FM in music synthesizers – for the home tinkerer
Basics – the 555 timer IC
In this article we cover basic modulation and tone generation circuits using the 555 IC for the DIYer. Many synth based projects are based on the 555 IC and young hobbyists are encouraged to read, experiment and play with these fascinating chips.
The generation of electronic signals which are converted to sound is possibly the easiest description of the synthesiser (UK) or synthesizer (USA). Being pedantic this would also describe an electric guitar which is not a synthesiser, or is it?
Moog and DX7 – analogue to digital
Most people in the music industry would have heard of Moog and the Yamaha DX7. When we think of Moog we think of two people, Robert Moog and Rick Wakeman. I would like to describe Rick Wakeman as possibly one of the most technically advanced musicians of our time, an absolute legend. Youngsters of today should switch off the rap n crap, pop and listen to The Six Wives of Henry VIII and Journey to the Centre of the Earth. Wakeman has played with the big names in industry, including Cat Stevens. Oh, Yes!
As an aside but big news if you are a Trevor Rabin fan (ex-Rabbitt, Yes) we have the new formed group, ARW – Anderson, Rabin and Wakeman. AWR Tour.
Robert Moog was king of the music scene in the early 60s with his analogue synthesiser development which was used by most of the big bands at the time. Where Moog was king of analogue, Yamaha became king of digital with the DX7. And no, Moog’s analogue models are still popular today and fetch a pretty penny on eBay. Look for one of his most famous models, the Minimoog of the 70s.
What’s in it?
In our previous articles on Ring Bridge modulation and FM Receivers it’s no wonder that it didn’t take engineers long to start using FM synthesis to generate unique sounds. Opportunities create wealth, right?
The common modules found in synthesisers are:
- VCO – voltage controlled oscillator
- LFO – low frequency oscillator, usually sub hearing <20Hz
- Envelope shaping – ADSR, attack, delay, sustain release
- VCA – voltage controlled amplifier, 2 quadrant multipler
- VCF – voltage controlled filter
- Gates – not a module but the electronic switching
- Arpeggiator – sequences the notes of a chord (minimum 2 notes). Up, down, up and down, randomise- after defining a chord.
- Sequencer – Plays notes in a series, mostly user defined.
- Ring Bridge Mixer – 4 quadrant multiplier, suppressed carrier AM
Building blocks – modules
Music synthesisers form great building blocks for DIYers and programmers. The initial build could carry for example two VCOs and a ring bridge mixer with additional modules added later to generate more effects. (I include unique tones, even noise here as an effect).
Your hobbyist free standing synthesiser with out any form of midi controller input would not need to follow any specific design criteria if the objective is just to get a unique sound, possibly generated through voice (vocoder). Your synthesiser modulation or carrier wave does not have to use sine waves either – triangle, sawtooth and square with variable mark space ratios are indeed great for changing frequencies to get unique sounds.
The Nuts and Bolts – FM Synthesis
Note that the circuit above was only simulated on LT-SPICE XVII and not tested. For best results see TI literature on the 555/556 chips.
Not intending to duplicate what one can find on the internet in droves, write-ups and warnings of sophistication on FM Synthesis striking fear into the hearts of many. It’s not to be, this is a DIY page, the hobbyists page. Buy a DX7 if you want but when fiddling there is nothing like having a Function generator on your workbench. Since you are working with audio frequencies, building your own 10Hz to 100kHz FG is not expensive neither difficult. You will need the frequencies to be stable though.
See Yebo Electronics – Audio Function Generators Kit52S – I have not built this circuit. Ask the shop assistant to check the diagram before shipping. Sometimes it’s not that legible. Pricing around R450.00 ex shipping.
The most practical circuits to build when experimenting usually incorporates the 555 or dual 555, the 556 from Texas Instruments.
Further reading: Possibly one of the best on the net, Indiana University – Introduction to Computer Music
Dintree – Synth DIY (interesting read – schematics)