MIDI Basics

MIDI Basics

The Musical Instrument Digital Interface (MIDI) protocol provides a standardized and efficient means of conveying musical performance information as electronic data. MIDI information is transmitted in "MIDI messages", which can be thought of as instructions which tell a music synthesizer how to play a piece of music.

The MIDI data stream is a unidirectional asynchronous bit stream at 31.25 Kbits/sec. with 10 bits transmitted per byte (a start bit, 8 data bits, and one stop bit). The MIDI interface on a MIDI instrument will generally include three different MIDI connectors, labeled IN, OUT, and THRU. The MIDI data stream is usually originated by a MIDI controller, such as a musical instrument keyboard, or by a MIDI sequencer. A MIDI controller is a device which is played as an instrument, and it translates the performance into a MIDI data stream in real time (as it is played). A MIDI sequencer is a device which allows MIDI data sequences to be captured, stored, edited, combined, and replayed. The MIDI data output from a MIDI controller or sequencer is transmitted via the devices' MIDI OUT connector.

The recipient of this MIDI data stream is commonly a MIDI sound generator or sound module, which will receive MIDI messages at its MIDI IN connector, and respond to these messages by playing sounds. Figure 1 shows a simple MIDI system, consisting of a MIDI keyboard controller and a MIDI sound module. Note that many MIDI keyboard instruments include both the keyboard controller and the MIDI sound module functions within the same unit. In these units, there is an internal link between the keyboard and the sound module which may be enabled or disabled by setting the "local control" function of the instrument to ON or OFF respectively.

The single physical MIDI Channel is divided into 16 logical channels by the inclusion of a 4 bit Channel number within many of the MIDI messages. A musical instrument keyboard can generally be set to transmit on any one of the sixteen MIDI channels. A MIDI sound source, or sound module, can be set to receive on specific MIDI Channel(s). In the system depicted in Figure 1, the sound module would have to be set to receive the Channel which the keyboard controller is transmitting on in order to play sounds.

Figure 1: A Simple MIDI System

Information received on the MIDI IN connector of a MIDI device is transmitted back out (repeated) at the devices' MIDI THRU connector. Several MIDI sound modules can be daisy-chained by connecting the THRU output of one device to the IN connector of the next device downstream in the chain.

Figure 2 shows a more elaborate MIDI system. In this case, a MIDI keyboard controller is used as an input device to a MIDI sequencer, and there are several sound modules connected to the sequencer's MIDI OUT port. A composer might utilize a system like this to write a piece of music consisting of several different parts, where each part is written for a different instrument. The composer would play the individual parts on the keyboard one at a time, and these individual parts would be captured by the sequencer. The sequencer would then play the parts back together through the sound modules. Each part would be played on a different MIDI Channel, and the sound modules would be set to receive different channels. For example, Sound module number 1 might be set to play the part received on Channel 1 using a piano sound, while module 2 plays the information received on Channel 5 using an acoustic bass sound, and the drum machine plays the percussion part received on MIDI Channel 10.

Figure 2: An Expanded MIDI System

In this example, a different sound module is used to play each part. However, sound modules which are "multitimbral" are capable of playing several different parts simultaneously. A single multitimbral sound module might be configured to receive the piano part on Channel 1, the bass part on Channel 5, and the drum part on Channel 10, and would play all three parts simultaneously.

Figure 3 depicts a PC-based MIDI system. In this system, the PC is equipped with an internal MIDI interface card which sends MIDI data to an external multitimbral MIDI synthesizer module. Application software, such as Multimedia presentation packages, educational software, or games, sends MIDI data to the MIDI interface card in parallel form over the PC bus. The MIDI interface converts this information into serial MIDI data which is sent to the sound module. Since this is a multitimbral module, it can play many different musical parts, such as piano, bass and drums, at the same time. Sophisticated MIDI sequencer software packages are also available for the PC. With this software running on the PC, a user could connect a MIDI keyboard controller to the MIDI IN port of the MIDI interface card, and have the same music composition capabilities discussed in the last two paragraphs.

There are a number of different configurations of PC-based MIDI systems possible. For instance, the MIDI interface and the MIDI sound module might be combined on the PC add-in card. In fact, the Multimedia PC (MPC) Specification requires that all MPC systems include a music synthesizer, and the synthesizer is normally included on the audio adapter card (the "sound card") along with the MIDI interface function. Until recently, most PC sound cards included FM synthesizers with limited capabilities and marginal sound quality. With these systems, an external wavetable synthesizer module might be added to get better sound quality. Recently, more advanced sound cards have been appearing which include high quality wavetable music synthesizers on-board, or as a daughter-card options. With the increasing use of the MIDI protocol in PC applications, this trend is sure to continue.

Figure 3: A PC-Based MIDI System