MIDI Mode 4 is used to control machines that produce sound in a fairly obvious way. But what are the benefits of applying it to a device that merely treats sound, like Yamaha's SPX90 multieffects processor?

THE MAIN USE we have seen so far for having instruments which are capable of "looking at" more than one MIDI channel at once, is to receive note information which is to be applied to different timbres or samples. We've not really concerned ourselves with program changes (though we did note in passing that the Casio CZs could alter program numbers separately for each voice if the program change came in on the appropriate channel).

With the Yamaha SPX90 that's under scrutiny this month, the situation is reversed. For while it can usefully interpret note information to specify transposition intervals (when in Sample or Pitch Change modes), the main use of its MIDI capability is to enable it to receive program change numbers, to change the signal-processing role it is performing. In this it is basically similar to most other MIDI effects units, though more versatile than many.

Originally, MIDI signal processors merely changed the number of the currently selected memory according to the incoming MIDI patch-change number. As musicians and engineers soon discovered, this was not always that useful. Just because you wanted patch 23 on your synth, didn't necessarily mean you wanted the effect set up on memory 23 of your DDL. You could always juggle patches around on either your synth or your effects unit (or both), but this was time-consuming and still not entirely satisfactory.

Manufacturers soon realised this, and the second generation of MIDI-equipped signal processors came with the ability to get round this problem. This was done by having an internal memory number assignable to each MIDI program change number. So, for example, you could make your DDL or reverb switch to program number 20 every time it received MIDI program change number 47.

This system has several advantages. First, you can make the effects unit switch to program 20 when it receives MIDI program change 3, 58, or 127. In other words, you can use that favourite flanging effect on several different synth patches. Second, if you have fewer than 128 memories on your effects unit (and few devices offer more than that), you can still usefully receive all 128 MIDI program changes (which many synths do send) to select from your more restricted pool of effects programs.

But there's still a problem. Suppose you are using several MIDI instruments, but only one MIDI effects unit. Via this unit, you are processing signals from different instruments at different times. What happens when you want to use program 22 on two different synths, with a different effect on each? Whichever memory you set to be selected when program 22 is received will automatically be called up when you select program 22 on either synth - assuming that the effects unit is connected to the output of both synths via a MIDI mixer.

The only alternative seems to be replugging the MIDI cables between using one synth and the other - unless you have an SPX90. Because the SPX can look at several MIDI channels at once, the same program change number can be interpreted differently, depending on which MIDI channel it's sent along.

How is this configured on the Yamaha? Well, the machine actually has four banks of program change assignment. Each bank works exactly as described above, assigning one of the 90 programs available on the SPX to each of the 128 program change commands which MIDI provides. So, each of these four banks has the flexibility to assign an SPX90 effect to every program on a certain MIDI instrument.

But each of the banks can be set to look at a different MIDI channel. So, you could set each bank to match the MIDI channel of a particular synth, allowing the SPX90 to call up a different set of program change assignments depending on which (of up to four) instrument's MIDI channel the instruction is received along.

There are three possible ways of configuring this system to get the best out of its MIDI flexibility, and each requires a particular piece of MIDI equipment to make it viable. The first (and cheapest) option uses a MIDI merger, the second a MIDI master keyboard, and the third a MIDI sequencer.

The following examples all show MIDI synthesisers, but any MIDI devices capable of sending and receiving program changes can be used.

In all these applications, it is assumed that synth A is set to MIDI channel 1, synth B to channel 2, synth C to channel 3, and synth D to channel 4. Similarly, the four banks of the SPX90 would be set so that A is looking at MIDI channel 1, bank B at 2, C at 3, and D at 4.

To set this up on the SPX90, start by hitting the Utility switch twice. This will get you to the MIDI control page. Here you'll see the Bank letter (A, B, C or D) on the left and the MIDI channel number (1-16) on the right. Use the left-hand up/down switches to change the bank to A, then the right-hand up/down switches to change the MIDI channel to 1. Then step up to bank B and set this to channel 2. Repeat this for bank C (channel 3) and bank D (channel 4).

The channel numbers you actually use are not critical, provided each bank is set to look at the same MIDI channel as the synth it is meant to work with. In other words, synth A must be set to the same channel as bank A on the SPX90, and so on.

Now you can move on to setting up the assignment of MIDI program number to SPX90 memory, by hitting Utility when bank A is showing (select this with up/down switches if necessary). This takes you to the MIDI program page. Here you use the left-hand up/down switches to select each MIDI program number (shown on the left of the display), then use the right-hand up/down switches to choose the corresponding SPX90 memory. Repeat this procedure for each program change you need to allocate for bank A.

To move onto allocating bank B, go back to the MIDI control screen and step up to bank B. Then hit Utility again and you'll be able to set the corresponding SPX90 programs for this bank. Repeat this procedure for the last two banks.

In Application 1, the four MIDI Outs from each synth are mixed together via a MIDI merger such as that made by US company JL Cooper. Whenever a program change is made on any of the synths, the SPX90 will know from the MIDI channel which synth the instruction came from, and will select the appropriate effect you've assigned to that program change number in that particular bank.

In Application 2, one master keyboard is used to control all the program changing and MIDI channel switching. If you send a patch change on MIDI channel 3, not only will synth C change to program 38, but the SPX90 will automatically set up the effect you have matched to that program.

Application 3 is basically an expansion of the second, using a MIDI sequencer to record whatever you play on the master keyboard. Note that to hear what you're playing, you'll need to set the MIDI Out/Thru to Mix mode on your sequencer. If you don't have this function, you'll need to reconfigure your system each time you switch between record and playback. Avoid this if you can: it's not a lot of fun.

When you start overdubbing on your sequencer, you won't be able to use two different effects processes at the same time. But you can still "timeshare" the SPX90 in the most efficient way, affecting one synth in the verse, another in the chorus, a third in the middle eight, or whatever. And the effects programs will be switched at the same time as the patches are being changed.

But, I hear the sceptical cry, why is this any better than sequencing the SPX90 on its own MIDI channel? Well this way, you're saving both a MIDI channel and a track (or overdub) on your sequencer, which can be used for other things. This may seem like an unnecessary economy, but when you start using multi-timbral synths and samplers in Mono Mode at the same time (as we've been doing in previous features in this series), 16 MIDI channels suddenly don't get you very far.

Remember: every MIDI channel you save is an extra sound you can sequence.

Note - this series started in "Electronics & Music Maker" magazine - click here to go the previous part