2 modules ideas

Grant Richter grichter at execpc.com
Tue Aug 15 18:54:34 CEST 2000


> 1) "additive" synthesis
> 

The Buchla Model 148 Harmonic Generator did this.

>From the catalog "Generates a fundamental and its first
nine harmonics (Harmonic Numbers 1-10 (sic))".

The method used was to start with a triangle oscillator,
followed by cascaded non-linear function generators to
generate harmonic triangles, then convert the triangles to
sines with the classic JFET waveshaper.
I never have found a schematic for this, but once talked
to a fellow at U of Cal. that had seen it.

The initial triangle can be turned into a 2X triangle by
full wave rectification followed by a gain of 2 and offset
back to a zero center either by AC coupling or DC summing.
This would allow you to generate 1,2,4,8 triangle wave
frequency multiples. Once again, the triangles are then
converted to sines.

The 3X NLF must have used some of that Buchla voodoo.
How you get a 3X nice triangle wave from a 1X triangle
wave, is not obvious to me. But if you can do it then
the 6 multiple just uses the FWR stage again. And 9 could
use the x3 circuit again.

5 and 7 would seem to be very tough. Perhaps a multiplier
between lower order terms (I can't visualize it).

Another method would use the phase detectors from 4046s
with a divider chip to drive EXAR VCOs with pulse and
sine wave outputs. (pulse for the divider and sine to
listen to). In this case the PLL slew rate could be
exploited for effect and a slew pot could be put on
each harmonic. That's bound to sound interesting.

The least expensive method I can think of would use a
multisampled EPROM look up table. A 8K EPROM divided
into 8 sections gives a sine lookup table of 1024 for
the fundamental. You would store 8 waveforms each with
a higher harmonic. You could use a 400 Khz 10 Bit National ADC
(ADC1061) for the 10 LSB of the EPROM.
You then phase lock an HC4046 at 8X the ADC one bit. That
goes to a 4024 which scans the eight different harmonics through
a 4051 to eight sample caps buffered by two TL074s.

Chip count 9 for 9 harmonic sines at multiples of input triangle
TL072 input buffer to ADC
National ADC1061 A/D
EPROM
DAC0800 D/A
4046
4024
4051
2 X TL074

This has one input for a triangle wave and eight sine outputs of the
frequency multiples 2-9 of the input.

Essentially what you are doing is what Buchla did above, but using a hybrid
digital approach. The EPROM is also being used for sine conversion. Better
results might be obtained by outputing triangles from this scheme and
keeping the analog sine shapers.









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