MIDI Merge Boxes
Philip Rees 2M & Groove Electronics MIIM MIDI Mergers
When the usual MIDI In, Out and Thru that appear on most of your MIDI gear are unable to manage your MIDI data, outside help is in order. Vic Lennard has the urge to merge with Philip Rees' 2M and Groove Electronics' MIIM.
If you've managed to connect more than a couple of MIDI devices together the chances are you've wished for a MIDI merger - how do the Philip Rees and Groove unit measure up?
MR REES' MERGER is a neat looking device measuring just over four inches square, and sporting the customary Philip Rees colours of black, red and white. There are two each of MIDI In, Thru and Out, labelled A and B in each case. Each MIDI In feeds the corresponding MIDI Thru, while the merged signal from the MIDI Ins is transmitted from both MIDI Outs. The top of the unit incorporates a graphic diagram of the data flow, and the 2M comes with a ready-fitted standard plug complete with a 1 amp fuse. Someone cares.
The design principle behind the 2M is that it is totally automatic in its operation, all necessary decisions being taken by the internal microprocessor, and that the internal buffers for storing information prior to transmission are large enough never to overflow - in fact there are 16kbytes of RAM available. The way the 2M works is like this: a 128 byte performance buffer is allowed to fill up with Note On/Off and controller data, after which it starts to filter out aftertouch and pitch-bend information, retaining any of the latter which is attempting to re-centre the pitch-bend wheel (without which you may find your music undergoing strange transpositions). Active sensing is completely filtered out and all note information is set as running status. This means that only two bytes are needed for each note event instead of the customary three, hence helping to prevent delays building up. Data transmission works on a priority system where Song Select and Song Position Pointer come first followed by MIDI clock and then other MIDI data. A lot of thought has been put into the Clock Interlock system to enable it to accomodate MIDI clock generating devices at each MIDI In, but only acknowledge clock pulses from the device which last sent a Start or Continue command. System Exclusive has its own 640byte buffer which will fill to 140 bytes and then start sending byte for byte, inserting MIDI clocks where necessary between blocks to keep the timing accurate. If MIDI Time Code (not yet really with us) is being input at MIDI In A, SysEx will be stopped to allow the code to pass. Pitch-bend information from two keyboards on the same channel is added together - start bending the wheel on one keyboard, hold it and the other keyboard's wheel will continue the bend - interesting bearing in mind that pitchbend has 8,192 possible positions, but probably the only practical approach without actually filtering out the data from one of the keyboards.
In use, the 2M is pretty well invisible, although the startup procedure is important. Receiving modules should be turned on first followed by the 2M and finally computers and master keyboards, otherwise, due to the running status, a Note On byte could be missed and playing the keyboard will result in not a great deal happening. (To escape from this situation, simply transmit a different status byte - move the pitch-bend or modulation wheel and all will be hunky dory again.)
One problem that I came across was in the requesting and receiving of a System Exclusive dump. As soon as the SysEx buffer reaches 140 bytes, all Note Off and pitchbend reset commands are sent on all 16 MIDI channels, creating an extra 96 bytes at the start of the dump. As a specific number of bytes are expected when dealing with System Exclusive you're in trouble - if status byte FO is not received a "time out" will occur. Philip Rees assures us this has now been rectified by removing the reset commands although, at the time of writing, the revised unit was not available for testing.
Most people buying a merge box will want it either for merging performance data or for visual editing of synths using a computer. A merge box allows the master keyboard and the computer to both control the synth while the MIDI Out feeds data from the synth back into the computer - in this way you can edit and play at the same time. Using the Philip Rees merger, I tried various visual editors with a TX7 and a D550 without encountering any problems at all.
Finally, an excellent manual - including applications and a full run-down of how the box works. And all from a company who are more than happy to troubleshoot over the phone.
THE UNIT RECEIVED for review, while having final release circuitry, was in a pre-release casing, but suffice it to say that there are MIDI In/Thru/Out sockets for each channel on the rear and a row of eight buttons labelled A to H on the top, each with its own LED, which can either be on, off or flashing.
Conceptually, the unit is very simple: Any input can be routed to any output and the choice of data filtering is left completely up to you. For setting up, MIIM offers three modes which are accessed by continued pressing of button A, the LED A will either be off (Route/Merge mode), on (Filter channel A) or flashing (Filter channel B).
The Routing and Merge mode is indicated by LED D flashing and is invoked by using buttons E and F to determine the output of channel A; buttons G and H do likewise for channel B. The two streams of data are merged if they are routed to the same output, the LEDs above each letter indicating whether that route is in use. Should a data overflow occur, LED D will go out and LED B will flash; this situation can be rectified by pressing button D to clear the backlog. Unfortunately this can lead to Note Offs being lost in the process, consequently button B also acts as a reset - turning off modulation, sustain, all notes, aftertouch and pitch-bend. This takes about 64 milliseconds.
In Filter mode, keys B to H filter out the following data respectively: System Exclusive, System Real Time (start, stop, continue and clock), pitch-bend, patch change, controllers, polyphonic/channel pressure and note on/off. Oddly, though, System Real Time does not remove active sensing (which is sent every 0.3 seconds to confirm that a controller is still connected) but passes through active sensing from both keyboards, should there be two connected. While I could not get this to misbehave, I certainly feel that it could cause problems in filling up the MIDI buffer in a sequencer.
In use, thought has to be given as to what needs to be filtered out. Channel pressure is an obvious choice unless you have a keyboard that doesn't require half a ton of pressure to respond.
If you're using the merge box to jam with two synths, you should bear in mind that many older synths only transmit on MIDI channel 1, and trying to pitch-bend on two such keyboards simultaneously can lead to highly non-musical glitches. A similar problem will also occur with cheap hardware sequencers which can only record one MIDI channel at a time.
On the other hand, the ability to re-route inputs to outputs at the press of a button makes this box an interesting proposition for two players each controlling a synth or two voices from a multitimbral expander immediate changeovers are possible without changing MIDI channels on the synths.
Another problem occurs with our old friend System Exclusive. While any system real time information politely waits until a gap occurs between data blocks, any performance information will force its way in and cause the dump to be aborted when in handshaking mode. The only way around this is to ensure that an end-of-exclusive signal has been received before other data is transmitted, and this will certainly cause delay where MIDI clocks are concerned.
For the visual editing tests, out came DX-Heaven and the trusty old TX7 - with the same result as for the Philip Rees box: no problems at all.
Groove Electronics have never been renowned for their manuals and the seven pages of photocopied A4 which accompanies the MIIM does nothing to improve their reputation.
BOTH OF THESE boxes suffer when asked to mix performance data with System Exclusive data which leads me to the following conclusion: only short, single voice or performance memory dumps should be attempted through a merge facility. MIDI may be fast, but, being serial, still only sends one byte at a time and this will inevitably lead to timing problems and/or the aborted dumps.
Another point to note is that it is possible to trigger the same Note On from different keyboards, and the results at Note Off time are unpredictable - either one or two Note Offs will be required to clear the two Note Ons. I found that the D50 and DX7 needed individual Note Offs, while Roland RD pianos and a JX10 cleared both Note Ons from a single Note Off. The only answer is to experiment.
Ignoring System Exclusive considerations, both these units do what they profess to. Your choice will have to depend on whether you want to be able to control data filtering with the MIIM or trust the automatic facilities of the 2M.
Prices Philip Rees 2M £79.95; Groove MIIM, £95. Both prices include VAT
More from Philip Rees, (Contact Details)
Groove Electronics. (Contact Details)
Review by Vic Lennard
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