[sdiy] frequency shifter

Richie Burnett rburnett at richieburnett.co.uk
Fri Jun 19 13:17:11 CEST 2020

One configuration gives inverted output compared to the other.


-----Original Message----- 
From: Tom Bugs
Sent: Friday, June 19, 2020 10:50 AM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] frequency shifter

I wanted to just give a thumbs up to the Quadnet software Dave suggested for 
phase-shift networks.
It took a few moments searching online to find - quite old & windows only, 
but useful!

One minor wondering - on other phase shift stages I've seen the resistor in 
series and cap to ground - QuadNet has it the other way around - am I right 
to presume it doesn't make a difference which way around?
(yes, I should try it - and/or - do some maths..!)


On 11/03/2020 07:28, David G Dixon wrote:

I built a frequency shifter following the Bode plan.  This is frequency 
shifting by manifesting certain trigonometric product-to-sum formulae using 
electronic circuits:

sin u sin v = 0.5 [cos(u – v) – cos (u + v)]

cos u cos v = 0.5 [cos(u – v) + cos (u + v)]

So, if you have two signals with their 90-degree quadrature signals (say, u 
is the audio you want to shift and v is the on-board quadrature oscillator), 
then if you multiply the signals together (using a four-quadrant multiplier) 
and also multiply their quadrature signals together (using a second 
four-quadrant multiplier) then you can sum the multiplier outputs together, 
and you will get the following (by adding the two equations together):

cos (u – v)

This represents the audio signal u which has been frequency-shifted downward 
by the frequency of the oscillator signal v.  Of course, the audio signal 
probably has many frequencies u occurring simultaneously, and they will all 
be shifted down by v.  That’s what makes frequency shifting sound so alien 
and weird.

By being clever with the summations of the multiplier output signals (based 
on a little bit of algebra), you can also recover the up-shifted audio:

cos (u + v)

With both the down- and up-shifted signals, you can get a stereo effect.

The entire circuit consists of a 90-degree phase displacement network to 
generate the cosine of the incoming audio (I designed mine with 12 stages 
from 15 Hz to 15 kHz using a little thing I found on the internet called 
QuadNet), a quadrature oscillator to generate both sine and cosine waves at 
frequency v (mine is TZFM and consists of two Rubicon cores with sine 
shapers, with one syncing the other in such a way that the two are always 90 
degrees out of phase), two four-quadrant multipliers (I built a dual unit 
from a single 2164 chip – two linearized VCAs), and a couple of output 
amplifier stages for doing the summing.  The key to success is to AC couple 
the signals into the multipliers to eliminate DC offsets in the incoming 
signal, which is the single largest source of error in the circuit.  If that 
is done properly, the multipliers require no trimming (if accurate summing 
resistors are chosen).

The circuit works great and sounds super freaky.  I’m going to be building 
another one for one of our members here shortly.

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of 
Sent: Tuesday, March 10, 2020 7:01 PM
Subject: [sdiy] frequency shifter

Hi, I got a little lost trying to understand what a frequency shifter is. I 
mean the pre-digital method used by Moog (I think?).

I recently built a 4pole APF, and was really impressed with some of the 
pitching effects I could achieve with it. I'm guessing this is an entirely 
different method than the frequency shifters like Moog and JH have done.

Is the APF method used by anyone? What are its limitations, and what is it 
even doing?

Well, I guess I'm just looking for some conversation on the topic of analog 
frequency/pitch shifting methods. If anyone has any thoughts/experience 
they'd like to share.







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