[sdiy] 1-quadrant multiplier with 2164

Thu Jan 20 19:07:07 CET 2022

Can be done various ways.

You could use a ShortTerm Fourier Transform, then simply reconstruct the signal by summing DFTs at different sampling rates for each frequency band.  This will have a few problems, such as discontinuities at the joins between the transforms which need filtering before feeding to the DFT bank.

Another way is to feed the signal to a wavelet filter bank to produce a Continuous Wavelet Transform, transform the mother transform reconstruction coefficients according to whatever pitch raise you want by frequency, then reform the audio signal from the new coefficients.  

-----Original Message-----
From: cheater cheater [mailto:cheater00social at gmail.com] 
Sent: 20 January 2022 17:35
To: Mike Bryant
Cc: David G Dixon; synth-diy at synth-diy.org
Subject: Re: [sdiy] 1-quadrant multiplier with 2164

How would you do it?

On Thu, Jan 20, 2022 at 6:17 PM Mike Bryant <mbryant at futurehorizons.com> wrote:
>
> Yes :-)
>
> But infinitely easier if you go digital
>
> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of 
> cheater cheater via Synth-diy
> Sent: 20 January 2022 15:25
> To: David G Dixon
> Cc: synth-diy
> Subject: Re: [sdiy] 1-quadrant multiplier with 2164
>
> I wonder if it's possible to build a frequency shifter that shifts higher harmonics more than lower harmonics.
>
> On Wed, Jan 19, 2022 at 6:48 PM David G Dixon <dixon at mail.ubc.ca> wrote:
> >
> > I must confess that I've lost the thread of this argument just a little bit.
> > However, what I like about my approach (which I have used many times 
> > in many different contexts) is that, in order to build a nice linear 
> > VCA from 2164, you really need to have a clean 5V source anyway.  I 
> > keep a pile of LM336Z5 for just this purpose, and use two opamps to 
> > buffer and invert this to get low-output-impedance +5V and -5V 
> > references on all my multipliers.  If one uses precisely matched 
> > resistors on the inverter, then one can get those references within 
> > a mV of each other -- the actual voltage doesn't matter (and it is 
> > usually around 4.90V), but as long as they are equal and opposite, 
> > then they can be used for precise multiplication.  This is one of the keys to the precision of my Freak Shift frequency shifter circuit.
> >
> > I don't really understand how adding a stable DC value to a signal 
> > increases the noise of that signal.  I must confess that I also 
> > don't care at all about it.  My method is the simplest.  You don't 
> > have to pre-condition the incoming signals at all.  The CV signal is 
> > unchanged, and the DC reference levels are simply summed to the incoming signal.
> >
> > If you want to change the actual levels, you can simply change the 
> > resistor values.  I do this all the time.  One of the keys to my 
> > one-VCA four-quadrant-multiplier circuit (of which there are two in 
> > the Freak Shift, made from a single 2164 chip) is to lift and 
> > diminish the CV such that the zero point of the multiplier is at +5V 
> > and full
> > +/- unity-gain multiplication occurs between +2.5V and +7.5V.  This
> > gives lots of headroom
> > -- it essentially makes it impossible for the CV in the multiplier 
> > to hit zero at the 2164 control pin (because the incoming CV signal 
> > will never be anywhere near 20Vpp), which would give a dead zone on 
> > the multiplication.  I achieve this simply by bringing the CV in 
> > through 200k while using 100k on the reference voltages.  Of course, 
> > the signal is now cut in half as well, so I simply double the 
> > feedback resistor on the I-V converter.  As long as all of these 
> > 100k and 200k resistors are within 0.1% of each other (and the 100k 
> > and 200k resistors don't need to be in a precise ratio -- they only 
> > need to be precise within their own values), and all incoming 
> > signals are AC coupled through big back-to-back electrolytics, then the four-quadrant multiplication is very tight, which is important for frequency shifting.
> >
> >
> >
> > -----Original Message-----
> > From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf 
> > Of cheater cheater via Synth-diy
> > Sent: Wednesday, January 19, 2022 4:23 AM
> > To: Neil Johnson
> > Cc: SDIY List
> > Subject: Re: [sdiy] 1-quadrant multiplier with 2164
> >
> > [CAUTION: Non-UBC Email]
> >
> > I wonder if it matters that Dave's version will create theoretically 
> > more distortion on the positive swing of whatever vs the negative 
> > swing, whereas my version will apply distortion (non-linearity) more 
> > or less symmetrically... do the numbers show that it matters at all? 
> > I bet it would matter with some, let's say, crappy devices.
> >
> > On Tue, Jan 18, 2022 at 1:57 PM Neil Johnson via Synth-diy 
> > <synth-diy at synth-diy.org> wrote:
> > >
> > > > This is certainly true but note also the importance of zero when
> > multiplying. The zero signal stays zero no matter what you multiply 
> > by. In Rutger's case that zero is in fact -5V, so the origin of 
> > Neil's graph should be at -5V signal and zero control voltage. That 
> > is why the level-shifting solution is so effective and it is also 
> > why I believe Rutger is correct to call this a one quadrant multiplier.
> > >
> > > Yes, this is just a bit of algebraic juggling.
> > >
> > > If we take Dave's approach:
> > > - convert the bipolar +/- 5V input to a unipolar 0 to -10V input
> > > - add a -5V offset to the output _after_ the VCA (so no bearing on 
> > > the quadrantiness of the VCA itself)
> > >
> > > With a unipolar CV and a unipolar signal ... a 1-quadrant VCA.
> > > And don't forget that as-drawn the linearised VCA is inverting.
> > >
> > > Cheers,
> > > Neil
> > >
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