[sdiy] Ring Mod (was Re: Hadamard Transform Network)

[Magnus Danielson]

Hi Tim,

On 05/20/2017 12:57 AM, Tim Ressel wrote:
> Here is my pontificating take on the subject.
>
> 4QMs are mathematically pure, aside from distortions, etc. An AD633,
> when given 2 and 3 volts on the input, will give 0.6V on the output (the
> actual function is (X*Y)/10 for scaling).  One excellent use for an
> AD633 is in an analog computer.

Indeed.

> Diode ring modulators use steering diodes to switch the polarity of the
> modulator input. The carrier is used to switch the diodes on and off,
> and must of a high enough level to turn on the diodes. In theory the
> diode ring mod is identical to a 4QM with one input being a square wave.
> The square wave has harmonics, of course, and these also interact with
> the modulation input.
>
> In practice, it gets muddy. The diodes don't just turn on and off. They
> go through a curve as they transition. This adds stuff to the sound. One
> can turn down the carrier so the diodes are just conducting to get an
> effect. Also the ring mod uses transformers (mostly) and that adds
> another dimension to it. If the diodes are not matched (and lets face
> it, they ain't), another type of non-perfection is introduced. So the
> diode ring mod is charmingly imperfect, mathematically speaking.

Diode mixers, DBMs, can be used in linear mode and in saturated mode. 
There is a whole bag of pros and cons. They do not "switch" in a digital 
sense, but rather each diode represents the mixing in each quadrant. The 
double-balanced mixer setup allows them to add or subtract as needed for 
the forward direction to act on the quadrant the two input signals 
operate in. In each quadrant there is a linear term and often also 
overtones that mix. The later is used in synthesis.

> There is a third circuit that makes that 'ring mod' sound: an exclusive
> OR gate. This produces a really dirty grungy raspy sound.
>
> So take your choice. I have and use all three.

Cheers,
Magnus

>
> On 5/19/2017 3:34 PM, Neil Johnson wrote:
>> Tom,
>>
>>>> David G Dixon wrote:
>>>>> Also, if a "ring modulator" and a "4-quadrant balanced modulator" give
>>>>> exactly the same transfer function, then why can't the terminology
>>>>> by used
>>>>> interchangeably?
>>>> They don't give the same transfer function, so you shouldn't
>>>> interchange the words.
>>> Further explanation please, Neil.
>> In the general case, discussed above, they don't have the same
>> transfer function.
>>
>>> Like David, it was my understanding that a diode ring modulator gave
>>> the same sum and difference frequencies when fed with clean square
>>> waves like you suggested. In which case, it would be doing 4 quadrant
>>> multiplication in that limited case.
>>>
>>> Is that not right? Why not?
>> In the *specific* case of driving both with a perfect square wave
>> carrier yes they do produce similar outputs.  But that's a specific
>> case, and David's comment was a general case (no mention of
>> specifics).
>>
>> The two main areas where they don't produce the same outputs are when
>> either of the carrier or signal has DC offsets (transformers are very
>> good at blocking DC), or when the carrier is not a square/pulse wave
>> (the diodes of the ring modulator add colour that don't get in a 4QM).
>> More on this topic in the MW thread (I appear on page 2):
>>
>> https://www.muffwiggler.com/forum/viewtopic.php?t=127474
>>
>> Neil
>