But under what circumstances is it needed? Let's take an example where a sequencer is used to control both a keyboard synth and a drum machine, with the latter providing the MIDI clock timing data necessary to keep it in sync with the sequencer. It has to keep together all the bytes that comprise each message and it has to ensure that the timing for each message is such that they cannot collide. If we want to merge data from two or more MIDI outputs, it has to be through a purpose-made device - the MIDI merge box.ĪS WE'VE ALREADY seen, a MIDI merge device has two principal tasks. This means that should any of the three bytes which comprise a Note On message (Note On status, MIDI channel, note number and velocity) be lost, the message would be incomplete and would be unintelligible to the receiving device.Ĭlearly then, MIDI data needs to be handled with some care though vast amounts of information may be transmitted down each cable, these cannot be simply joined together in an effort to mix information from separate sources. To further complicate matters, the bytes for each message are, in effect, mutually dependent and cannot normally be separated. Sending the entire set of program parameters for all the sounds in a synth, on the other hand, could involve some 64,000 bytes or more. Different types of messages are made up of differing numbers of bytes: Note On and Off, for example, need three bytes each to convey all of the necessary information, while a program change requires only two bytes. Each message consists of a series of MIDI bytes which, essentially, are made up of strings of numbers. Pressing a key, for example, sends a Note On message, while releasing it transmits one for Note Off. THE "PACKAGES" OF MIDI data sent down each cable are referred to as messages. And just as in our example the tubes must be kept separate if the balls are to emerge in the correct order, so each MIDI line must be kept independent if data is to be interpreted correctly by the receiving device. Here, data transmitted down MIDI cables takes the place of the coloured balls rolling through tubes whilst the "push" provided by the compressed air is generated electronically by what is known as a current loop. ![]() ![]() The flow of MIDI information follows broadly similar principles. If, however, they are joined so that they share a common outlet, the balls will emerge mixed together, making it impossible to determine the order in which they entered, or even which ball came from which tube. With the tubes separated, the balls will obviously emerge in the same order as they were sent in. THERE CAN BE few people taking their first tentative steps into the world of MIDI who haven't wondered at some time or other "Why can't I simply splice together two MIDI leads when I want to use two different keyboards to play a single synth?" With a MIDI Out socket on each of the keyboards but only a single MIDI In on the synth, this, on the surface, wouldn't appear to be too unreasonable a request.īut of course, life isn't ever that simple.Ĭonsider the example of two tubes each with different coloured balls being pushed through them by means of compressed air.
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