Cheap ring modulator

[Don Tillman]

   Date: Wed, 20 Mar 1996 14:20:34 +0100
   From: thierry.rochebois at ief-paris-sud.fr (Thierry Rochebois)

   I had an idea for a cheap Ring Modulator/VCA.

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

			 CMOS switch

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

Except that VCAs are typically exponential.

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

Not to my knowledge.

   Is it a good circuit ?

"Good" in a moral sense?  :-)

It's okay in that it will work and it's slightly more flexible than an
XOR gate.  But limiting one input to a digital signal just to save
money is pretty "user hostile" (opposite of "user friendly").
Remember that a 1496 modulator chip only costs about a dollar.

Typically a circuit like yours would be implemented as a variation on
the previously discussed +/0/- gain circuit, replacing the pot with a
single (SPST) cmos switch and a resistor.

  -- Don