[sdiy] frequency shifter

Tom Bugs admin at bugbrand.co.uk
Wed Mar 11 11:05:45 CET 2020


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.
>
> ------------------------------------------------------------------------
>
> *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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