Cheap ring modulator

Thierry Rochebois thierry.rochebois at ief-paris-sud.fr
Wed Mar 20 14:20:34 CET 1996


Hello,

I had an idea for a cheap Ring Modulator/VCA.
In the MS20 Korg, ring modulation of 2 state signals (pulse or square)
is achieved using a xor gate.
There is a simple solution for ring modulating a 2 state signal with
an analog signal (such as sine, sawtooth, triangle):


Pulse signal -------------+
                          |
                      +---+---+
                      |       |
Analog signal +-------+--O    |
              |       |   \   |
              | |\    |    +--+-----O out
              | | \   |       |
              +-|-1>--+--O    |
                | /   |       |
                |/    +-------+

                      CMOS switch

When the pulse signal is 1, the analog signal is "multiplied" by 1.
When the pulse signal is 0, the analog signal is "multiplied" by -1.
Here is the ring modulation.



When using an ADSR as analog signal, this simple circuit behaves like a VCA
(dedicated to pulse or square signals).


Is there anybody who knows this circuit as part of an existing synth ?
Is it a good circuit ?

Thierry Rochebois

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