MIDI (Part 2)
The second part
Following on from last month's introduction to MIDI Jim Betteridge begins by taking a closer look at its various 'Modes'
As MIDI was designed to be fitted to more or less all types of electronic keyboards, sequencers and drum machines, its facilities had to be somehow 'layered' in terms of complexity. For example, if a simple drum machine were to be interfaced with the Prophet T8 or some other keyboard with velocity and pressure sensitive keys and a variety of other advanced features, the drum machine would only be able to deal with a fraction of the information available from the synth and there would inevitably be a high degree of redundancy within the communication.
This is where the 'modes' which were touched upon in the first article come in. There are three modes of operation in which a MIDI-equipped unit can function. In order of increasing sophistication they are: The Omni Mode, the Poly Mode and the Mono Mode. The simplest instruments will be capable of functioning in the Omni mode only, whereas the most sophisticated will be able to operate in all three depending upon the requirements of the other instruments to which it is interfaced. When powered up, each instrument will 'default' to Omni mode. In other words, in the absence of any other external commands, it will automatically select that mode. Here's how the Sequential Circuits (SCI) MIDI specification talks about the system.
"Each unit connected to the MIDI has separate transmit and receive ports. 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.
Normally transmitters will periodically send out a Mode Select command for the most powerful mode to which they can be configured. However, the actual data transmitted will be in the mode to which a second transmitter may have switched the receiver. For example, synth A by default transmits in Omni Mode to Synth B. Synth B, being capable of Poly Mode operation, periodically transmits Omni Mode select codes to Synth C. But the data sent from Synth B to C will be in the Omni format (because Synth B's receiver is constantly getting Omni Mode Select commands from Synth A.) Synth C may or may not respond to the Poly Mode Select commands from Synth B, because if a receiver is capable of operating in the requested mode, it switches to that mode. Otherwise it ignores the Mode Select command. (By the way, the Mode Select commands double as "All Note Off" commands, and therefore can only be sent while all notes are off, or when it is desired to turn all notes off.)
At power up or reset all instruments default to Omni Mode. Regardless of the system configuration, Omni transmitters always send polyphonic data on channel one. Omni receivers respond to On/Off events sent over any channel (1-16). These notes are handled according to the internal assignment scheme of the synthesizer. So this configuration allows any number of polyphonic synthesizers to play in parallel as soon as they are interconnected.
A receiver's mode can only be changed by a Mode Select command transmitted in the channel(s) to which it is currently assigned. If the receiver is not capable of operating in the requested mode, it ignores the Mode Select command. No unit may switch its own modes. Even though a receiver in Omni Mode receives in all channels, it will respond to mode select commands in only one channel: the one to which it is assigned.
Receivers and transmitters without channel selection capabilities are always assigned by default to channel 1.
Omni Mode addresses all units with the same data. Poly Mode allows individual addressing of each unit. In other words the master controller can send separate parts to each synth, whereas in the Omni Mode they all play the same part.
The master controller in the channel network sends all commands, which are encoded with their destination channel number, over one line. This requires each unit to include an address selector switch to define its channel of operation.
The channel definitions having been made, the master controller must issue the command to the receiver on that channel to switch to the Poly Mode. Thereafter, the receiver listens for keyboard data encoded with its channel number. Any number of notes can be sent, to which, again, the polyphonic synth will respond according to its own priorities.
Poly Mode will be useful for sequencing multi-part arrangements of standard synths, for example, which can't be done in Omni Mode.
When a synthesizer has Mono capability, and it receives a Mono Select command it configures itself to receive on the channel it is assigned to and above, up to the number of voices it has. For example the Prophet T8 in Mono Mode will transmit and receive on channels 1-8. (Future synthesizers could contain more elaborate channel selection capability.) Channelling each voice provides fast transfer of individual pressure (also called 'after touch') data for each key. It also makes true legato possible because the note value (= voice pitch) can change without having to first turn the note off (as in the Poly Mode)."
A system of components, ie synths, drum machines, and so on, can be interconnected in either a 'Star' or a 'Chain' configuration.
In the case of a Chain Network the master computer or sequencer in the line sends its commands down the line via the 'MIDI In' and 'MIDI Thru' (excuse the American spelling) sockets on each unit. Remember, the 'Thru' port (socket) gives a replica of the information received at the 'IN' port. Only one other instrument in the chain is able to 'talk back' to the master controller via its 'MIDI Out' port.
With a Star Network the master controller has two way communication with each of the units configured via their In and Out ports.
In each case the master could be a complex sequencer or a micro computer. This latter option is one of the most exciting possibilities existing with MIDI as it brings the considerable power of the already established computer market into play. At present there is a very limited availability of software for such interfacing, and the manufacturers involved are keeping disturbingly quiet about future projects.
SCI have on the market a package which allows the relatively inexpensive Commodore 64 micro computer (plus a datasette or disc for sequence storage) to be interfaced with either the Prophet 600 or the newer Prophet T8 to provide a very flexible sequencer claiming these features:
Synchronisation with most drum machines (Linn, Oberheim, Drumulator, and contrary to the suggestion in our T8 review, the Roland Drumatix and TR-808).
Expanded sequencer memory; up to 6,000 notes.
Up to nine sequences storable, each with several tracks (overdub).
Auto-correction of the sequence played.
Transpose and use of pitch bend and modulation wheels (also storable, which isn't the case with the Prophet T-8's built-in sequencer).
The ability to save your sequences on cassette or disc.
It is also apparently capable, in the case of the T-8, of storing velocity and pressure information from the keyboard, which is obviously of major importance with an instrument of this sophistication.
Apart from this though, there is very little to be had at present. The English offices of all the Japan based companies are being kept largely in the dark, with only the bare minimum of information being given out to allow maintenance of current models. Yamaha in Japan have apparently written a software package which is at present doing the rounds of the computer companies in search of some takers to adapt it for use with specific micros. All they will say is that there is a great deal of interest, but nothing more specific. The future of MIDI is largely reliant upon the software written for it, and the manufacturers involved will obviously be guarding against pirating.
MIDI has its critics, mostly within the ranks of the upmarket competition. New England Digital (Synclavier II) or NED for short, consider its maximum baud rate of 31.25K to be too slow, and suggest that under certain conditions, when using a Chain Network, the time delay between notes played on instruments at either end of the chain might be significant. Their research has shown them that a top, professional rhythm section is likely to be accurate to within 3-4ms either side of the beat, and that any inaccuracies beyond that will have a detrimental effect on the music in terms of feel rather than noticeable delays.
The maximum baud rate for MIDI was originally to the 19.2K to be compatible with the micro computer standard serial interface RS232, with which it is hopefully to be extensively used. It was upped to 31.25K specifically to avoid any problems with delay, and whilst the majority of manufacturers tend to think that it's fast enough and that any further increase in speed would cause problems in interfacing with the RS232, it has yet to be fully put to the test.
NED have got around the problem with their own interface design by going for a parallel format, although this was rejected for the MIDI proposals because of the significantly higher cost of implementation.
CBS/Fender also have a few reservations and are working on their own serial interface, Triad, to supercede the system in operation on the Chroma. It has the same connectors but uses a maximum baud rate twice that of MIDI's at 62.5K, with a microprocessor controlled buffer to allow the RS232 to cope with the increased speed. Also, by means of an asynchronous format, their system uses all eight bits of each word for data, as opposed to the MIDI which, as described last month, uses the most significant bit to discriminate between 'data' and 'status' bits. Another difference is that with the MIDI system, intercommunication between all instruments and the controller (ie a micro) can only be achieved with the Star Formation, in which case the micro will need as many interfaces as there are units: that could be expensive. With Triad, a standard feature is that any unit can talk to any unit, and for instance change the tempo of a sequence.
A very important point is that the Triad theories have not yet been proved. The project is still to a large degree at the drawing board stage, but on the other hand, there is no apparent reason why it shouldn't work. The next few months promise to be very interesting.
Feature by Jim Betteridge
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