Thru zero FM

[mbartkow at ET.PUT.Poznan.PL]

Martin Czech wrote:

> Yamaha synths do phase modulation, not FM.

That's why I proposed to exploit the Wiard Waveform City idea for this
very purpose. In order to achieve the possibility of phase modulation,
the input of the Wiard's module has to be expanded by adding a second
A/D converter and a digital adder, so that the wavetable
EPROM is effectively addressed by a sum of two numbers taken modulo 2^8
(or whatever the number of bits in wavetable address is)

MAIN IN --> ADC -|
                 V
                ADDER MOD 2^8 --> WAVE ROM --> DAC --> OUT
                 ^
MOD IN ---> ADC--|

The arithmetic modulo 2^8 operation is simply done in digital domain by
rejecting the "carry" bit of the adder. This results in "phase wraping".
If the remainig 8 bits are used to address one sine cycle (or other 
waveform), its instantaneous pitch is determined by the accumulated pitch 
of both the input waves. Considering the MAIN IN is feed with a ramp, we
call a "carrier" and the MOD IN is zero, the output is simply the wave
stored in the WAVE ROM. Now, putting some signal to the MOD IN, the phase
of the wave is being modulated forth and back by the instantaneous value
of the mod signal. Note that the modulation depth (I mean the range of phase
swing) is not dependent on the frequency of the "carrier", but only on the
amplitude of "modulator". This is exactly how Yamaha DX synthesis works.
Note the symmetry of both inputs, which justifies calling it
"cross-modulation".

Yes, I know it's not exactly analogue, but you still are free to use VCOs,
instead of digitally generated waveforms like in DX-7.

BTW, I have an idea for purely analogue approach, more on that in my next
mail.

regards,

MB


--

Maciej Bartkowiak, PhD
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