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Tutorial: The Technology of MIDI

Part 2: MIDI Cables & Connectors

Overview
MIDI Messages
Standard MIDI Files

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 most 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 needed.

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.

USB and FireWire

All recent computers are 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 on one end and MIDI DIN on the other, or as complex as a 19 inch rack mountable CPU 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. 

Current USB technology generally only supports communication between a host (PC) and a device, so it is not possible to connect two USB devices to each other as it is with two MIDI DIN devices. (This may change sometime in the future with new versions of USB). 

Since USB MIDI devices only communicate through the PC, two devices can use different schemes for packing up MIDI messages and sending them over USB, and each device's driver on the host will know how to unpack those message and convert them back to MIDI messages. That way all USB MIDI devices can talk to each other (through the host) even if they do not all use the same translation format. However, most devices actually follow the format specification for MIDI 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 so can talk to other FireWire MIDI devices even if they use different methods for formatting their MIDI data. But FireWire itself also supports "peer-to-peer" connections, so MMA (with the 1394TA) produced a specification for transport of MIDI over IEEE-1394 (FireWire), which is available for download on this site (and incorporated in IEC-61883). 

Ethernet

If you are connecting a number of MIDI instruments to one or more computers, using Ethernet seems like a great solution. In the MIDI industry there is not yet agreement on the market desire for MIDI over Ethernet, nor on the net value of the benefits vs. challenges of using Ethernet, and so MMA has not yet adopted a standard for MIDI over Ethernet. 

However, other Standard Setting Organizations have specifications for MIDI Over Ethernet, and we think it appropriate that people know about those solutions. There are also proprietary solutions for MIDI Over Ethernet, but because they are not open standards they are not appropriate for discussion by MMA.

  • IETF RTP-MIDI

The IETF RTP Payload Format for MIDI was developed with significant input from MMA, and is the foundation of Apple's "Network MIDI" implementation of MIDI over Ethernet. Though not officially adopted or endorsed by MMA, the RTP-MIDI approach is acceptable to MMA members and is already being used in hardware and/or software products. More information about RTP-MIDI can be found at www.rtp-midi.org.

  • IEEE Ethernet AVB

For the past several years, the IEEE has been developing a suite of protocols for low-latency audio and video transport over Ethernet with high quality of service, collectively known as Audio/Video Bridging, or AVB. The effort is part of the larger IEEE 802.1 Working Group, which develops networking standards that enable interoperability of such ubiquitous devices as Ethernet switches. The AVB protocols provide precision time synchronization and stream bandwidth reservation at the network level. (However, the AVB protocols do not provide a standard means for interoperable communication of content such as a live video stream.)

The IEEE-1722 AVB Transport Protocol (AVBTP) standard uses the AVB protocols and provides content encapsulation in an evolutionary manner by adopting the existing IEEE 1394 (Firewire) audio and video streaming mechanisms already in use by millions of A/V devices. However, the AVBTP specification also supports other media formats (Payload Types), so the MMA has collaborated with IEEE to define a separate Payload Type identifier for MIDI (and possibly future content formats defined by the MMA). The P1722 standard references an MMA specification that is not yet published, but will be completed in the near future.

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