Data Communication/MIDI: Juan P Bello

Data Communication/MIDI
Juan P Bello
MIDI
The Musical Instrument Digital Interface (MIDI) is a standard for
communication between electronic musical instruments, which has
also been applied to a larger range of equipment (Fx units,
mixers, light control, etc)
Main functions: transmission of performance/control data around

a digitally-controlled music systems and of other data such as
timing info, set-up parameters, samples, etc.
The MIDI standard was formulated by agreement between the

manufacturers in the early 80s
While use of the hardware interconnection scheme is in steady

decline, the communications protocol is still widely used.
Background (1)
Electronic musical instruments, and the need to control them remotely,

predated MIDI
Old instruments used analog voltage control instead of microprocessors
These often used one port for timing and another for note triggering and
pitch info (as a DC control voltage)
Background (2)
With the advent of microprocessor-based control in musical
instruments a number of digital control interfaces appeared
Incompatibility amongst them created the need for
standardization and agreement between major manufacturers
This resulted on the MIDI 1.0 specification released in 1982/83.
The standard core functionalities remain largely unchanged,
although several others have been added over time, e.g.: MIDI
files, general MIDI, sample dump, MIDI timecode, etc.
The MIDI Manufacturers Association (MMA), is the governing body
regulating modifications to the standard (http://www.midi.org/).
MIDI vs Audio (1)

In digital audio, the waveform is converted to the digital domain,
where it is processed and stored before being converted back to
analog
Sounds are stored and replayed precisely, but we have no access

to control data (the parameters that generated that sound)
Since we need high temporal and amplitude precision to properly
represent an audio waveform, digital audio uses a lot of memory
space.
MIDI vs Audio (2)

In MIDI, processing and storage also occurs in the digital domain,
but the information being processed is not the audio signal but the
control data used to generate it.
An electronic instrument is needed to reproduce the sound, which
means that unless we use the exact same synthesis engine, MIDIgenerated sounds are never the same.
Because it comprises control data only, MIDI uses significantly

less memory space than digital audio
Data communication (1)

Serial vs Parallel: 1 bit per clock vs n bits per clock
Serial:
Transmitter Receiver
101001
11
Parallel:
Transmitter Receiver
1
1
1
1
1
1
0
0
0
0
1
1
0
0
1
Parallel communication can be extremely fast, but also bulky and

expensive for use over long distances (common within computers)
Serial interface, although slower than parallel, allows for simple
connectors and cabling (simple implementation and low cost).
Speed of communication is measured in # data symbols per
second (bauds). This is commonly equivalent to the information
(data) rate, but not always.
Data communication (2)

In synchronous communication, the data is accompanied by a
clock signal (on a separate wire or modulated with the data)
This is used to tell the receiver the time slot in which it should
register each arriving data bit.
In asynchronous communication, transmitter and receiver clocks
are not connected but should oscillate at nearly the same rate.
Start and stop bits are used in the communications protocol to
allow phase adjustment
Data communication may be unidirectional or bidirectional.
Bidirectional interfaces can be further divided in half duplex (only
one direction at a time) and full duplex (capable of simultaneous
transmission/reception).
MIDI hardware (1)
MIDI was designed to be simple and easy to install in economic

equipment, and widely available to as many users as possible.
It uses an uni-directional serial interface and asynchronous communication
(no clock is embedded in the transmission)
Conversion between parallel and serial is performed by an Universal
Asynchronous Receiver/Transmitter (UART) unit.
MIDI hardware (2)

Parallel/serial conversions (in UART) use shift registers:
Serial interface is slower than parallel but allows simple connectors and
cabling (simple implementation and low cost). Speed of MIDI
communication: 31.25 kbits/s (bauds). As a reference USB 1.1 low-speed
transmit at 1.5Mbauds, USB-2 at 480Mbauds
Serial to Parallel
Parallel to Serial
MIDI hardware (3)
In asynchronous serial communications the transmitter sends nothing but

a serial data stream and (usually) needs no acknowledgement.
Uses start and stop bits to define the data boundaries. Every MIDI byte
transmitted/received is coded as a 10-bit byte.
In MIDI a binary 0/1 is defined by current flowing/not flowing on the
current loop. Thus the idle state is binary 1, start bit is always 0 and stop
bit is always 1.
Clocks must run at exactly the same rate (1% tolerance in MIDI)
Data period
MSb
LSb
1
0
Idle state
Start bit
One clock period
Stop bit
MIDI hardware (4)
There are 3 kinds of MIDI ports: IN, THRU, and OUT. The IN port accepts
input to a device, the THRU port passes an amplified copy of the input
signal along, and the OUT port is used to transmit the devices output.
MIDI interconnection (1)

The hardware uses cables terminated in 180-degree 5-pin DIN
connectors, of which only three pins are used (5, 4 and 2).
Pin 2 is connected to earth in OUT and THRU only

MIDI data transmission is slow enough to avoid transmission
losses and fast enough to make any transmission delay musically
negligible.
In the MIDI specification, the opto-isolator is required to have a
maximum rise-time of 2s
time
The rise-time refers to the speed of response of the device, if too

slow it results on a roll-off of sharp edges (also for fall time) that
can introduce errors in the data transmission.

When several devices are connected in series the data passes
through as many opto-isolators, resulting on an accumulation of
rise-time distortion.
Additionally, long cables cause unwanted distortion (a max. length
of 15m is often recommended)
MIDI/computer interface (1)

Computers are often the central controllers of a MIDI system
Therefore there is need for a MIDI interface
In the past, this was often achieved by means of an expansion
slot card, or using the so-called MIDI-joystick port
More recently, external MIDI/USB devices have been used.

These are widely used for multi-port interfaces, which are able to
handle a number of MIDI streams (each controlling up to 16
channels) and distribute separately.
They allow the synchronized handling of several devices (video
recorder, automated mixer, effects, samplers, etc).
MIDI/computer interface (2)

Recent devices are using universal interfaces (USB, firewire) to
transfer MIDI data directly
There is a USB class definition for MIDI devices that wraps MIDI
messages within USB packages.
It virtualizes the concept of MIDI IN/OUT connectors, allowing the

conversion to take place in software.
A device that receives and transmits USB MIDI events is called a
USB MIDI function. It separately process MIDI events from other
data transfers (bulk dumps).
Useful References
Francis Rumsey and Tim McCormick (2002). Sound and Recording: An Introduction, Focal
Press.
Chapter 13: MIDI
Francis Rumsey (1994). MIDI Systems and Control, Focal Press.

Chapter 2: Introduction to MIDI Control
MIDI Manufacturers Association (2002). The complete MIDI 1.0 detailed specification
(www.midi.org)

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Data Communication/MIDI

Main functions: transmission of performance/control data around

The MIDI standard was formulated by agreement between the

While use of the hardware interconnection scheme is in steady

Electronic musical instruments, and the need to control them remotely,

MIDI vs Audio (1)

Sounds are stored and replayed precisely, but we have no access

MIDI vs Audio (2)

Because it comprises control data only, MIDI uses significantly

Data communication (1)

Parallel communication can be extremely fast, but also bulky and

Data communication (2)

MIDI hardware (1)

MIDI was designed to be simple and easy to install in economic

MIDI hardware (2)

MIDI hardware (3)

In asynchronous serial communications the transmitter sends nothing but

One clock period

MIDI hardware (4)

MIDI interconnection (1)

MIDI interconnection (2)

The rise-time refers to the speed of response of the device, if too

MIDI interconnection (3)

MIDI/computer interface (1)

More recently, external MIDI/USB devices have been used.

MIDI/computer interface (2)

It virtualizes the concept of MIDI IN/OUT connectors, allowing the

Francis Rumsey (1994). MIDI Systems and Control, Focal Press.

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