So, after just building my Phase 45, I was thinking about the differences between flangers and phasers. I did read the verbage on phasers/flangers at GEO, but I still have a question about it. Depending on how I adjust that trimpot on the Phase 45, it sounds like a flanger to me - albeit, not as exaggerated sounding as a flanger, but still sounds much like a flanger. Is the difference strictly how many notches there are in the circuit? And do flangers also use feedback to exaggerate the notches?
Flangers do use feedback to create sharper effects. Often it's labeled Resonance, but it's still feeding the effected signal back into itself.
Flangers and Phasors do a similar, but different thing. Flangers work on a principle of a small delay (usually a BBD analog delay circuit) modulated and blended back in with the dry signal. Phasors do not use a delay, but (and I'm sure someone else can explain it better) a series of filters that are essentially shfiting the phase at a varying frequency. The circuits do a similar effect electronically, but with two different approaches, which gives them unique sounds compared to each other.
Roger
The is a great article on phasing vs flanging by Marvin Jones in a 1978 issue of Polyphony that I need to scan and post one of these days.
Essentially the deal is this. Phase-shift (or allpass) produced notches remain fixed in number, while time-based notches change in number. Most other things flow from that. Both can use regeneration to enhance or exaggerate the notches and peaks, and both can use overall phase inversion of the signals to produce negative/positive phasing/flanging or simultaneous sum/difference outputs. The fixed vs variable number thing is crucial, though.
With phase shifters, the number of notches/humps produced depends on the number of stages used (divided by 2), and the cumulative phase shift produced by those stages. Most of us will experience phase shifters as having 12 or fewer stages (typically 2-6), typically yielding just a small handful of notches. Because there are just a few notches and peaks clustered together, there is a more pronounced "focus" to the sound as it sweeps up and down, because it is all happening at one "place" in the audio spectrum (although that place moves around).
In contrast, the number of notches produced in the audible spectrum increases as delay time increases, so that when you get out to 10-12msec delay times, the signal is covered with notches and peaks, justifying the term "comb filter". The key to the flanging sound is that as the swep starts from its "highest" point (i.e., shortest delay), the signal starts to get increasingly "infected" with notches, until you reach that point where you get well over a dozen and the delay starts to sweep the other way, gradually removing notches and "uninfecting" the signal. With phasers, there is a striking similarity to sweeping a bandpass filter up and down, whereas that does not appear with flangers unless there is some serious regeneration around a particular delay time.
In theory, if you had enough phase shift stages to produce a LOT of notches, then as you swept it higher, many of those notches would move outside of hearing range, leaving just a few audible. Then as you swept down again, the spectrum would start to include more and more notches, producing that "infected signal" phenomenon. In fact, this is not only true in theory but in practice. Mike Irwin made and demonstrated a 24-stage phase shifter to me (I have a sound sample somewhere), and it starts to sound very much like flanging at that point.
On the other hand, the demo uses a white noise signal, in which it is not possible to also detect the detuning that comes with varying the time by as much as 10msec. Had he used a music signal, I think the phase/flange difference would still be perceivable. Although phasing CAN mimic the psychoacoustic effects of changing the number and ditribution of notches, the flanging effect also comes with audible time delay, which phasing does not mimic.
So, phasing and flanging can live in the same neighbourhood and block, but they can't pass for each other flawlessly.
Thanks guys. That gives me a much better idea about the differences between the two. Going by what my ears tell me, it sounds like you could end up at almost the same place with a multi-stage phaser as you could with a flanger, except that the phaser would probably become a little impractical...
I would love it if you did scan that article, Mr. Hammer. 8)
I'd be happy to, but it'll have to wait for a while. We're heading off on cross-country minivan trek for 3 weeks, and it'll take at least a week to clear out all the corporate spam from my mailbox when I get back.
BTW, one of the things that phasing CAN do, which flanging can't is to change the distribution of phase shift and the resulting notches. The UNiVibe is one example of this in action. The disparate cap values distributes the phase shift differently such that the notches are shallower and spread farther apart. Conversely, it is possible to produce notches that are closer together than those produced in the usual P90-type pedal/design, and all of that is independent of whether one is higher up or lower down in the sweep. With flanging, the spacing of notches is determined by the time-delay, period.
Mark-
Thanks for the succint addendum, that makes perfect sense to me now.
I think I may just breadboard a phaser circuit and mess around with it just for fun...
Ok, one last question, though. There is a cyclical effect with my Phase 45, which sounds kind of like a time delay effect. What causes that? Is it the LFO that does it?
Couldn't tell you. I suspect I'd need to hear it to know what part of the sound catches your attention.
Mark,
what about the April/May 78 article by Gary Bannister??
"To Phase or Flange"...
tone
That is the very article, and yes, the correct author. I was confusing it with Marvin Jones' article on Reticon chips. That's the one I'm hoping to scan and post when I get back from holidays.
Do you have it available or do you know of it posted anywhere already? If so, that would save me a heap of work.
Yeah, if it's posted somewhere, please let us know! 8)
QuotePhasors do not use a delay, but (and I'm sure someone else can explain it better) a
Everything mentioned after this quote helps clarify, however, it is good to remember that phase shift
is time delay. The difference is that it is not
constant time delay for all frequencies unless the phase shift is linear from 0 Hz to infinity Hz. For audio purposes, linear phase from 50 Hz to 5 kHz would be effectively the same as a flanger delay with enough phase stages...although the phaser cannot change the time delay, rather it changes the frequencies at which the time delay occurs.
The difference with a flanger, however, is that the linear phase function
slope is modulated, whereas with the phasor, the slope (constant time delay) is set by the number of filter stages, then the spectral location of this linear phase change is modulated. So the phasor has a constant delay on a certain group of frequencies, and modulates which frequencies these are. The flanger modulates the slope of the linear phase line.
If you don't quite follow this, go back up the Hammer's posts as he did a better job of simplifying the matter. My entire point is that phase IS delay.
Quotelinear phase from 50 Hz to 5 kHz would be effectively the same as a flanger delay with enough phase stages...
you can get there with n "univibes", where the 4 staggered univibe stages resemble 1 linear phaseshift stage, which all in all takes 4n phase-inverter/RC combo`s...., where n should be >2, better 4 or more; :wink:
note: a phase-inverter (like in tubeamps) does
not shift the phase,
but just inverts (swapping polarity) it with
no delay involved.
Yes, phase IS delay, although what counts as delay on the scope is somewhat different than what counts as delay in the ear. That is probably the source of much confusion, as the delay amounts that are worthy of mention to EE-types simply won't register in the experience of most musicians.
The one exception to this principle is that the amount of "delay" created by phase relationships varies with frequency and when you get down low in the spectrum, such delays can be heard AS delays. For instance, were I to play a 100hz tone out of the left speaker, and send its inverted version to the right, there will be an approximate 5msec difference between when the peak of the waveform hits one ear vs the other, and inter-ear differences of that sort ARE consciously detectable by humans. The same is not true when we move up into 1khz and beyond.
Quote from: Mark Hammer on August 09, 2004, 10:58:43 AM
The is a great article on phasing vs flanging by Marvin Jones in a 1978 issue of Polyphony that I need to scan and post one of these days.
...
https://www.muzines.co.uk/images_mag/pdf/pl/pl_78_04.pdf (https://www.muzines.co.uk/images_mag/pdf/pl/pl_78_04.pdf) (p.12)
Hah! You beat me to it, Ton! :icon_lol: Scrolling down through the thread, I thought "If it was still 2004, I'd have to scan it, but I know it's posted, so I can just link to that, now." And there, when I got to the bottom, you had already done that, with the exact link I was going to use. :icon_biggrin:
That article makes clear what I think of as the key difference between phasers and flangers: Flangers have the notches arranged harmonically. If you happen to play a note that hits a notch, all of the harmonics of that note will fall in notches too, and the note will completely disappear. That doesn't happen in phasers because the notches aren't spaced like that.
That said, sweeping a *lot* of notches produced anyhow you like is going to sound "flangey", and sweeping a few notches is going to sound "phasey". The rest is largely details, hohoho ;)
QuoteThat article makes clear what I think of as the key difference between phasers and flangers: Flangers have the notches arranged harmonically. If you happen to play a note that hits a notch, all of the harmonics of that note will fall in notches too, and the note will completely disappear. That doesn't happen in phasers because the notches aren't spaced like that.
If you listen to the delay/phased signal they also sound different, and there's no notches since we haven't mixed the signals yet. (There's many extraneous reasons that can cause this as well eg. sweep shape.)
Quote from: ElectricDruid on October 02, 2022, 04:52:14 PM
That article makes clear what I think of as the key difference between phasers and flangers: Flangers have the notches arranged harmonically. If you happen to play a note that hits a notch, all of the harmonics of that note will fall in notches too, and the note will completely disappear. That doesn't happen in phasers because the notches aren't spaced like that.
That said, sweeping a *lot* of notches produced anyhow you like is going to sound "flangey", and sweeping a few notches is going to sound "phasey". The rest is largely details, hohoho ;)
It does.
If we use the starting point that a 10-stage phaser using the common first order allpass filters starts to sound like a flanger we can see what matches up in terms of notches and delays.
An all-pass filter is an analog circuit. However, it has an effective delay. The effective delay (in time) is the group delay. The group delay is defined as Tg = - d(phase)/dw. LTspice can plot group delay.
https://en.wikipedia.org/wiki/Group_delay_and_phase_delay
I took four circuits to compare:
- a straight delay 0.68ms to 3.4ms, representing the flanger
- a conventional phaser constructed from 10 first order allpass stages.
From experience, this is the minimum phaser which starts to sound like a flanger.
- a cascade of 5 *identical* second order all pass filters.
The Q was chosen to be 0.6 which gives a fairly flat group delay over a wide bandwidth.
The second order all-pass circuit are discussed in this thread.
(It doesn't matter which one we used as we are only interested in the response.)
https://www.diystompboxes.com/smfforum/index.php?topic=129676.0
- a full-blown 10th order Bessel allpass. This is made up of the same form circuit
as the previous one. The difference is the f0's and the Q's are not equal.
They are tuned to given the flattest group delay and widest bandwidth.
Basically these are the best approximation of a delay by an analog circuit.
What I've done is tune the delay and lower notch frequency to roughly match between the
Flanger (delay) and each of the Phasers (all-pass).
Schematic:
(https://i.postimg.cc/233X8bRY/Phaser-vs-Flanger-10th-order-sch-V1-0.png) (https://postimg.cc/233X8bRY)
Group Delay:
- we can see at the low end of the sweep the delay is flat in the lower part of the guitar spectrum.
- at the high end-of the sweep we can see the delay extends to nearly to whole guitar spectrum.
- in short the all-pass base-line first order pass circuit just approximates a delay over the sweep.
That kind of lines up as the 10 stage circuit does start to sound like flanger.
- We can also see the 5x identical stage Q=0.6 circuit provide a match over bandwidth.
- The Bessel has further improvements. Nice and flat over a substantially wider bandwidth.
(https://i.postimg.cc/LnBKkFJ8/Phaser-vs-Flanger-10th-order-group-delay-V1-0.png) (https://postimg.cc/LnBKkFJ8)
Notches:
- this very much follows the pattern of the group delay.
The frequencies where the group delay is flat are exactly where the notches agree best.
- the 10 stage simple circuit just keeps up with the flanger at the lower notch points (both lo and high sweep)
and spreads over a reasonable part of the guitar spectra.
(https://i.postimg.cc/v1npBRwy/Phaser-vs-Flanger-10th-order-response-lo-sweep-V1-0.png) (https://postimg.cc/v1npBRwy)
(https://i.postimg.cc/tZH8nr9x/Phaser-vs-Flanger-10th-order-response-hi-sweep-V1-0.png) (https://postimg.cc/tZH8nr9x)
As far as the high frequencies go. I suspect our ears are less able to decode the mess in the high frequencies from the flanger. From the guitar spectra we might expect we need to get some degree of matching upto 3kHz to 4kHz. The circuits are crudely holding on at the top of sweep. So it does support to some degree why the 10th order filters sound like flangers.
If use higher order filters then we would expect an even closer match between the flanger and phaser.
For common low-order phasers these clearly sound like phasers. The above matching patterns definitely fall apart for low filter orders (small numbers of stages.) This follows from the group delay not having a flat region within the range of guitar frequencies.
What about the statement that a 10+ stage phaser might be used in a rotary woofer part sim, but a delay is slightly more simple anyway?
At faster speeds (e.g., >0.8hz or so), perceptual differences between flangers and phasers tend to disappear. I suspect this is largely due to both sweeping over their entire range quickly enough that we tend not to pay attention to how and where they sweep. They tend to pull apart, perceptually, when the sweep is slow. A big part of that is the manner in which audible notches increase as the sweep descends. I like to describe this as the audio signal becoming "infected" with notches, as the delay time goes from absolute minimum to its maximum. Although the sweep ratio of minimum to maximum delay is important, it is the minimum delay time that plays a huge role in the dramatic quality of a good flanger. If the minimum delay time is very short, notches created at that delay are either well above audibility or the spectral content of the source material, or else simply difficult to hear/notice because there may be frequency content "up there", but it's such low amplitude, relative to the rest of the signal, that we simply don't perceive those notches. Whatever the case, as the sweep descends, more and more of the signal is amenable to audible notches, and more notches are created.
One of the more common flangers, the Boss BF-2, does not sweep to anything shorter than 1msec, such that there are always audible notches at every point in the sweep cycle. It never achieves that "infection" quality as more desirable flangers do. According to the experts, one needs to have a longest-to-shortest delay time ratio of about 35:1 or greater for "dramatic" slow flanging. The BF-2 is around 13:1. That said, if one has a preference for using flangers like a chorus or ersatz slow Leslie, you don't want a very wide sweep range, so the BF-2 is just fine. Similarly, if you listen to the always-phased sound of Donald Fagen's P90-modulated Rhodes, the sweep speed is jiggly-but-not-bubbly, and the sweep width is actually pretty shallow. The WIDE range is for very slow sweeps.
Okay, put that on hold for the moment. Let us say that we have a 10-stage phaser, yielding 5 notches. As has been noted by others, the spacing of the notches will be different than a flanger, yielding a different tonal quality. The shape of the sweep can be made equal between phaser and flanger. Assuming we are able to get the lowest notch produced up into the inaudible range, as it starts to descend in the sweep, making more of the notches audible, I suspect there will be a certain tonal similarity between phaser and flanger. But as it sweeps further in a downward direction, making all notches audible, it would kind of runout of drama, relative to a flanger, where the number of notches produced by time delay just keep getting larger and larger.
In some respects, and in theory, one could force a flanger to sound more like a phaser by severely lowpass filtering the delay signal, such that the number of notches created and audible at, say, a third of the downward sweep, are pretty much the same number as when one reaches the bottom of the sweep (i.e., longest delay time). I leave it to you empiricists to do that experiment and tell me if I'm right or totally wrong.
All of this is based on a perceptual analysis of the two effect types, and what each nudges to pay attention to.
Thx for the addition Mark.
Thing is, my question was completely theoretical. Phasers become huge and impractical at a certain point where a single delay line does the job.
However .... afaik phasers where the first compact effects to mimick rotary ... where freq split delay lines are the elaborate approach. That is intriguing and confusing.
Being freq-based, a phaser gives larger "delay" on lower freqs and this becomes shorter as freq goes up. That is a bit what a rotary system does.
Quote from: Steben on October 03, 2022, 02:44:42 PM
...
Being freq-based, a phaser gives larger "delay" on lower freqs and this becomes shorter as freq goes up. That is a bit what a rotary system does.
Swap the position of the "phasing" R & C, and its the other way round.
(But you sit there with the "floating" (i.e.: not: "to ground") variable resistor problem - which can be solved, though ...).
One thing about flangers is that the notches are always going to be harmonically related. So in the case where you have something like a single guitar note, with or without distortion, those notches can quite easily sweep through all frequencies with energy in them at once, making the note "disappear" for a moment. This is a bit different than a TZF where the cancellation is complete for all signals because it simply amounts to a phase inversion.
DL
Phasing it to death:
When NYQUIST played with his 736-stage (!) Phaser - 93 (!) years ago:
190ms delay (with inductors!)
Unmodulated, however ...
https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=1726578A&KC=A&FT=D&ND=4&date=19290903&DB=EPODOC&locale=en_EP (https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=1726578A&KC=A&FT=D&ND=4&date=19290903&DB=EPODOC&locale=en_EP)
Ununderstandable!
(I would have loved to play with a BIG LFO-modulated hi-current coil near all those inductors, and mix the phaseshifted output with the dry signal ...).
In contrast thereto:
Early modulated delay (ideal for Flangers) from 107 years ago.
(Again it served to mangle communication)
https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=1325574A&KC=A&FT=D&ND=7&date=19191223&DB=EPODOC&locale=en_EP (https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=1325574A&KC=A&FT=D&ND=7&date=19191223&DB=EPODOC&locale=en_EP)
(Again: just mix it with the dry signal).
It`s all about secrecy!
As I said before, if you listen to the unmixed signal from a flanger and a phaser there is clearly a difference in the sound. The unmixed signal has *no notches*. So focusing on notches isn't the whole picture.
I found some samples of the Sobbat PB-2 Phasebreaker II. This unit has a switch for vibrato mode which I'm fairly sure disconnects the dry signal from the mixer. For the sake of this argument it would be nice if it did or if that could be confirmed. To me it's not a big deal as the vibrato mode is representative of listening to a phaser with the dry signal removed from the output mixer.
Phaser:
Sobbat PB-2 Phase Breaker II
https://www.youtube.com/watch?v=xwo8gdab34c
4 -stage ?, 3 modes
time Mode Comment
0:00 Phase vintage, no feedback
1:33 Phase 2 small amount of feedback
2:30 Vibrato no clean mix
Sobbat PB-2 and MXR Phase 90 (1974 script, bud box )
https://www.youtube.com/watch?v=EpOIl4Zzxno
1:44 Vibrato mode.
There is phaser clearly a phaser-ish characteristic in Vibrato mode.
Vibrato mode is less pronounced than phaser mode.
What's missing is the notches. The missing character is very similar to when you dial-up notch on a parametric equalizer.
A Flanger with the dry signal removed is very much like this:
Boss Vibrato VB2/VB2W
Waza run-down
https://www.youtube.com/watch?v=b3ldgDN179Q
Sounds at 2:19
It's fairly clear the unmixed sound of the phaser has a phaser characteristic. The vibrato/flanger sounds very much like a tape machine with a slipping belt.
So why's that?
For a flanger the perceived pitch is,
p = w * [1 - (d td / dt) ]
where (d td / dt) = the rate of change of the delay. Basically modulation of the delay.
w = angular frequency [rad/sec].
Just think of that as the *signal* frequency, w is related to frequency from w = 2* pi * f .
The important thing here is all signal frequencies are pitch shifted by the same (relative) amount. So
if the pitch shift is one semitone at 100Hz it shifts one semitone at 1kHz. The pitch of the
entire signal is shifted up and down in a way which preserves the relative tuning, like a tape.
For a phaser the perceived pitch is,
p = w * [1 - (Tg/Tg0) * (d Tg0 / dt) ]
The group delay for a phaser is plotted back in Reply #18.
Tg0 is the group delay at DC (the intercept on the axis at frequency 0)
Tg is the group delay at the signal frequency w.
Look at the group delay graph and you can see at low frequencies Tg is approximately the same as Tg0.
In this case Tg/Tg0 = 1 the pitch shift is,
p = w * [1 - (d Tg0 / dt) ]
Virtually the same as the flanger.
But at higher frequencies Tg drops off and the amount of pitch shift becomes less and less.
If you have trouble with the idea of group delay just think of it as the effective delay
which is stretching or contracting the signal due to the phase-shift of the allpass filter.
We can relate it back to a phaser circuit as follows:
- When we build a phaser we modulate say a resistor value using the LFO.
- The modulation of the resistor value modulates the frequency of the allpass filter.
f0 = 1/(2*pi*RC) ;or w0 = 1/RC.
- The w0 value and the number of allpass stages sets the DC group delay Tg0.
It shouldn't be too hard to accept that when the all-pass filter is set to a low frequency
the amount of phase shift is more and that produces a larger delay.
We can even calculate the group delay. For a basic phaser,
Tg0 = n * 2 / w0 = n * 2 / (2*pi*f0). where n is the number of first order stages.
(you can see this in the group delay plot in reply #18)
- Modulating the resistance modulates f0 and that modulates the group delay Tg0 (and Tg).
- w0 and number of stages also set to total phase shift and the notch locations.
IMHO the difference between phasers vs flangers is that the phaser has different delays
at different frequencies. A flanger is pure delay whereas a phaser has a dispersive nature.
When the phaser has a small number of stages the group delay is not constant at all over the audio
spectrum - it is very dispersive.
What that means is for a given signal different parts of the signal are shifted by different amounts.
If the pitch shift is one semitone at low frequencies it is not shifted one semitone at higher frequencies.
Well not unless there are so many phaser stages that it looks like a delay.
The point reply #18 is there is enough stages to sound like a flanger (by ear and from experience on a real unit).
We can also see the group delay of the all-pass filter looks like a true delay over a reasonable portion
of the spectrum.
Here's the plots for a 4-stage phaser. (I left the 2nd-order all-passes in for completeness.)
It's pretty clear from the group delay that it hardy looks like a delay over any part of the guitar spectrum.
We can't ignore the fact the notches don't line up with a flanger (true delay). That's also part of the sound. However, as the sound samples above show the notches don't explain everything either.
Schematic:
(https://i.postimg.cc/BjXrwcm6/Phaser-vs-Flanger-4th-order-sch-V1-0.png) (https://postimg.cc/BjXrwcm6)
Group delay:
(https://i.postimg.cc/gXqQ6M2M/Phaser-vs-Flanger-4th-order-group-delay-V1-0.png) (https://postimg.cc/gXqQ6M2M)
Frequency response of mixed signal:
(https://i.postimg.cc/MXqh889g/Phaser-vs-Flanger-4th-order-response-lo-V1-0.png) (https://postimg.cc/MXqh889g)
(https://i.postimg.cc/yWj21r5x/Phaser-vs-Flanger-4th-order-response-hi-V1-0.png) (https://postimg.cc/yWj21r5x)
Some phasers have unmodulated all-pass sections these don't count for pitch shift. The extra stages do shift the notches but not in the same way as if they are modulated. Pitch shifting only applies to modulated allpass stages. If you listen to a Boss phaser with 12 stages it still sounds quite a bit like a phaser. That's because only 8 stages are modulated and modulated 8-stages is on the transition where phasers start to sound like flangers.
Quote from: Rob Strand on October 05, 2022, 06:44:41 PM
For a flanger the perceived pitch is,
p = w * [1 - (d td / dt) ]
where (d td / dt) = the rate of change of the delay. Basically modulation of the delay.
Where did you find this, Rob, or how did you derive it? It doesn't fit with my own research into this, so I'm curious.
It's a minor point though - I agree with all of your conclusions re flangers vs phasers, and removing the dry signal and looking at just the effect on the wet signal is a very good idea for helping understand the differences between them.
Quote from: ElectricDruid on October 06, 2022, 06:12:31 AM
Quote from: Rob Strand on October 05, 2022, 06:44:41 PM
For a flanger the perceived pitch is,
p = w * [1 - (d td / dt) ]
where (d td / dt) = the rate of change of the delay. Basically modulation of the delay.
Where did you find this, Rob, or how did you derive it? It doesn't fit with my own research into this, so I'm curious.
e differences between them.
I derived it but I thought it was reasonably well known. You can get different results if you use the control voltage or the delay *implied* from the control voltage "now" (as BBD's accumulate the sample delays). The way I've phrased it is td is the actual delay from when the sample went in to when it came out, doesn't matter what happened in between to get there.
The idea stems from the instantaneous frequency. The place this appears most in textbooks is under Phase Modulation, see p17. You take the time derivative of the whole sine argument. Without modulation that's (wt+theta) and the time derivative gives w, the fixed frequency of a sine wave.
https://www.csun.edu/~skatz/katzpage/sdr_project/sdr/FM_and_PM.pdf
For the time delay case you can see J.O.Smith uses the same equation, see equation (10),
https://www.researchgate.net/publication/2568326_Doppler_Simulation_And_The_Leslie
For the phaser I haven't seen the form I posted but it comes from the same process. The equation doesn't just fall out. A number of steps are required to massage the equations so the modulated part is the DC group delay. I did that deliberately so you can see the common thread between group delay and time delay, and also the differences from the frequency dependent Tg(w)/Tg0 factor.
FWIW, the notch positions are also linked to the group delay. The phaser notch frequencies deviate from the Flanger notch positions when the group delay isn't flat. If the allpass filter has zero phase shift at DC then the DC group delay pretty much forces the position of the lowest notch. That means all phasers with same group delay have approximately the same lowest notch frequency. You can see that in the plots. For a 4th order phaser you can see the group delay starting to droop at the notch frequency. The DC group delay predicts the lowest notch at 0.39*w0 and the actual is 0.41*w0. So notches and group delay are linked. Adding fixed all-pass stages is way to shift the notches.
QuoteWhere did you find this, Rob, or how did you derive it? It doesn't fit with my own research into this, so I'm curious.
OK I think I see where the difference is creeping in.
From here,
https://electricdruid.net/investigations-into-what-a-bbd-chorus-unit-really-does/
Here your derivatives are in terms of clock frequency and not delay so you end-up with different results. (It's not incorrect, but it is a step along a long chain of similar things.)
For a flanger:
- We start with an LFO waveform. The shape of that can vary.
- The LFO feeds into the VCO resulting in a clock frequency.
- The clock frequency sets the instantaneous delay time.
- The BBD averages all the delay times to produce a delay.
The whole process of how the LFO waveform translates to a delay affects the shape of d td /dt. In reply #16 I mentioned shaping affects the final result. I mean it's no surprise sine modulation sounds different to triangle or exponential waveforms. Sine is often presented for theory, and in DSP, but triangle and exponential are more common in circuits. The way the control voltage affect the VCO frequency is something that needs to be factored in, and not all VCOs do the same thing. Moreover flangers often use transistors to create an exponential current at the input to the VCO.
Phasers can have a similar set of variables. We could even make a waveform that mimics what's going in a flanger.
All those factors contribute to why one flanger/chorus/phaser sounds different to another.
I guess the point is those factor aren't what makes a flanger sound different to a phaser.
There's something more fundamental separating them.
These analyses enforce my idea a rotary sim with low count stage phaser for the low pass region might work enough. But is not that useful. I mean a 2 stage phaser is not good enough and 4+ stages are just as complex to build as a delay stage.
A 4 to 6 stage phaser is perhaps the most simple rotary sim. Univibes are completely into that game.
They do not have amplitude wobble however to make things complete. Afaik that wobble is mostly percieved in the lower freqs.
One thing that strikes me is the amount of rotary sim tips and hints across the net that can be sumarised as chorus/flanger for high speed and phaser or vibes for low speed.
A fast chorus has a cheap but noticable rotary organ thing. The shine on you crazy Diamond sound definitely has a slow leslie thing. And that is a phase 90.
Thing is, I tend to agree.
But why?
The only thing that pethaps comes to mind is the fact low mid speed accentuates the difference in low and high rotary delays which is suited to a phaser. The high speed accentuates the lfo above the delays, which is suited for chorus style effect.