Using MIDI

MIDI lets you transfer musical information between a sequencer, musical instruments and other devices. You don’t need to understand any technical details to use it, although most computers also need a special MIDI adaptor box, sometimes known as a MIDI interface.

• Modern MIDI adaptors connect to a spare USB port on your computer, although older devices employ one or two serial port connections, as found on earlier machines.
• A special kind of USB to serial adaptor, such as the Griffin gPort or g4Port, can be used to connect an elderly serial-based MIDI adaptor to a modern USB-equipped computer. Hence that old Mark of the Unicorn Midi Time Piece can be connected to your new machine and, with suitable software, can even be used with CoreMIDI-based applications in Mac OS X.

A MIDI sequencer records how you performed on a musical keyboard. Alternatively, you can employ manual editing to create an entirely new song or to modify an existing sequence. And with suitable hardware you can synchronise the results to a video recording by means of timecode.

• A sequencer can be a stand-alone device, although most modern sequencers consist of sequencing software, which is installed on a computer that has a MIDI adaptor.
• A sequence created in a sequencing application is frequently stored as a Standard MIDI File (SMF). However, this kind of file doesn’t include any of the sampled sounds that may be used in the sequence. So, if you want to transfer information containing both audio and MIDI data, you must use a different kind of file, such as the OMF Interchange (OMFI) format.

MIDI Connection Systems

The diagram below shows a simple MIDI system using a basic adaptor:-

As you can see, the output of the MIDI keyboard is connected to the input of the adaptor whilst instruments, effects devices, or MIDI-controlled audio mixers are connected to its output.

You can create a daisy-chain on the output, connecting up to three devices via their MIDI Thru sockets. This means that if your adaptor has three outputs you can use a total of nine devices.

The following diagram shows a more sophisticated system in which a MIDI Thru box has been added to expand the number of outputs available from the adaptor. In this example a MIDI merger has also been incorporated. This combines MIDI Timecode (MTC) information with the output from the keyboard. It also allows editing of material within a device (usually in the form of sound samples), providing that both the MIDI inputs and outputs of the device are connected to the system.

SMPTE timecode can be extracted as an audio signal from a recording track on a video cassette recorder (VCR) or audio tape machine. This form of timecode, known as longitudinal timecode (LTC), is converted into MTC by means of an LTC to MIDI converter.

• If you can’t afford a MIDI merger you can use a MIDI switch box, although this isn’t as convenient, since you’ll need to move the switch whenever you go in or out of editing.

Finally, here’s an example of a system using an intelligent adaptor:-

As you can see, this particular adaptor also generates and reads LTC. In addition, the system supports timecode stored as vertical interval timecode (VITC) within the actual video signal of the VCR. This is converted to standard LTC by means of a VITC to LTC converter.

Using a MIDI Keyboard

Often one instrument or a dedicated keyboard-only device is employed as a master keyboard. Clearly, you’ll want to hear a sequence as it’s played on the keyboard and recorded into the sequencer. Unfortunately, the keyboard is usually on a different MIDI channel to the instrument. Most sequencers overcome this problem by replacing the channel number of the keyboard by that of the instrument, a process known as auto-channelising or echo. Of course, if you’re using the keyboard as a synthesiser the sequencer must also ‘echo’ data back to the keyboard via its own channel.

Most sequencing applications let you record and echo to any channel, irrespective of the keyboard’s channel. If all synthesisers are in Omni off mode (see below) and are using unique voice channels, you can select and play any sound from the keyboard by selecting an appropriate echo channel.

• If your master keyboard is also a synthesiser, and you want to use the synth itself, you must turn Local Control off (see below). Any notes you play will then go to its MIDI output but not to its sound generators. However, data arriving at its MIDI input will produce sounds.

Introduction to MIDI Messages

Standard MIDI messages can be divided in the following groups:-

Voice Messages

These include the most common type of message, such as Note On or Note Off. Each instruction of this kind refers to a MIDI channel, identified by a channel number which is normally assigned to a specific voice in each musical device. Any track that’s created in a sequence can be used to play the required voice by sending a message to the appropriate MIDI channel.

Voice Modes

Each musical instrument can be set up to respond in one of several modes:-

Controls

Control Change messages are generated from switches, foot pedals, or other controllers that are provided on some MIDI devices. This data can then be used to adjust other devices whilst playing a sequence. Typically, such controls have 14-bit resolution, stretching the capacity of MIDI to the limit. However, the transmitted data can be thinned, usually with little audible effect, or the receiving device can be programmed to do the job itself on receipt of a simple MIDI command.

System Exclusive Messages

These messages, also known as Sysex messages, often contain manufacturer’s proprietary codes for controlling or editing data in their products. Although details for System Exclusive codes are usually supplied, you’ll often encounter special software that obviates any need to understand them.

Synchronisation

Several different types of message are used for synchronising other devices to a sequencer. Initially, the sequencer should send a Song Select message to indicate which song it’s about to play. A Start message should then be sent to the sequencer to initiate playback of the song.

Once it’s running, the sequencer sends regular Timing Clock messages at a rate of 96 clocks per whole note (semi-breve). By using these clocks to modify a ‘song position pointer’ register, each device knows how far the sequence has progressed. To return to a particular point in the sequence the sequencer must send a Song Position Pointer message to all devices.

In addition to the above messages, it’s also possible to convey timecode, the actual time of day of a recording in hours, minutes, seconds and frames. These messages allow other devices such as video and audio equipment to be synchronised to a sequencer. Two systems are used:-

MIDI Channel Numbers

MIDI requires every voice or instrument in a device to be assigned a MIDI channel number. However, most drum machines use only one channel for all of the sounds in the device, which is a useful feature when you’re short of MIDI channels. In this instance, each sound is assigned to a different note: this is possible because percussion sounds aren’t normally played at a particular pitch.

Traditionally, any instrument can be assigned to any number, sometimes causing unsuitable sounds to be heard in a sequence. However, devices designed to the General MIDI (GM) convention use a standardised program numbers for common voices. Such GM instruments, containing 128 standard GM sounds, can play sequences without the user having to worry about assignments.

Further extensions to the GM standard include GS, devised by Roland, and XG, developed by Yamaha. Both overcome the limit of 128 sounds provided by GM by using alternative sounds known as variations, as well as special effects. Note that although GS or XG sequences can be played on ‘standard’ MIDI instruments, none of the special GS or XG content will be reproduced.

MIDI and Timecode

Many MIDI devices and sequencers can be synchronised to timecode. Timecode indicates the time of day of the original sound or video recording or its duration in hours, minutes, seconds and picture frames in the form hh:mm:ss:ff. The standard is defined by the SMPTE, although the number of frames per second is set by your television standard, usually NTSC or PAL.

• Setting a starting point of 00:00:00:00 can upset some sequencers. If this happens, try a non-zero starting point, such as 10:00:00:00. Fortunately, timecode recorded as time of day (rather than programme duration) usually begins with a non-zero value.
• Timecode can include user bits that convey extra data in the timecode, such as a portion of ASCII text describing picture content. Unfortunately, these bits aren’t standardised and special equipment is usually required to provide them within an LTC signal.

Timecode can come in various forms, each suited to a particular purpose. When conveyed over a MIDI circuit it’s usually in the guise of MIDI Timecode (MTC), Direct Time Lock (DTL) or Direct Time Lock enhanced (DTLe), as described above. However, it can also be transported in the form of an analogue audio signal, can be embedded in the data carried over a digital audio link or incorporated within a related video signal. These types of timecode are described below.

MIDI in Control

Sometimes, you’ll encounter a non-MIDI device that can be controlled via MIDI. Such a device can be electrically compatible with MIDI, allowing a connection to be made via a simple adaptor cable. In other instances, a special interface box may be required to provide the connection.

• Although a device’s hardware can be made suitable for a MIDI connection, its control codes can still be incompatible with your MIDI adaptor or software.

One elderly device, the Akai DP3200, an audio router with 32 inputs and outputs, can be controlled using MIDI instructions, even though it wasn’t originally marketed as a MIDI device. Its control input appears on a 4-pole XLR socket, although this connection is electrically compatible with MIDI. A suitable adaptor cable should be wired as shown below:-

If you want to loop the data onto another router you can use a cable wired as follows:-

©Ray White 2004.