[sdiy] XOR as 'digital' ring modulator

[cheater cheater]

Hi Tom,
You're missing non-linearities in the analog multiplier.

You're also missing the fact that no waveform is a true "square"
waveform: from what I understand, in a XOR gate, the output signal is
re-generated. It is not a derivative of either input signal. This
means that any deviations from ideal squares on the input will not
show up in the sound of the output.

D.

On 09/11/2010, Tom Wiltshire <tom at electricdruid.net> wrote:
> 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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