What Is Midi - Akai S950 Operator's Manual

WHAT IS MIDI?

Back in the early days of synthesizers, a system known as voltage control was
used. Basically, when you played a note on the keyboard, you sent a voltage
to voltage controlled oscillators and the value of that voltage determined
the pitch of the oscillators. At the same time, the keyboard sent out a pulse
(known as a trigger or gate pulse) to devices known as envelope generators
which also produced a rising and falling voltage which would sweep through a
voltage controlled filter and open and close a voltage controlled amplifier.
A device known as a low frequency oscillator also generated rising and
falling voltages and this could be routed to the oscillators, filter and/or
amplifier for pitch, tonal and amplitude sweeps.
Most synths at that time were only monophonic - that is they could only play
one note at a time and so it was easy thing to stick four sockets on the back
of the synth. One socket had the keyboard voltage output, another had the
gate pulse output whilst the other two sockets were for voltage and gate
pulse inputs. Using these sockets, it was possible for one synth to play
another and it all worked rather well. The problem was that different
manufacturers used different methods of triggering the envelope generators
and so, without using special interfaces, it was not possible to have a Moog
synth playing a Roland or ARP synth or vice versa. Neither was it possible
for a Korg synth to play a Moog or a Sequential synth. This was also true
with the simple sequencers that were around in those days and, similarly, it
was not possible to use a Moog sequencer with anything other that Moog
equipment - likewise with Korg.
This problem was further complicated with the advent of polyphonic synths
such as the Prophet 5, Oberheim OBX and Roland Jupiter 8 and compitcated even
further with the advent of digitally controlled synthesizers.
In 1983, Dave Smith, president of Sequential Circuits, put forward a proposal
to the Audio Engineering
manufacturers were
sequencers and drum machines, a standard digital interface could be used that
allowed any manufacturers
interfacing problems. The Musical Instrument Digital Interface or MIDI had
been born and over the following few years, the specification of MIDI changed
quite radically to the powerful interfacing system it is today. But what is
MIDI?
MIDI is basically a digital communication system. At the heart of every
synth, sampler, drum machine and sequencer is a microprocessor. By getting
the microprocessor
instruments microprocessor it !s possible to play the two instruments. as if
they were one. The most basic use of this is to layer two instruments sounds
on top of each other to create a richer, bigger sound.
If you refer to Fig. 1, you can see the layout of a typical synth or sampler.
As you can see, at the heart of this is the microprocessor or Central
Processing Unit (CPU) and this is controlling practically every aspect of the
instrument from keyboard control to sound editing - some even allow sequencing
and other facilities. Observe the position of the MIDI outputs. If the MIDI
OUT is taken to the MIDI IN of another instrument, anything you do on the
controlling synth is duplicated on the other.
Society (AES)
using microprocessors
equipment
from one instrument
which suggested
at the heart
to be used with
to directly
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that as most
of their synths,
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'talk' to another