.  Unfortunately,
Laurent Coray, the designer/implementor of the controller in the
PowerRoll, passed away before completing the software in the controller.
Hopefully the new owner will continue his work.  "WindPlay" already has
the support.  It is enabled by the "Optimize UM1 data stream" option on
the MIDI configuration.

The point of all this is all systems have limits.  The editors of the
music rolls for our pianos knew the limitations of the systems and
frequently modified the coding to compensate.  Any system can be pushed
beyond its limits with bad results.  Conversely, with unlimited money
one can design a system to remove almost all limits.

Richard Brandle

(Message sent Sun 19 Mar 2000, 16:21:21 GMT, from time zone GMT-0700.)

Key Words in Subject:  Limitations, MIDI, Systems

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