Developing a well tuned chromatic series of notes for DIY multi octave keyboards can be achieved using top octave generator (TOG) and simple frequency dividers.
The TOG originally developed in the 1970's was used by music synthesizer manufactures to overcome tuning issues related to temperature sensitive resistor networks.
There are several varieties of TOG chips (ex. MK50240, MK50241 ) with the difference being high octave note starting points and clock duty cycles. Due to the advent of digital processing these chips are no longer manufactured and are getting harder to find
For analog synth builders designing a top octave generator with discrete components can be a daunting task. Fortunately there are retro versions of the MK50240 available from makers such as Flatkeys. This projects uses Flatkeys MK50250-Retro as the TOG for a 5 octave synthesizer project.
TOG OverviewThe Flatkeys MK50240-Retro is pin compatible with the original MK50240 chip. An external clock running at 1 - 2 mhz determines starting pitch. The retro chip has its own FPGA that creates 12(13) equally divided frequencies consisting of a full octave for the highest octave set on the keyboard. Note: only the last octave uses high C pin (13th note).
The 74LS93 is an up/down module 16 binary counter which divides top octave frequencies to produce remaining lower octaves.
Each chip provides 4 outputs representing 4 octaves of one note. Chips are cascaded to provide additional octaves. The synth project in this example has 5 octaves requiring 2 chips per note.
8 octave divider circuit for C note
(Each note is repeated with duplicated circuits connected to corresponding TOG pin)
A total of 12 dividers are used to create 5 octaves of notes (48+1 for last high C). Frequency (note) outputs are enabled by the keyboard controller. Output signal from all dividers is mixed together to produce a 49 key polyphonic synthesizer.
Pictured below is the completed prototype chromatic note generator board. Oscillator is connected to input. Keyboard presses turn on opto isolators which connect selected note to common bus for output. Further details for bus mixing and keyboard latching will be provided in future articles.
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