[sdiy] frequency shifter

David G Dixon dixon at mail.ubc.ca
Wed Mar 11 21:40:33 CET 2020


I’d be lost without the maths.  I always derive the transfer functions of any filters I design, for example, and I have found that to be invaluable.

 

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From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Tom Bugs
Sent: Wednesday, March 11, 2020 12:49 PM
To: *SYNTH DIY
Subject: Re: [sdiy] frequency shifter

 

Don't feel you *have* to get tangled up in maths - I could do maths up through my technical university course, but frankly it never conveyed spirit to me & I rarely go anywhere deep with it in any of my designing.
I was also going to suggest looking up the CGS Dome Filter (the phase shift network mentioned) but don't think schematics were presented - it does, however, state the Electronotes article I referred to - #83
https://sdiy.info/wiki/CGS_dome_filter

On 11/03/2020 15:22, ColinMuirDorward wrote:

I'm still working on these replies, folks, thanks for the discussion! 

OK, TBH, I'm pretty lost. It's probably beyond me without having the math. Is it anything like barber-poling a series of APF filters?

 

 

On Wed, Mar 11, 2020 at 9:43 AM Quincas Moreira <quincas at gmail.com> wrote:

This is a celebrated one, by JHaible. I have the pcb, waiting for the day when i’ll have time to source the parts... 

 

http://jhaible.com/legacy/frequency_shifter_fs1a/fs1a

 

Sent from my iPhone





On 11 Mar 2020, at 4:11, Tom Bugs <admin at bugbrand.co.uk> wrote:

 

My brain is not awake enough to closely follow Dave's description! But I remember there's a good basis in an old Electronotes - I didn't follow it exactly by any stretch but it really helped me design my own, the theory I took being:
1) make a quadrature sine/cosine VCO
2) audio input splits to two 6-stage all-pass filter
3) each filter-chain is followed by a RingMod/Multiplier with the modulating input coming one from sine / one from cosine
4) then you do sum & difference of the two ring mods to get up & down shifts.

What I really enjoyed was adding feedback! In fact, redeveloping the ideas at the moment & adding in a bit of extra control + output mixing/panning. Really great audio processor, even at LFO rates where it becomes a wonderful phaser type machine.

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.

 

 


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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


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VAT No. GB 988 2629 57
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