A point which isn’t quite clear either here or in some of the pictures of the ARPs is what the function of the S&H external clock input is. But the later (orange) ARPs clarify that in the panel layout — the jack is an input to the S&H circuit, replacing the internal clock. Consequently it can be used to time the circuit separately while the internal clock is still triggering the transient generators.
Wondering what the actual transient times were, I connected up to an oscilloscope and found that the ADSR attack is surprisingly short in proportion to the other times. This remained the case at all time settings.
Since it’s a matter of occasional confusion, even amongst people doing synth repairs for a living, I thought I might try a simple description of how divide-down polyphony in analogue instruments works.
An RCA CD4024AE 7-stage frequency divider IC.
The basis of the technology is a circuit called a frequency divider. [1] This can be done differently with digital manipulation, but a typical analogue frequency divider responds to a (significant enough) change in input voltage by waiting until a similar change recurs one or more times before changing its output. The simplest and commonest division is the one-half or first suboctave, where the count is every two cycles. (A binary divider. n.b. divider circuits are often also known as counters.)
A simple binary divider triggering on the rising edge of an input waveform will switch its output state with every voltage rise (“edge”) detected. Here the falling input edges do not affect the output. Divider circuits which trigger on the falling edge can also be built.
In electronic instruments, input to dividers typically comes from an oscillator circuit. Originally, each of the notes in the top octave of divide-down instruments were generated by a separate individually tuned oscillator. The collection of oscillators this required is usually referred to as an oscillator bank.
I’ve been somewhat reluctantly acquiring Eurorack modules, and recently decided that to save a bit, I’d be as well to make my own case out of an old wooden box and some bits I could buy. This didn’t work out as, once I had racks and power distribution, it became apparent that the box would need rather more modification than I’d planned. But I still wanted to rack some modules. So with a bit of rummaging around I found some scrap plywood and some aluminium sheet I took off something a few years ago. [1] Careful measuring shows that there’s a section of the sheet that should do the job, in which there’s minimal corrosion and only one hole, and I might be able to get the power socket in that space. So let’s go.
I want this case to be as compact as possible, though I don’t really know quite how this is going to fit together. I’m thinking it should be possible to fold the aluminium sheet so that it clips into the middle screw groove in the top and bottom extrusions of the (captive-nut) racks, thus avoiding any exposed metal edges. If it works, it should also be possible to fold a front and back plate over a base and then hold them in with a small number of screws.
the plan
I cut the ends out of plywood with mostly a handsaw, but using an electric jigsaw for the curve (which I drew freehand on the wood since it wasn’t trying to be circular anyway). The curve didn’t quite work out thanks to problems with the saw and clamps, but some work with a flap sander improved things. [2] The racks have countersunk ⌀3·5mm holes at either end, presumably on the basis that minimising protrusion is good. I had to buy some bolts, but couldn’t get 3·5mm so I’ve drilled the holes out to 4mm. Washers and Nylock nuts on the outside.
At this point things are a bit rough; the plywood could use a little filler but the aluminium can just be sanded and primed. Both will be painted. Black, because, black.
I’m also intending to line the wooden parts with aluminium foil. I wasn’t entirely sure but eventually decided to go with a thinner plywood scrap for the base. The front and rear support strips are another offcut of something, cut longitudinally. In practice the plywood I had for the base turned out to be a bit curved so I used a larger offcut for the rear support strip to help straighten it. The ends screw into these, making the assembly quite solid already.
I transcribed the music in the Drawing In (v.3) video to MIDI and tried it out with a few standard library instruments. Which wasn’t all that impressive initially but something odd happened when I added the timing track in — what had been a fairly basic delay effect on the Glockenspiel (substituting for the bell sound from the MS‑20) started doing some fairly wild and wonderful things. I’m not sure quite why, but this is clearly adding some modulation to the delay timing, so it probably qualifies as a flanging effect. After listening to it for a while I realised it is similar to some effects I heard long ago, possibly on some Pink Floyd albums from about 1971. So presumably you can do this sort of thing with tape delays.
This is the audio output I’m getting. Not a finished track, just for illustration.
To be clear, I wouldn’t expect audio arrangement software to do this. It seems like a bad idea if timing changes can affect your sound. But I’m starting to like the result. I just want a way to do it deliberately without having to change the timing.
A little while ago I obtained a stack of old electronic instrument paperwork which I’ve been sorting through and am planning to scan. To start with, this is a single sheet giving what seems to be an update for the Armon “A61S-A492-K4009-ARMONPIANO”. I’m not clear whether that’s one or three separate models. The update is for a revision to a tone generator board, now AR-27. It’s undated but I’d guess late-70s. Perhaps this will be of interest to someone?
