[sdiy] frequency shifter

ColinMuirDorward colindorward at gmail.com
Wed Mar 11 20:05:44 CET 2020


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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