Music synthesisers

see copyright notice. Page created 26-Apr-2001 updated 8-Nov-2019. Use the button groups above to navigate quickly around the site.

MOSS (Music Oriented Sound Synthesiser)

I never actually owned a Stylophone, the musical toy famously played by David Bowie on Space Oddity. Instead, circa 1977, I made this monster based on how I thought a stylus-controlled instrument should look. Much to my surprise and delight it's still in full working order!

A note on the block diagram proudly records that it contains 37 transistors, 18 ICs and 26 diodes! There's also a strange device called an RPY13, of which I once had access to a vast supply (don't ask...), comprising a light bulb surrounded by 4 Cadmium Sulphide Light Dependent Resistors, all contained in a blackened glass envelope looking very much like a valve. This forms part of the VCF (Voltage Controlled Filter) where two of the LDR elements are in a sort of Wien Bridge circuit to set the filter frequency. As the light bulb is the controlling element it doesn't have a fantastically good response time!

There are two separate sound generators. The stylus controls the main one, which is structured much like other monophonic analogue synths of the day, and accounts for nearly all of the front panel controls. The stylus itself is made from a biro case and the springy bit of a pair of National Health Spectacles that went round the ear - it was just the right component for the job. Each pad on the 3-octave "keyboard" is made from tin. Not tinplate, but solid sheet tin. It's nice and flexible and doesn't oxidise much, and the oxide is in any case conductive.

A resistive divider ladder supplies analogue control voltages to the stylus pads, each one being individually trimmed by its adjacent preset. Superimposed on the DC control voltages is an HF signal of a few hundred kHz, which is detected by the stylus input circuit to control envelope generation and sample-and-hold of the control voltage. The sine/square/triangle generator is an ICL8038CC, which at the time seemed like God's Gift to Synth Designers.

The contraption to the left of the stylus pads is the controller for the other sound generation department. This is polyphonic, based on an "organ divider" chip (AY-1-0212) whose 12 semitone outputs are further divided down to create 5 octaves of tones. The master oscillator frequency is adjustable over a full octave so the note range can be matched to that of the stylus section. The 60 tones are brought out to small wire loops within circular holes in the top plate of the controller, which like the stylus pads is made of tin. The plate is connected to a high-impedance audio amplifier. You play the thing by electrically bridging the gaps with the finger tips, or for a nice gutsy sound you can use a whole finger to select several octaves at once. The output can be mixed either before or after the stylus-controlled envelope shaper; with practice and a supple left hand one can make quite complex chords. I think the inspiration for this setup came from my vague memories of playing (with) a piano accordion as a child!

The construction technique could politely be described as informal. The front panel was the only planned area; the electronics grew in a glorious, chaotic suspension behind the controls (note the RPY13 dangling by its socket just to the right of the power supply). As with many other bits of kit I've built, I never got round to making a proper case for it. It's quite safe though, the live contacts of the mains switch are protected by large blobs of Araldite :)

MOUSE (My Own Unique Sound Engine)

Following on from the analogue MOSS came the mainly-digital MOUSE, created in 1981 as my first hobby application for the Z80 microprocessor. It's interesting that this instrument, although 4 years the junior of MOSS, didn't ackle when I switched it on in 2001. In fact it was so broken I had to fetch a scope probe to it. One of the Z80 CPUs turned out to have a permanently-low WR pin... how very fortunate it was in a socket. All is now well.

Yes, I did say one of the Z80 CPUs. There are two, designated "A" which looks after the low-frequency stuff (keyboard, indicators and sound modulation) and "B" which does the real-time wave synthesis. The latter can construct sine, square and pulse waveforms and operates in 4-voice polyphony. A single 8-bit DAC is demultiplexed into four analogue voice channels, each with its own LPF and digitally-controlled analogue VCA (thus the effective audio performance is rather better than 8 bits!)

Four buttons control most of the audio functions in conjunction with some of the keyboard keys (I do love multi-purpose controls!). To transpose the keyboard for example, you hold down LOCAL and press the TRANSPOSE key (actually this is the default selection at power-up), then hold down the up- or down-arrow button whilst repeatedly pressing any keyboard key; the note plays up or down in semitones until you release the arrow button. Other functions include envelope parameters and three types of width modulation for the pulse waveform.

The original plan was to allow the keyboard to be split into a number of zones or segments, each playing in a different configuration. Hence the GLOBAL/LOCAL buttons and the Segment controls; it isn't fully implemented though.

Inside, the customary wooden system box houses digital stuff on the base and analogue on the top; they have separate power supplies. The keyboard is connected by a 14-way ribbon cable. The system clock is nominally 3.46MHz, adjustable ±3% by a front panel variable capacitor for fine tuning of the absolute pitch. The analogue channels use passive L-C anti-alias filters and series-connected JFET VCAs (the cermet presets on the VCA board are adjusted to null out the harmonic distortion caused by FET nonlinearity - it's quite a nifty circuit!)

The best feature of all is the built-in program editor. If you hold down both GLOBAL and LOCAL buttons and press the PROGRAM EDIT key, the instrument goes into a mode whereby any location in the "A" CPU memory can be interrogated and, in the case of RAM locations, modified. Extensive use of RAM breakpoints and jump tables in the firmware means it's possible to patch most parts of the running program fairly easily.

So how do you read out the contents of memory without a display? By ear, of course. If for example you press the ADDRS= key, enter a 4-digit hex address (using the lowest 16 white keys) then press DATA? the audio will play out the 8-bit contents of that memory location, using a sequence of 8 tone bursts - a high note for logic 1 and a low note for logic 0. If you feel really adventurous, the RUN key will start execution at the current address. Ok, I know, I'm a nutter.

BORING (Breeze-Operated Random-Interval Note Generator)

(You wouldn't believe how much effort went into that acronym).

When I wrote this article in 2001, the opening words were "Have you seen the price of windchimes?". A more pertinent intro now might be "Have you seen the price of copper tube?". Back in the days when plumbers couldn't use the latter price to justify their charges, I spent some playtime hitting short lengths of tube to see what sounds they made. I concluded that the frequency (pitch) emitted is, roughly, inversely proportional to the square of the tube length; the best place to support it is one quarter of its length from the top; and the best place to hit it is at the bottom. Fascinating fact: the note emitted in this mode is higher for a given length of 22mm tube than for the same length of 15mm!

After further experiments I devised a pleasing chord, made from the notes E5, A5, B5, C#6 and E6. The final lengths of 22mm tube were 390, 337, 318, 298 and 274mm. It's best to cut slightly too long then trim bits off with a pipe cutter until it sounds just right. The striker is a glass Baby Bio plant food bottle wrapped with pvc tape to sweeten the sound a bit (they make them of plastic now so you'll have to find something else!). A few bits of card, stuck on the striker chord in the manner of an old-fashioned kite tail, catch the breeze nicely when the adjacent window is open.

Please, if you make or buy a windchime, use it only indoors unless you live in the middle of at least an acre of private ground!

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