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Yamaha MEP4 Processor

Simon Trask somehow finds time to examine the most sophisticated MIDI event processor of the lot. Are its facilities usable?

Yamaha's MEP4 is the most comprehensive MIDI processor yet devised. Its black-box exterior conceals a wealth of facilities to manipulate MIDI data, but are they sufficiently easy to use to be worthwhile?

Now that expanders and mini-keyboard synths are entering into common usage, the role of a master instrument is becoming increasingly important, and increasingly demanding. Many synths are simply not well suited to fulfilling this role, and in a sense they shouldn't be expected to.

A dedicated controller keyboard is one way of solving this problem, a new-generation electronic piano is another, while a third is to interpose a MIDI 'black box' between your main instrument and the rest.

The last option is where Yamaha's new MEP4 comes in. There are similarities of front-panel operation and styling (including a 16-character backlit LCD, thank goodness) between this and Yamaha's other new signal-processor, the SPX90 (reviewed in last month's E&MM).

But that's about as far as those similarities go, because apart from the obvious difference that one processes audio data while the other manipulates MIDI data, the MEP4 is conceptually a far harder nut to crack than a unit like the SPX. And the 30 factory preset effects that Yamaha have thoughtfully included on the MEP4 (in a 60-voice RAM) may cause confusion to many, simply because the unit's effects demand that your MIDI instrument setup is configured appropriately, and that all your instruments are capable of transmitting and receiving the controller codes that some of these presets make use of.

When you disentangle the MEP4 from its packaging, you discover an example book with program charts and system setups for all 30 presets, which is invaluable - though it doesn't suddenly make the MEP4 a breeze to use.

Fortunately, you don't have to plunge in at the deep end with the MEP4. At its most basic level, the machine acts as nothing more sophisticated than a simple five-way MIDI Thru box. This is achieved by pressing the Bypass button on the unit's front panel, thus sending the data on MIDI In straight to the four MIDI Outs on the back panel as well as to the Thru. So the MEP4 can start working for you as soon as you get it out of its box, which is no more than you should expect, after all.

Pretty soon, however, you'll be wondering why you paid £345 for a simple Thru box, and that's the time to start finding out what the MEP4 can really do for you.

Figure 1

Figure 1 shows the basic layout of the MEP4. Four independent processors receive data from the single MIDI In, and can send the results of their own operations to any one of four MIDI Outs; the Thru fulfils its normal role of passing on incoming data unchanged, which means you could, for instance, chain two MEP4s together. As well as directing the output of each processor to a separate Out, you can route up to four processors to one Out, allowing the creation of composite effects.

What you can't do is send the output of a particular processor to more than one MIDI Out, so if you want to layer an effect, you have to double the effect on two processors or daisy-chain your synths.

Figure 2

But what are these enigmatic processors? Figure 2 illustrates the components that go to make up each processor: remember there are four of these running in parallel, each with its own settings of the same parameters. Each processor can be switched on and off from dedicated front-panel buttons, which is a useful way of removing a particular instrument from the action, or of isolating part of a composite effect.

Starting at the left-hand end of the diagram, the Channel Filter allows you to assign a processor to any number of MIDI channels, from one up to the full 16. At the other end of the chain, the Output Assigner allows the result of the processor's actions to be sent on any single MIDI channel, and/or offset from its current channel number by a constant amount, and then sent to any one of the four MIDI Outs.

It's worth bearing in mind that the MEP transmits its processed MIDI data on a maximum of four MIDI channels; should you require further channel capacity (which you may well do if you're using multi-timbral synths or samplers), you'll need a second MEP4 connected via the Thru.

The Data Presetter allows you to select a patch number, pitch-bend value and two controller-code values, which are all sent on the selected MIDI channel whenever a new MEP4 program is called up. Thus, you can trigger up to four different patch numbers, on any channels, for every incoming patch number. This is the same facility offered by Dynacord's MCC1, but with the added advantage that MEP4 memories don't have to be aligned with specific patches - more on this later.

The pitch-wheel setting is perhaps most useful for zeroing the effect of the wheel, while Yamaha's examples commonly use the two controller codes for zeroing the modulation wheel and setting the volume pedal to maximum. Resetting controllers is a useful task, and it's a pity Yamaha haven't allowed for a few more of these codes to be transmitted.

However, what is useful is that the MEP4 transmits sustain pedal-off messages together with note-off messages for all notes currently active, in a healthy variety of circumstances including whenever a new program is selected or a processor is turned on or off.

All of which should ensure you don't get droning notes at any time when using the MEP4 - a reassuring thought.

The Message Filter allows you to filter out any of nine categories of MIDI message from the data stream. These are: note-off, note-on, polyphonic aftertouch, control changes, program changes, channel aftertouch, pitch-bend, channel mode messages and system messages. Aside from choosing which data goes to which slave instruments during performance, you can also use this section as a means of removing unwanted data (say, pitch-bend data) from a sequencer track.

The Delay Processor allows you to delay transmission of all MIDI data passing through it for up to three seconds, selectable in millisecond increments. Like Akai's ME10D MIDI Digital Delay (reviewed E&MM November '85), this is only a single delay. But unlike the Akai, the MEP4 allows you to group all four processors together for a single four-note delay - still within the three-second limit. This makes the delay quite flexible, but the MEP4's routing limitations mean you can't send the composite effect to more than one MIDI Out. Pity.

I've left till last what is probably the most flexible and currently unique aspect of the MEP4: the Data Modifier. Unfortunately, this is also the section in which you come most intimately into contact with actual MIDI data, so you really need to know about MIDI if you're not to get lost.

With the Data Modifier, you can alter any MIDI messages (one per processor) apart from channel mode and system messages, in up to four ways. 'Altering' can mean anything from changing a particular value to changing the entire message. And you can convert messages which have specific data values, so that one particular note, for instance, can be used to trigger any result the MEP4 is capable of creating.

There are six ways in which you can modify a message (and remember you can combine them in up to four steps): Expand, Step, Offset, Reverse, Limit and Convert. The first five of these modify data bytes only; Convert is the one that allows you to change a message completely.

"Presentation: When you disentangle the MEP4 from its packaging, you discover program charts and system setups for all 30 presets — which are invaluable"

Expand allows you to multiply a data value by any value from 1/16 to 16. This could be a useful way of, say, altering the velocity range of notes on a particular channel.

Step allows you to filter particular messages whose data byte isn't a multiple of a specified value, which could come in handy for cutting down on data generated by continuous controllers.

Offset allows you to add a fixed value (-127 to +127) to any data byte. The most obvious use for this is to create parallel octave, fifth or fourth effects - though any interval is possible.

Reverse allows you to reverse MIDI data for any specified message around a selected fixed middle-point. You could use this, for instance, to crossfade between two instruments (which would require more than one processor), or to create reverse keyboard effects á la Zawinul Prophet 5.

Limit is where you get to set up four-way splits. For each processor you can define an upper and a lower limit, which gives you the sort of flexibility otherwise only found on Oberheim machinery. It's worth bearing in mind that such a composite effect doesn't allow you to create any other effects, as each processor's Data Modifier can only process one MIDI message at a time.

Convert, as mentioned earlier, allows you to change one message into another. For instance, channel aftertouch can become volume or pitch-bend data; a sustain pedal-on message can become a note-on instruction to trigger a bass drum on a MIDI drum machine; data ± keys can be used to trigger keyboard octave-shifts (by sending minimum or maximum pitch-bend values); and a specific note-on or velocity value can be used to trigger any effect - options which introduce an interesting pseudo-random procedure.

It's a pity, though, that you can't convert to System messages such as start, stop and continue or song-select, for remote control of sequences from any MIDI code.

Being able to specify up to four operations (a Convert function must always come last) allows for even more sophisticated possibilities, while you can always send the same data to two processors, treat it differently, and send the results out of just a single MIDI Out.

I'll repeat the warning, though: you really do need to know your MIDI codes (in hexadecimal, too) if you're not to get hopelessly lost in this area of the MEP4. Even then, it can require some effort to think through exactly what's happening, or what you might be able to do.

But what's so sensible about the MEP4 is that you can use it in a modular fashion, utilising all its other capabilities and only approaching the Data Modifier section when you feel ready for it.

Maybe Yamaha should have included a quick way of switching each section in and out for the currently-selected processor. That would have allowed you to isolate particular sections while trying to find out why that reverse keyboard effect with multiple delays is causing your keyboards to give a convincing interpretation of Mount Vesuvius erupting.

One facility which may help you diagnose such drastic errors is the MIDI monitor, which allows you to see the output of any processor and any of the four MIDI Outs. As the user manual honestly points out, it's feasible that you could overload the MIDI bus by sending multiple continuous controller data over a single MIDI Out - in which case the MEP4 will dutifully cry on your shoulder and inform you of its problems.

All the above settings are storable in 60 programs, each of which can be given its own nine-character name. Given the variety of uses you can put the MEP4 to, I wonder if even this many will be enough.

You can assign program-selection to any one of the 16 MIDI channels, which is obviously necessary when running the MEP4 from a sequencer or an instrument with multi-channel sending of data. You can also dedicate a particular track on a sequencer to selecting MEP4 programs, thus making patch-changing independent of any particular instrument part.

For maximum flexibility, any MEP4 program can be assigned to any incoming MIDI program number from 1-128, with settings being stored in a program assignment table. There's also a footswitch input for stepping through programs - as they are either in memory or in the program assignment table.

Yamaha have given the MEP4 the ability to send and receive parameter data for all 60 programs over MIDI, which is obviously useful for transferring data between two MEP4s. The other possibility lies in storing the MEP4's programs via computer, but that of course needs someone to come up with the right software for the right computer (ie. your one). Still, Yamaha have provided a clearly laid-out data chart on which you can inscribe your most profound data-manipulation thoughts.

All in all, the MEP4 gives you a MIDI Thru junction box, a multiple patch-selector box, a fine set of sound layering and splitting abilities, the most comprehensive MIDI delay line currently available, a MIDI data filter, channeliser and re-router, and a MIDI data manipulator that gives you so many options, it'll be a long time before you work through all its practical applications.

At the same time, the unit's ability to create seemingly unpredictable (though ultimately understandable) effects is another of its attractions - though maybe you need to be a bit perverse to appreciate that.

Front-panel operation of the MEP4 is extremely straightforward (helped by the informative display), and allows you to find your way around all the parameters with the minimum of fuss. The only annoyance is the Yamaha's inability to send each processor's output to any, rather than one, of its MIDI Outs.

If you're prepared to make the not inconsiderable effort to become conversant with the MEP4, you will not be disappointed. It's the most flexible and powerful MIDI signal processor currently available, and as such, will allow you to manipulate your keyboard setup in ways you'd never thought possible. Who said technology gets in the way of creativity?

Price RRP £345 including VAT

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jun 1986

Scanned by: Stewart Lawler

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Review by Simon Trask

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