There seems no end to the ingenuity of Roland's MIDI accessory department. If there's something your current synth system can't achieve, chances are they now have a box that'll let you do it.

Given the inherent flexibility and open-ended nature of MIDI (or as cynics might say, its lack of a rigorous, consistent specification), it's not surprising that a whole new industry is fast taking shape to cope with the wide variety of MIDI problems that need solving. These four Roland units under review are part of this trend, with three of them providing purely MIDI processing and the fourth allowing the CV/Gate and MIDI worlds to talk to each other - in one direction, at least.

First of all, let's clear up any confusion over the designation of these units. Originally intended to be part of the Boss range of products and labelled - in prototype form - as MI10, MI30, MI40 and MI50, they've now been added to Roland's own MPU (MIDI Processing Unit) range, which up till now has consisted only of the low-profile (on this side of the Atlantic, anyway) MPU401 intelligent MIDI interface unit. The new designations are MPU101, MPU103, MPU104 and MPU105.

Quite a number of people have already confused the MPUs with the new Boss Micro Rack signal processors, which provide more traditional studio fare in the same compact casings as MPU103-105, but are not MIDI-compatible. The fact that both types of processor will fit the same 9.5" Micro Rack (or, two-by-two, Roland's own 19" 1U Rack Mount Adaptor) has probably contributed to the confusion.

Anyway, all four units are further evidence that more than any other synth manufacturer, Roland are keen to develop peripheral devices to run alongside the instruments themselves. They've led the way in interfacing and processing boxes since MIDI arrived - probably before.

Let's take the simplest unit first. The MPU 104 MIDI Input Selector allows you to select between any one of five inputs (synth, controller keyboard, sequencer or drum machine) to act as a master unit. Another application would allow you to have up to five instruments (which could include expanders) connected to a computer with patch- and sequence-dump software.

Technically, a MIDI receiver should not be driven by more than one transmitter at any one time, so quite correctly, it's only possible to select one MIDI In at a time on the MPU 104. You can press more than one input selector at once, but the unit always defaults to the lower number. It's also possible to switch out all the selectors, so you can use the MPU 104 as a MIDI master on-off switch - useful when you suddenly want to play your master keyboard by itself.

The MPU105 MIDI Output Selector is, not surprisingly, similar in design to the 104. As it's outputs you're selecting with the 105's buttons, you can of course have all five selected simultaneously. In fact, this new machine is to all intents and purposes an upgraded version of Roland's earlier MM4 MIDI Thru Box, adding one more Thru and the ability to select which Thrus are active over and above its forerunner's specification. Thus you can switch in or out any combination of connected MIDI instruments - at the touch of a button or two.

Taken separately, the 104 and 105 are useful enough, but taken together they would allow you, for instance, to have a flexible five-instrument-plus-sequencer MIDI system, with any instrument able to be switched in as the recording instrument, and all for a fairly modest outlay. Alternatively, you could set things up to allow any one of five MIDI instruments to control up to another four, with the exact configuration alterable almost as quickly as it takes to figure out what's going where in the first place.

What these units aren't (and this also applies to the other two MPUs reviewed here) is 'intelligent'. Or to put it another way, they can't stop you from altering their configuration whilst music is actually being played over MIDI. This could actually be a bit more serious than it sounds: imagine a situation where you switch out an instrument, or switch to a new master keyboard, before releasing all the notes on your current master keyboard. The result would be the dreaded MIDI drone, as the wrong sound is left hanging on the other end of the line.

Sycologic have succeeded in getting round this problem with their MI4 MIDI matrix unit, and so have Yamaha on their new DX21 polysynth, which won't allow you to change MIDI channels till all MIDI note-on commands have been balanced by note-off ones.

Now to more complex connections. Whereas the above two processing units simply distribute incoming data without affecting it in any way, the MPU103 MIDI Channel Filter/Converter allows you to do rather more in the way of MIDI signal customising. You can switch it to read data only from a selected channel, to change channel numbers to a specified number, and to filter out the odd section of channel data, such as pitchbend info.

The 103's front panel looks similar to the 104's and 105's, but has the addition of two calibrated knobs for Filter and Converter MIDI channel selection (any one of channels 1-16 in each case). As with the Output Selector (but not, for some reason, the Input Selector), a small green LED flashes on whenever MIDI data is passing through the unit - a neat, informative touch that Roland first used on their MKS series of voice expanders.

The third processing option available on the 103 is Key Event Only, accessed via a simple on/off button. When the option is activated, the only data allowed to reach MIDI Out is note-on and note-off information, which allows for attack and release velocity values but not aftertouch - slightly self-defeating, if you ask me. What you get, then, is the ability to filter out pitchbend and patch changes, for instance, but not the ability to choose one without the other. And virtually everything else goes overboard as well, which seems a bit inflexible.

When Key Event Only is off, all System (ie. non-channel) messages other than active sensing are transmitted over MIDI Out as soon as they're received. Similarly, all channel data is supported - subject, of course, to the choices you make with the filtering and converting options.

The Filter control allows you to isolate a particular MIDI channel for processing, but remember that all MIDI data is passed through the unit unprocessed on MIDI Thru, so the Filter will only block data on the MIDI Out path.

"Design: What these units aren't is intelligent — they can't stop you altering their configuration whilst music is playing over MIDI."

It might not sound that way, but the 103 is actually a very straightforward unit - it only deals with a single channel, and a single channel conversion, at any given time.

An obvious use for the 103 is to allow the omnipresent DX7, which as we all know can only transmit on MIDI channel 1, to transmit on any channel - rather handy if you want to use a DX7 as a master keyboard for inputting multi-channel sequences for instance. Conversely, it could be used for reassigning a particular MIDI channel on playback of a multichannel sequence, so that music recorded with one instrument can be played back over another with no tedious rewiring for anyone to have to worry about.

All in all, though, the 103 doesn't strike me as being a wonderfully useful unit. It's certainly not worth its asking price of £225. My feeling is Roland would have been better off leaving out the Key Event Only option, which isn't particularly versatile but which must have accounted for a big slice of the R&D budget cake. More useful would be a processor that allowed you to define a note-range to be passed through to slave instruments. Or better still, one which allowed you to allocate user-definable note-ranges to different MIDI channels; that way, you could effectively give multiple splitpoint abilities to MIDI keyboards that don't, ordinarily, possess them.

As for the MPU101 MIDI-CV interface, it's a different kettle of fish altogether. At £275, it's not only the most expensive of the four Roland units, it's also some £75 more costly than its only competitor, the Jellinghaus CGX MIDI-CV interface. But as we shall see, it's received a very thorough implementation which, happily, puts it in a class of its own.

Like the Jellinghaus, the MPU101's main function in life is to give people with older, CV synthesisers the chance to connect them up with present-day MIDI ones. Seeing as Roland produced an awful lot of the former, as well as considerable numbers of the latter, it makes sense for them to unite the analogue and digital worlds with an interface of their own. However, the 101 can function within a broader MIDI system, in addition to tying-up your controlling MIDI synth or sequencer in a dedicated MIDI/CV role. Alternatively, if you're into driving multiple analogue synths in a big way, you can hang a second MPU101 off the MIDI Thru.

The MPU101 has provision for connecting up to four 1V/octave monophonic synths, which can then be driven in a variety of ways from the data on MIDI In. Each CV/Gate output on the 101 has an associated Dynamics output, which allows MIDI attack velocity values to control volume and/or filter amount on your CV synth, depending on its internal patching facilities. Four other outputs are provided to control pitchbend, modulation (MIDI controller code 1 provides the mod value here), aftertouch and volume. These are recognised in the basic channel in Poly and Special modes, but not Mono mode (more on these modes later).

The 101 can also be set to recognise information from a sustain or hold pedal. This is assigned the value 64, which is the value Yamaha introduced with the DX7 and which, thankfully, seems to be gaining general acceptance among the various manufacturers.

The above four outputs are all assigned to the currently-selected MIDI channel, which can be any one of the 16.

At the heart of the MPU101 lie five modes of voice assignment: mono, 2voice, 3voice, 4voice, and the enigmatically-named 'Special'. The first of these is basically MIDI Mode 4, namely Omni Off/Mono. Incidentally, the MPU 101 won't respond to mode messages over MIDI; Omni is always off, while Mono or Poly modes are selected from the 101's front panel. This is a shame, even if the provision of Poly submodes (2voice, 3voice and 4voice) isn't catered for at all by MIDI.

In Mono mode, the MPU101 responds to note data on the selected basic channel plus the next three consecutive channels, automatically allocating one channel to each of the four CV/Gate outputs. Obviously, selecting too high a base channel will result in no input on channels above 16. Each channel can handle just a single voice; play a chord and only the latest Note On is played.

Poly mode, as you may already know, allows data to be received on the assigned basic channel only, and allocated polyphonically according to the voice-assignment configuration of the receiving instrument. In the present context, you're able to select 2-, 3- or 4-voice reception, and one of these voices is then allocated to each of the CV/Gate outputs. Thus, if you've selected 4voice mode (and assuming you've got four monosynths hooked up to the 101) and you play a four-note chord over MIDI, each monosynth will be sent one of those notes.

Interestingly, if the number of simultaneously incoming note-ons exceeds the number of voices you've set, the extra notes are sent out over MIDI Out, which explains the presence of such a socket on a MIDI-CV interface. What this means is that you can link up two 101s so that the second handles the 'overflow' from the first, giving you up to eight notes sounding forth on your collection of CV synths, always assuming you've got enough of them.

Of course, two 101s tied together afford more flexibility than just that. For instance, one 101 could be put into Poly and one into Mono mode (you'd need to use MIDI Thru for this), so that one could play up to four-note chords from one MIDI channel, whilst the other played up to four monophonic lines from four consecutive MIDI channels. Now that's what I call a thorough interchange between the CV/Gate and MIDI worlds. And what's more, you can invert the Gate logic from its default positive-going pulse, so that synths with negative Gates needn't be consigned to the scrap-heap of incompatibility.

It's worth noting, by the way, that all the output sockets on the MPU101 require mini-jacks - once part of common usage, but an unusual sight in these days of the mighty five-pin DIN. They're still easy enough to find, though, while Roland themselves market a selection of leads that includes the requisite mini-to-mini jacks.

Unfortunately, this review's word-count is getting a bit high (and the Editor shows no mercy in these matters), so I'll just briefly mention a few more features of what is, after all, a rather impressive unit.

To put an end to the enigma, 'Special' mode assigns the highest, latest and lowest notes of a chord sent over the current MIDI basic channel to CV outputs 1, 2 and 3 respectively. The Assign Mode three-way switch allows any one of these notes to be assigned to CV output 4, and to MIDI Out, which means you can 'pull out' a particular musical line that you might wish to highlight in some way. Other useful assignment modes come, into play in the Special option, but lack of space precludes their discussion here.

"Facilities: The 101's main function is to give people with CV synthesisers the chance to connect them up with present-day MIDI ones."

The Clear button turns off all Gate outputs and sets the pitchbend, mod, aftertouch and volume outputs to default values. The Tune control works on all four outputs simultaneously and is variable up or down a semitone, whilst the three-way Transpose switch allows all four outputs to be transposed up or down an octave simultaneously.

There's also a sub-function (Roland's word) which allows you to synchronise an old-style CV/Gate sequencer with a MIDI one. This is achieved by converting incoming System Real Time codes (ie. the codes that normally control stopping, starting and synchronisation within a MIDI system) to good old-fashioned trigger pulses. Four different trigger rates are available from the MPU101 via the four Gate outputs: semiquavers, quaver triplets, quavers and crotchets. Trigger pulses are sent from each Gate output when the MIDI Start or Continue message is received and carry on until the arrival of a MIDI Stop signal, while tempo is controlled by the master MIDI drum machine or sequencer.

And as if all that wasn't enough, the trigger pulse can be set to either positive or negative gate, so the MPU101 really should please everybody, though obviously, the unit won't function as a MIDI-CV interface when in MIDI-Trigger mode.

In the practical part of the exam, the office Roland SH101 received a thorough going-over sandwiched between a transmitting DX21 and a receiving DX7, hung off MIDI Out and MIDI Thru respectively. The MPU101 didn't really get the chance to shine forth in its full glory, but in every respect that I was able to test it, the interface performed admirably. I don't think you could ask for much more than the 101 gives you.

What the 100 series as a whole won't give you is programmability - the ability to store multiple configurations as patches for subsequent instant retrieval. I guess that'll be the next (and more expensive) step.

There's no doubt these units perform a very valuable function in a musical instrument world that's coming increasingly under the spell of MIDI. (The latest achievement is a MIDI accordion, incidentally.) The Output Selector in particular performs a fundamental function within just about any MIDI system, given that manufacturers are not yet generally disposed towards providing multiple MIDI Outs on their equipment.

I like the modular approach Roland have adopted, as it means you can buy, say, the Output Selector first (and I reckon it will be the obvious first choice for a lot of musicians) and the Input Selector at a later date when finances allow. Both units are sensibly priced and perform their simple functions well, and the Output Selector qualifies instantly for 'once you've got it, you'll wonder how you ever managed without it' status.

The compactness of the 103, 104 and 105, and their ability to fit into the 19" rackmounting format, only adds to their appeal. It's a pity that the 101 wasn't designed in the standard 19" width, as its present shape doesn't fit in with anything else, and is consequently something of a pain in the neck to house.

The thoroughness with which Roland have implemented the MIDI-to-CV idea on the 101 is impressive, though. To my mind, its way ahead of its present competition, will probably remain ahead of the field for some time to come, and comes with a realistic price-tag.

By contrast, the MPU 103 Filter/Converter strikes me as being more than a mite overpriced. At this level, I'd have preferred to see a more carefully thought-out (and consequently more useful), data filtering facility than Key Event Only. Is it too much to hope that Roland may someday heed my advice and produce a cheaper, dedicated channel-converter unit?