Climb Aboard The MIDI Bus (Part 1)
The amazing musical instrument interface
MIDI is the new word — an important development and the cause of many a tentative discussion. In the first of a two part feature, Jim Betteridge takes a look at why it is, what it does and how it does it. Hear the word...
You only have to witness the horrors of the domestic video format war to appreciate the dilemma faced by companies with innovative ideas. A quick glance at JVC's (who developed the VHS format) bank balance might give an insight into the possible monetary gains involved.
The development of the synthesizer is typical of this phenomenon. With the idea and implementation of voltage control (VCOs, VCFs, VCAs etc) came the inevitable differences of opinion and a number of variant approaches to a similar set of objectives. Thus we are stuck with a mixed bag of incompatible standards, such as different levels and polarities for gate and trigger voltages, and logarithmic as opposed to linear VCO voltage/pitch ratios. The time taken to interface a selection of synthesizers began to dwarf the time spent actually composing with them!
An instrument's inability to communicate with new equipment means potential obsolescence. It has been agreed for some years that a system which would allow comprehensive interfacing between different makes and models would be of great benefit to all manufacturers, and would allow the musician to purchase an instrument knowing that it won't be made redundant as soon as a new wave of equipment is made available.
It was Sequential Circuits Inc (SCI), purveyors of the legendary Prophet series, who started the ball rolling. They first became interested in the idea of using micros for interfacing in connection with a sequencer designed for use with the Prophet 10. The unit was later made available as an option for the Prophet 5, but was considered too clumsy for general use.
The Universal Synthesizer Interface (USI) was the immediate predecessor to the MIDI, and following discussions with the presidents of Oberheim and Roland, the specification was presented by SCI's Dave Smith and Chet Wood at the 1981 Audio Engineering Society Convention.
Having modified the USI spec in accordance with certain points arising from discussions at the convention, Smith sent out a questionnaire to everyone he could think of in the industry who might be concerned with the project. Following a strong response, all those interested were invited to attend a conference held to coincide with the January 1982, Western National Association of Music Merchants (NAMM) convention in Anaheim.
Subsequent to this, a group of Japanese companies who had been conducting their own research along the same lines, presented an alternative proposal, and it was the integration of the two ideas by Smith and Wood, which gave birth to the first MIDI (Musical Instrument Digital Interface) specification.
Although a copy was sent to all NAMM participants, it precipitated no further comment, and so after a number of discussions between SCI and Roland (which is serving as a liaison with Yamaha, Kawai and Korg) the final document was drawn up.
At present, the main source of information relating to MIDI is the SCI document entitled "The Complete SCI MIDI", and with the kind permission of SCI, I shall quote a couple of paragraphs to be found in the introduction:
'The purpose of the specification is to enable the easy integration of synthesizers, other electronic keyboards, sequencers, drum boxes and home computers from various manufacturers, into one programmable system. In being made compatible with foreseeable microcomputer technology, the useful lifetime of the musician's equipment is thereby multiplied. The realisation of complex, electronic assisted music hitherto reserved for well financed professionals becomes more widely available. For example, Synthesizers can be easily configured "in parallel", with instruments played simultaneously or remotely. Entire compositions, consisting of monophonic and polyphonic sequences and rhythm, can be played at one touch. The computer terminal can be used for composing, sequence creation and editing. Graphic quality printers can print the "hardcopy" manuscript of an improvisation or composition. Video synthesis can be integrated with music synthesis. Those parts of musical education requiring drill, eg, learning to read music, scale recognition and ear training, can be automated. This frees the teacher's time to concentrate on technique.'
As the name suggests, the MIDI system is digitally based, and the different components of a MIDI interface system communicate by means of digital codes.
Digital information is sent in the form of binary encoded numbers and is transmitted electrically by representing the binary digits, ie 0s and 1s with 'low' and 'high' voltages respectively. From the term "Binary Digit" has been derived the word "bit", so that a bit is a 0 or a 1 in a binary encoded number. Digital systems can be referred to in terms of the number of bits which they use in each unit of information, eg 4, 8 or 16. The more bits used, the larger the number which can be represented, and therefore the greater the accuracy or resolution of the system.
A group of 8 bits is called a byte (for 8 bit machines), and the MIDI uses a kind of modified 8-bit system wherein the first bit, called a flag bit, simply defines the nature of the information included in the remaining 7 bits: "Status" (1) or "data" (0). If the first bit, referred to as the most significant (MS) bit, denotes "status", then the following 7 status bits will give information defining the specific command being addressed, eg "key velocity". This will then be followed by a new byte with a flag bit denoting "data", wherein the 7 data bits will define the exact extent of the operation, eg the actual key velocity. At either end of each byte is a "start" and a "stop" bit which simply indicates the beginning and end of a byte of information.
Bytes of information come under five main headings within the MIDI format. So having laid the foundation I shall once again quote from the SCI information:
'There are five categories of MIDI data. Channel, System Common, System Real Time, System Exclusive, System Reset. Each category encompasses a number of "status bytes" which define specific commands under that category, and which precede data bytes which specify the exact operation. Status bytes are distinguished from data according to whether the most significant (MS) bit is set (1 = status) or reset (0 = data). The status bytes in each category are defined below. Note that any data sets (eg Note On event data) which are sent successively under the same status, can be sent without a status byte until a different status byte is needed.
Channel information performs most of the routine work. Commands are addressed to specific channels by a 4-bit number which is encoded into the status byte. The associated data bytes can identify keys going down (on) and up (off), their on or off velocities, and pressure or 'after touch' (on keyboards so equipped, such as the Yamaha DX7).
System Common, Real Time and Reset information is intended for all channels in a system. System Common information identifies song selections and measure numbers for all units. Real Time information is used for synchronising everything (perhaps to a master sequencer). Therefore Channel and System Common information is interruptible by System Real Time information.
System exclusive information allows the exchange of data which can be formatted as the manufacturer wishes. Only devices which recognise the manufacturer's format will attend the exchange. Reset simply initializes all equipment to power-on condition.'
The microprocessor in each piece of equipment deals with each byte in "parallel". That is it looks at all 8 bits at the same time through eight different inputs. To send the information in this parallel form would require the use of multi-way connectors and cabling which would add quite considerably to the overall cost of the equipment to which it was fitted. To make this a truly universal interface it had to be inexpensive enough to be fitted to low cost models as well as upmarket equipment, and so a serial format was decided upon.
The data is converted into serial form and the start and stop bits are added, so that the whole set of information can be sent down to a single line to the next piece of equipment where it will be converted back to a parallel format.
The original, pre-MIDI specification had suggested a serial transmission rate of 19.2kBaud (19,200 bits per second) which is the bit rate used by the standard micro computer serial interface "RS232". However, this was criticised for being too slow, in that it could introduce noticeable delays between instruments especially when connected in a complex or extensive configuration. Therefore the rate was upped to 31.25kBaud, which is fast enough for most applications and can easily be derived from dividing a 1MHz crystal by 32.
To allow the specific addressing of individual units or, for example, the individual voices of a polyphonic synth, 16 channels of communication have been provided which can be utilised in different ways depending upon the "mode" selected.
Each unit will include separate transmit and receive ports, generally using standard 5-pin, 180 degree DIN sockets, and these will be marked "MIDI IN" and "MIDI OUT" respectively. In addition the manufacturer may add a third port labelled "MIDI THRU", which will allow a duplicate of the information received at the MIDI IN port to be sent on to another unit in the case of a "chain" interconnection configuration.
A further quote from the SCI information will briefly clarify the situation regarding modes:
"There are three modes of operation for transmitters and receivers: Omni, Poly and Mono. Omni mode is the most general level of operation, interfacing all units. Poly mode allows each unit (synth, sequencer or drum box) to be addressed separately. Mono mode is the most specialised, allowing individual addressing of (for example) each synthesizer voice."
At the time of writing, the industry as a whole is remaining somewhat 'mum' on the subject of the practical implications of the system, although there is no doubt that the potential is great. Further Information, in the form of copies of the complete SCI Information booklet may be obtained from: Sequential Circuits Inc., (Contact Details).
Feature by Jim Betteridge