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Short Circuit

Time Machine Revisited

Article from Electronics & Music Maker, May 1985

An add-on for last month's Time Machine project that allows direct synchronisation of the Yamaha RX15 and MC202.

As a follow-up to last month's Time Machine project, here's a simple circuit that lets you start and stop RX drum machines from devices fitted with the Roland DIN sync standard.

Paul White's thoroughly wonderful Time Machine design (<1881>E&MM April) solved a lot of electronic music's most troublesome syncing problems, but what it can't give you is a usable method of starting and stopping a Yamaha RX drum machine direct from equipment fitted with Roland's 24ppqn sync standard, henceforth known as 'Sync 24'.

The problem is a simple one. Sync 24 incorporates a clock signal that's present all the time, and if you take that signal from a suitable machine and connect it to a Yamaha RX, the two instruments will run in sync only if your reflexes are agile enough to press the two Start buttons at exactly the same moment. And if you're trying to play a couple of synths at the same time, that's not a particularly easy task to accomplish.

The extraordinary feat of modern technology you see before you solves this problem by taking the Start signal from the Sync 24 bus and using it to operate the footswitch input of the RX. Simple, huh?

Sync 24

Before we go any further, a few words about the Sync 24 standard are probably in order. The interface - designed by Roland but subsequently implemented by a good few rival manufacturers as well - appears from the outside to be similar to MIDI, since it uses identical five-pin DIN connectors. But don't be deceived - the two standards have precious little in common besides their connecting plugs, and are completely incompatible. The Sync In and Out sockets are wired identically, so a straight-through connecting lead (ie. Pin 1 to Pin 1, and so on) is all that's needed to link up a couple of compatible machines.

Pin 1 carries a +10V signal for the whole time a sequence or drum pattern is being played; if a Continue button is used to start a pattern, a pulse is produced on Pin 5. Pin 3 carries a TTL-compatible clock signal, and this runs at a rate equivalent to 24 cycles per quarter note, hence Sync 24. Finally, Pin 2 is the common ground.

Figure 1. The circuit diagram.
(Click image for higher resolution version)

The Circuit

There's no way of starting and stopping either the RX15 or its costlier brother, the RX11, externally other than by using a footswitch. However, the RX footswitch input works in a slightly unusual way in that the machine in question starts and stops at the point of transition from closed circuit. We can make use of this by operating the normally closed section of a relay each time we want the RX to stop or start, but the only signal we can use is the start signal on Pin 1, and if we connected it directly to the relay, it would only be able to start the RX, not stop it.

What we need to do is generate a switching pulse each time the start signal changes state and, luckily, this can be achieved by inserting a capacitor in series with Pin 1 and the input of the relay driver. This produces a short positive pulse at the 0V-to-10V transition and a short negative one at the 10V-to-0V change. Unfortunately, the relay driver will only respond to positive pulses - hence the full wave rectifier to ensure all pulses are converted into positive signals.

We also need a high-gain amplifier to drive the relay properly, and this is achieved thanks to the presence of two transistors (TR1, TR2) in the Darlington pair configuration. D5's purpose in life is to prevent the large backward EMFs produced by the relay coil from damaging the transistors, while R3 is the current-limiting resistor for the LED (D6), which should light each time the relay operates.

Whilst the relay is operating, the current drawn is about 80mA, but since this happens only for short periods and the current is negligible the rest of the time, average current drain should be fairly small.

Figure 2. Maplin (left) and RS Components relays in detail.


As you can see, the circuit is extremely simple and can most easily be put together on a small piece of veroboard, though the exact size of this will depend on whether or not you're mounting the relay alongside everything else.

The usual rules apply. Solder the resistors, capacitor and semiconductor in that order, making sure you've got everything the right way round - this is of vital importance in the case of the electrolytic capacitor (C1). Choice of output sockets (SK2, SK3) is up to you; I used quarter-inch mono jacks because I already had suitable leads made up. The RXs use a 3.5mm jack socket for the clock input and a quarter-inch mono jack for the footswitch connector.

The whole thing should fit neatly into a small plastic box, assuming you have one to hand.

Figure 3. Prototype veroboard layout.
(Click image for higher resolution version)

In Use

The connections that have to be made are as follows. The five-pin DIN socket (SK1) should be linked via a straight-through cable to the Sync Out socket of the master machine. SK2 should be connected to Cassette In on the RX, while SK3 is hooked up to the footswitch socket. You now need to set up the RX for use with an external clock (beware - some RX functions don't work in this mode), with the timebase set to 24ppqn for Roland equipment and 48ppqn for Korg machines like the Super Drums and Super Percussion.

Make sure you use the Start button, not Continue, to set the sequence or drum pattern in motion, because the RX11 and 15 are always started from the beginning of a pattern when the footswitch is brought into play. And beware: if you use the Continue facility, start and stop too quickly or switch on the master controller with the interface and RX already powered up, the two machines may become nicely out of step. with each other - exactly the situation we've set out to avoid. If this happens to you, press Stop on both machines and start all over again.

May your patterns be eternally synced.

Parts List

R1 100K, 10% ¼W
R2 10K, 10% ¼W
R3 1K, 10% ¼W

C1 10μF, 100V electrolytic

D1-5 IN916
D6 Red LED
TR1-2 BC109

SK1 Five-pin chassis-mounting DIN socket
SK2-3 ¼" mono chassis-mounting jack socket
SW1 Single-pole changeover sub-min toggle switch
RL1 6V 100ohm changeover relay, single-pole (Maplin FM91Y; RS 346-637)
Connector for PP3 battery LED clip
Box, veroboard to suit

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

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Electronics & Music Maker - May 1985

Feature by Steve Hartwell

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