[sdiy] XOR as 'digital' ring modulator

Harry Bissell harrybissell at wowway.com
Tue Nov 9 22:14:41 CET 2010


It is indeed possible to convert two analog waves to PWM, exclusive-or them, and filter the output to achieve true ring modulation. In practice you'd need to band limit the input signals (LPF) and
reconstruct via a post filter. There will be phase shifts with this method, but it would make for
a semi-easy DIY project. You'd need a high frequency triangle wave generator, two compatators,
an x-or gate and an RC filter at the most simple...

It looks kind of cool, probably not worth the extra effort imho...

H^) harry



----- Original Message -----
From: Tom Wiltshire <tom at electricdruid.net>
To: Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 09 Nov 2010 14:26:35 -0500 (EST)
Subject: [sdiy] XOR as 'digital' ring modulator

Hi again,

Continuing the series of "things Tom doesn't understand that he'd like to get cleared up for once and for all", I've got another question.

I've read in a couple of places (quoted below) that an XOR gate is not strictly equivalent to a ring modulator. 

Wikipedia says: "Though not equivalent to ring modulation, with square waves the resulting sound is quite similar." (Stinks of fudge to me!)

Hal Chamberlin says (about the Rhodes Chroma): "The ring modulator is really just an exclusive-or digital gate (using CMOS logic that provides exact 5V amplitude outputs) and generates true ring modulation only when the input waveforms are square. Nevertheless, the audible effect of ring modulation is adequately produced even when the signals are nonsquare."

The output from an XOR can be viewed like this:

	A	B	Q
	-1	-1	1
	-1	1	-1
	1	-1	-1
	1	1	1

This looks to me like it's a perfect ring mod (multiplication function) as long as you stick to digital signals, square or otherwise. So XOR could multiply two PWM'd pulse trains no problem according to me, but not according to Chamberlin.

So what am I missing?!

Thanks everyone,
Tom

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Harry Bissell & Nora Abdullah 4eva



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