This is a very old concept of an
electronic organ I though of some years ago which uses simple
direct digital sound synthesis for tone generation.
Several so called "programmable logical devices" = PLDs are used to do this
the digital way.
As with usual organ concept, the
strategy is to generate all 12 required base frequencies with one
tone generation unit and then derive the sub tones by simple dividing
the frequencies by 2 each to obtain an octave shift. In PLDs this
can easily be done by programming. In fact it is only a vector
cutout. The sound generator comes with a 10 bit vector so 4 shifts of
tone layers are possible.
wave generation modules use a 7 bit wide digital vector to generate
saw tooth, triangle and square waveforms from out of it. This is
done by a virtual vector transformation.
The vector finally is
into an analog value using a 6 bit DAC consisting of a classic R2R-network.
The R2R-network here is used to adjust individual harmonics by
distorting it's transmission curve. A little low-pass-Filter
The final mixing is done by a
The design has been initially done
for Lattice PLDs, but will be brought to XC4000 PLDs from Xilinx, which are newly available. They
are around 30 times larger than the 3020 type, I once started with
nearly 10 years ago.
3 of such PLDs are required for one tone bank
because of the limited umber of pins and CLBs. Additional
functions might still be added, because there are still CLBs free.
12 channel tone generation
6 Bit Tone Generation Module
4 frequency levels by 3 sub modules possible
frequency setup table