[sdiy] XOR as 'digital' ring modulator

[Magnus Danielson]

On 11/10/2010 11:24 PM, cheater cheater wrote:
> Magnus,
> I'm afraid I didn't understand what you said there. Could you elaborate please?

A ring-modulator... the double-balanced diode ring, which is used for 
modulation and demodulation, a radio device... approximates a 4-quadrant 
multiplication. It does not have "a selectable gain of + or - 1".

The original ring-modulator isn't particular "linear", it has quite a 
bit of distortion in fact. It is also quite bulky for audio use due to 
the transformers. The Gilbert cell is another approach to achieve the 
same, using transistors rather than diodes. Linearisation is also attempted.

As for 4-quadrant multiplication is means multiplication covering the 4 
quadrants (of a circle) i.e. having both polarities on both input 
signals, but it assumes continuous scale to the respective inputs.

So, when I talk about ring-mod, I mean that... 4-quadrant multiplication.

Similar effects may be achieved using for instance switch-able gain, but 
it isn't a ring-mod, a ring-mod doesn't do that unless you use a 
balanced digital signal as input, but then you can't cover all the 
aspects of a ring-mod which a triangle and sine would cover for instance.

This is why I disagree with Tim.

Cheers,
Magnus

> Thanks
> D.
>
> On 10/11/2010, Magnus Danielson<magnus at rubidium.dyndns.org>  wrote:
>> On 11/10/2010 09:27 PM, Tim Ressel wrote:
>>> As a hopefully interesting side note, there are 2 ways to obtain the
>>> "ring
>>> modulator" function. The popular one for the synth crowd is a  linear
>>> multiplier
>>> like the AD633. The other way is by polarity reversal.  This is how true
>>> "ring
>>> modulators" work: the carrier signal goes  through an amplifier that has a
>>> selectable gain of + or - 1.  The  modulation signal's sign determines the
>>> gain
>>> polarity: when the mod  signal is positive the gain is +1, when negative,
>>> -1.
>>>
>>>
>>> This is of course not nearly as clean as the linear multiplier, but hey,
>>> clean
>>> is over-rated.
>>>
>>>
>>> The XOR ring mod works as a true ring mod, as an XOR gate is a
>>> digitally-controlled inverter.
>>>
>>> We return you now to your regularly scheduled programming.
>>
>> No. The true ring mod does not have gain which shift between +1 and -1.
>> It has a gain shifting between those two extremes. A true ring mod
>> actually always have losses.
>>
>> Modern ring-mod equivalents such as Gilbert cells can also be made to do
>> 4-quadrature multiplications like the old double-balanced diode rings.
>>
>> Cheers,
>> Magnus
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