# [sdiy] frequency shifter

Tom Bugs admin at bugbrand.co.uk
Fri Jun 19 11:50:19 CEST 2020

```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..!)

Tom

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 *ColinMuirDorward
> *Sent:* Tuesday, March 10, 2020 7:01 PM
> *To:* *SYNTH DIY
> *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.
>
> Cheers,
>
> Colin
>
>
> --
>
> https://www.instagram.com/colinmuirdorward/
>
> -
> <https://www.instagram.com/colinmuirdorward/>
>
> https://www.instagram.com/ssdp_synthesis/
>
>
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