Many different "transports" can be used for MIDI messages. The speed of the transport determines how much MIDI data can be carried, and how quickly it will be received.
Each transport has its own performance characteristics which might make some difference in specific applications, but in general the transport is the least important part of MIDI, as long as it allows you to connect all the devices you want use!
5-Pin MIDI DIN
Using a 5-pin "DIN" connector, the MIDI DIN transport was developed back in
1983, so it is slow compared to common high-speed digital transports available
today, like USB, FireWire, and Ethernet. But MIDI-DIN is almost always
still used on a lot of MIDI-equipped devices because it adequately handles
communication speed for one device. Also, if you want to connect one MIDI
device to another (without a computer), MIDI cables are usually
USB and FireWire
Computers are most often equipped with USB and possibly FireWire connectors, and these are now
the most common means of connecting MIDI devices to computers (using
appropriate adapters). Adapters can be as simple as a short
cable with USB or FireWire connectors on one end and MIDI DIN
connectors on the other, or as complex as
a 19 inch rack mountable processor with dozens of MIDI and Audio In and Out
ports. The best part is that USB and FireWire are
"plug-and-play" interfaces which means they generally
configure themselves. In most cases, all you need to do is plug in
your USB or FireWire MIDI interface and boot up some MIDI software and
off you go.
With USB technology, devices must connect to a
host (PC), so it is not possible to connect two USB MIDI
devices to each other as it is with two MIDI DIN devices. (This could
change sometime in the future with new versions of USB). USB-MIDI
devices require a "driver" on the PC that knows how the
device sends/receives MIDI messages over USB. Most devices follow a specification
("class") that was defined by the USB-IF; Windows and Mac PCs already come
with "class compliant" drivers for devices that follow the
USB-IF MIDI specification.
Most FireWire MIDI devices also connect directly to a PC with
a host device driver, and the host handles communication between FireWire MIDI devices
even if they use different drivers. But FireWire also supports "peer-to-peer"
connections, so MMA (along with the 1394TA) produced a specification for
transport of MIDI over IEEE-1394 (FireWire), which is available for download on this
site (and also part of the IEC-61883 international standard).
Ethernet & WiFi (LAN)
Many people have multiple MIDI instruments and one or more
computers (or a desktop computer and a mobile device like an iPad),
and would like to connect them all over a local area network (LAN).
However, Ethernet and WiFi LANs do not always guarantee on-time
delivery of MIDI messages, so MMA has been reluctant to endorse LANs
as a recommended alternative to MIDI DIN, USB, and FireWire. That
said, there are many LAN-based solutions for MIDI, the most popular
being the RTP-MIDI specification
which was developed at the IETF
in cooperation with MMA Members and the MMA Technical Standards Board.
In anticipation of increased use of LANs for audio/video in the
future, MMA is also working on recommendations for transporting MIDI
using new solutions like the IEEE-1722
Transport Protocol for Time-Sensitive Streams.
Everything is becoming "mobile", and music creation is no
exception. There are hundreds of music-making software applications
for tablets and smart phones, many of which are equipped with
Bluetooth "LE" (aka "Smart") wireless connections.
Though Bluetooth technology is similar to WiFi in that it can not
always guarantee timely delivery of MIDI data, in some devices
Bluetooth takes less battery power to operate than WiFi, and in most
cases will be less likely to encounter interference from other devices
(because Bluetooth is designed for short distance communication). In
2014 MMA formed a working group to investigate Bluetooth MIDI
performance and consider developing a recommended practice
(specification) for MIDI over Bluetooth.
It used to be that connecting a MIDI device to a computer meant
installing a "sound card" or "MIDI interface" in
order to have a MIDI DIN connector on the computer. Because of space
limitations, most such cards did not have actual 5-Pin DIN
connectors on the card, but provided a special cable with 5-Pin DINs
(In and Out) on one end (often connected to the "joystick
port"). All such cards need "driver" software to make
the MIDI connection work, but there are a few standards that
companies follow, including "MPU-401" and
"SoundBlaster". Even with those standards, however, making
MIDI work could be a major task. Over a number of years the
components of the typical sound card and MIDI interface (including
the joystick port) became standard on the motherboard of most PCs,
but this did not make configuring them any easier.
Serial, Parallel, and Joystick Ports
Before USB and FireWire, personal computers were all generally
equipped with serial, parallel, and (possibly) joystick ports, all
of which have been used for connecting MIDI-equipped instruments
(through special adapters). Though not always faster than MIDI-DIN,
these connectors were already available on computers and that made
them an economical alternative to add-on cards, with the added
benefit that in general they already worked and did not need special
configuration. The High Speed Serial Ports such as the "mini-DIN"
ports available on early Macintosh computers support communication
speeds roughly 20 times faster than MIDI-DIN, making it also
possible for companies to develop and market "multiport"
MIDI interfaces that allowed connecting multiple MIDI-DINs to one
computer. In this manner it became possible to have the computer
address many different MIDI-equipped devices at the same time.
Recent multi-port MIDI interfaces use even faster USB or FireWire
ports to connect to the computer.