[sdiy] frequency shifter

Tom Bugs admin at bugbrand.co.uk
Wed Mar 11 20:49:22 CET 2020


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 
> <mailto: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
>>     <mailto: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.
>>>
>>>     ------------------------------------------------------------------------
>>>
>>>     *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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