Electronic organs - circuits, experiences and interests

[ppppenguin]

By the time we get to the 1970s, there will be only one oscillator running in the MHz, divided down by peculiar-looking ratios in a custom 'Top Octave Generator' IC to create the 12 semitones, each of which is then divided in the customary fashion as above. We'll see in later posts what was done to disguise the thin, dull sound of a locked-phase generator system.

The "peculiar-looking ratios" can be done with non-integer dividers. This may sound nonsensical but it's a common if little-known technique. For audio frequencies there is the option of starting with a sufficiently high frequency that integer division ratios will give you the top octave. Not sure how high you would have to start to get reasonable equal temperament tuning (that's another big subject, just ask JS Bach).

Non-integer dividers are simply adders and accumulators. Take a 4 bit adder/accumulator. Try adding 3 on every clock cycle. The result will be uneven in the short term but the average division ratio will be 3/16. This method is also called a discrete time oscillator and is a very valuable trick. It allows accurate frequency synthesis, generates correct interpolation in standards converters and much more.