[sdiy] frequency shifter

ColinMuirDorward colindorward at gmail.com
Wed Mar 11 20:10:54 CET 2020


oh, I didnt notice JH shared his schemos, and some sound clips there, too.

On Wed, Mar 11, 2020 at 3:05 PM ColinMuirDorward <colindorward at gmail.com>
wrote:

> Ok, I see now why I've never heard of this dome filter aka phase shift
> network, because it's patented/protected material.
> Tom, do you have any audio clips to share?
>
> Re the maths, what's the quickest route to get there? I just wrapped up
> ten years of art school, maybe it's time I got a real education now.
>
> C
>
> On Wed, Mar 11, 2020 at 4:44 PM David G Dixon <dixon at mail.ubc.ca> wrote:
>
>> 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.
>>
>>
>> ------------------------------
>>
>> *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.
>>
>>
>>
>>
>> ------------------------------
>>
>> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org
>> <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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>> https://www.instagram.com/colinmuirdorward/
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>> <https://www.instagram.com/colinmuirdorward/>
>>
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>>
>>
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>>
>> UK company No. 07199808
>>
>> VAT No. GB 988 2629 57
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>>
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>>
>> United Kingdom
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>
>
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