[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..!)


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/
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy

BugBrand LTD
UK company No. 07199808
VAT No. GB 988 2629 57
1 Ninetree Hill
United Kingdom

This email has been checked for viruses by AVG.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20200619/7e8ddb97/attachment.htm>

More information about the Synth-diy mailing list