The MIDI Workstation
The MIDI (Musical Instrument Digital Interface) protocol was
established in 1983. The brainchild of Dave Smith (Sequential Circuits),
Ikutaro Kakehashi (Roland), and Tom Oberheim, MIDI was a pioneering
cooperative effort, uniting internationally competitive companies in the
quest to create a global platform to assist in the commercialization of
their individual products.
However, the unification of vastly differing technologies necessitated
compromise, and resulted in several MIDI limitations, primarily a low
data transfer rate (bandwidth protocol), unidirectional communication
between devices, and lack of control over subtle nuances of sound.
The MIDI Workstation
The establishment of MIDI for keyboard interface
prevented (or made extremely difficult) non-keyboard
related gestures. Regardless, MIDI became an important
platform for developments in interactive performance
systems; proprietary hardware devices could now be
interconnected, facilitating the design of robust, custommade portable interactive environments. Though the low
data transfer rate limited the representation of complex
musical structures, it nevertheless permitted real-time
processing of musical information.
MIDI (Musical Instrument Digital Interface)
consists of both a simple hardware interface, and a more elaborate transmission
MIDI is an asynchronous serial interface (meaning messages are sent
out one after the other). The baud rate (transmission speed) is 31.25
Kbaud (+/- 1%) or 31,250 bits per second.
The MIDI circuit is current loop, 5 mA. (m = Milli (10-3) & mA =
Milliampere). Logic 0 = current ON. One output drives one (and only
one) input., The MIDI specs. recommend the use of an optoisolator to
avoid grounding problems. An optoisolator is a sealed device which
contain both an LED and a photosensor (see the diagram that follows)
. These require less than 5 mA to turn on. Rise and fall time for the
optoisolator should be less than 2 microseconds.
Schematic of a MIDI Interface
MIDI connectors and cables
The standard connector is 5 pin DIN. Separate ports and
cables are used for input and output. 15 metres is the
recommended maximum cable length. Cables are
shielded twisted pair,. Pins 4 and 5 form a shielded
twisted pair, with the shield connecting pin 2 at both
ends. Pins 1 and 3 are not used.
A device may also be equipped with a MIDI THRU jack
which is used to pass the MIDI IN signal to another
device. Time delays caused by the response time of the
opto-isolator may result in timing errors , therefore there
is a limit to the number of devices that can be daisychained.
Other numbering systems (cont.)
Hexadecimal: expresses quantities in powers of 16
using the symbols 1,2,3,4,5,6,7,8,9,A,B,C,D,E,F.
60 = 3Ch
or 60 = 3(161) + 12
10010000 = 144 = 90h
00111100 = 60 = 3Ch
01000000 = 64 = 40h
Converting from decimal to binary
Convert the value 44 from decimal to binary
Let D= the number we wish to convert from decimal to binary (44)
Find P, such that 2^P is the largest power of two smaller than D (32)
Repeat until P<0
So…….If 2^P<=D then put 1 into column P
subtract 2^P from D
Otherwise put 0 into column P
Continue until complete!
Try a few out and use this conversion tool to check your answers