WHY IS IT that analogies are often drawn between audio systems and MIDI systems? Perhaps it's got something to do with the number of MIDI sequencers which use terms common to audio: tracks, filters, transposition, and other favourites.

Synth modules tend to have one MIDI input, but are often driven by more than one MIDI controller at various times. Perhaps you need to play a synth module from a keyboard one minute, from a sequencer the next minute, and at some point, from both. A MIDI switcher will only help matters in the first two instances, but come to the third, and it's time to scratch our collective heads. For unlike audio signals, which can be hardwired together, MIDI signals cannot - unless you really want to confuse your synth module. So here the audio analogy breaks down, right where 360 Systems' MIDIMerge+ comes along.

The function of a MIDI merger is to monitor two MIDI signals, and funnel them down into a single MIDI signal. Since MIDI mergers are required to prevent two MIDI messages from conflicting with one another, they must actually keep track of the data that flows through their circuits, so a microprocessor is usually required to handle this task at the required speed.

The bad news is that microprocessors cost money. The good news is that microprocessors are capable of doing a lot more than just sitting around all day keeping tabs on what sort of MIDI data is assaulting their inputs, and this makes most mergers much more than their name implies. Hence the "+" tacked on to 360's model. We'll see what this signifies in the following paragraphs.

The MIDIMerge+'s back panel requires little explanation. There are two input sections, each comprising a MIDI In and a MIDI Thru. Then there are two "Merged" MIDI outputs which provide identical MIDI signals, should you need to drive more than one MIDI device.

A ¼" input socket accommodates a momentary footswitch which, when closed, causes the unit to send out All Notes Off messages over all MIDI channels - ideal whenever your synth module is leaving a note hanging on. The All Notes Off messages can also be initiated from a similarly named front panel switch.

If the MIDIMerge+ was a mere merger, you wouldn't really expect much of its front panel. But the "+" sports a host of switches and LEDs, so it must be up to something more. Like six preset filtering options, eight user-definable filters, and a transposition function.

The filters help clear up the MIDI output signals by removing unwanted or unnecessary MIDI data. I won't go into why you might want to filter out data - there are too many reasons, and it doesn't take much experience with MIDI to discover what several of these reasons are.

Suffice it to say that you can filter out pitch-bend, modulation, aftertouch, program-change, System Real Time (clock, and related messages), and System Exclusive (patch dumps, sample dumps and so on) data, and any eight continuous controllers of your choice.

Each of these filters has a dedicated front panel switch with two LEDs (one red, one green, for inputs 1 and 2 respectively) above it to indicate whether the filter affects either or both MIDI inputs. This ability to filter one input and not the other comes in handy when, for example, you want to send a sequencer's MIDI note messages to a drum machine, but don't want its MIDI timing messages to affect that drum machine's internal sequencer. Simple - just switch on the Real Time filter.

If filtering isn't the only thing you need, look to the Channel Bump-Up switch, which allows all incoming messages to be shifted up by one MIDI channel. Too bad you can't shift them up by a variable number of channels, though...

The MIDIMerge+ allows incoming MIDI notes to be transposed up by a variable interval, and individually for each input, too. For this function, and for the user-definable filters, the unit provides a Set switch which tells the processor when to look at either MIDI input for relevant note intervals (for the transposition), or for continuous controller data (for the user-definable filters).

There's not a lot you can say about this unit that isn't indicated by its front panel, except perhaps: "How many patches does it have?" And there's the rub. For although the MIDIMerge+ remembers all the various parameter settings when it's switched off, it only remembers one set of settings.

Still, the MIDIMerge does what it sets out to do; it does so in the usual 360-style rack-mount chassis; and it does so with a full assortment of switches, avoiding the sort of "hidden function" lunacy that plagues so many other MIDI devices.

Sounds simple enough. And if you're being harassed by multiple MIDI sources, simple may well be all you need.

Price £225 including VAT

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