CHORUS vs. FLANGER: What's the difference?

[Thomeeque]

Hi!

I'd like to clone Boss CE-2 and now I wonder, what it would take to infiltrate bit of BF-2 inside too and make it switchable to FLANGER mode.

What is obvious when comparing schematics is added adjustable feedback of delayed signal back to BBD input (RES pot). What is less obvious to me are differences around VCO and clock-generator modulation - can somebody tell me what is it about? Is it about higher BBD clock (smaller delay times and higher sampling frequency) and wider VCO range (MANUAL pot function is clear)?

Or from the other side, is BF-2 VCO+clock circuitry able to generate same signals as VCO+clock circuitry of CE2 (by some adjusting of Depth, Speed and Manual) or would I loose some of original CE-2 range if I use BF-2 VCO+clock circuitry instead of original?

Thanks, Thmq

[Salvatore]

Basically  CHORUS and FLANGER's are a modulated delay line mixed with the original sound.
The difference is that a flanger has a shorter delay time.

[Mark Hammer]

There are design differences, time range differences, and psychoacoustic differences.

Design-wise, yes, they both modulate time delay, although the flanger uses some recirculation of the time-delay signal where the chorus does not.  The flanger also uses a shorter average time delay, although there is overlap between the range of flangers and choruses.

One of the principal differences between the two is that choruses tend to produce notches across the entire spectrum all the time, where flangers tend to be designed to leave much of the audible range un-noptched during portions of the sweep, and then "infect" the signal with an increasing number of notches as it sweeps downward.  It is this cyclical un-notching and re-notching of the audible signal that creates the most significant difference between the two effects.  One of the psychoacoustic consequences of this is that one's attention is drawn to the filtering effects of the flanger (since the notches come and go), but drawn to the pitch modulation effects of the chorus (since the notches, though moving around, are always present).  This is, of course, helped along by the amount of pitch modulation that occurs, or is easily produced, by the respective delay ranges.  For flangers, unless they sweep to a long-ish delay (i.e., into chorus territory) and have lots of modulation depth possible, the vibrato produced tends to be modest.  Once into the chorus delay-time range, significant pitch wobble is easily produced.

To a limited extent, it is possible to transform a chorus into a flanger by simply chopping down the delay-time range.  If the default range is, say 3-10msec, and you cut that by half, down to 1.5-5msec (for example, by dropping the clock capacitor from 100pf down to 50pf), you'll get something that starts to sound swirlier and vaguely flanger-like.  But it's a far cry from the real deal.  First, the chorus won't have recirculation built in, so the flange-ey sounds will have limits to their intensity.  Second, the lowpass filtering needed to keep clock noise out of the final output will be set lower the longer the delay times aimed for.  So, there will be a portion of the sweep where you may not even get to hear the notches because they are above the cutoff frequency of the filtering.  Though, that would depend on the specific make of chorus.

The other aspect is the type of sweep.  Because the notches only jiggle around a bit, you can't really notice them moving much in a chorus, so there is little need for slow speeds.  Consequently, you'll find that the range of sweep speeds tends to be much wider in a flanger than in a chorus.  That can be fixed, but a stock chorus will never really sweep slow enough to produce pleasing flange tones unless it is deliberately designed to cover both types of functions.  The other thing is that at slow sweep speeds, whether it is a phaser, or filter, or flanger, the ear tends to prefer nonlinear sweep that gets slower as it reaches the "turnaround" near the bottom of its sweep range and speeds up as it reaches the turnaround at the top end of the sweep range.  After a certain point, however, once the speed gets fast enough (and we'll arbitrarily designate "fast enough" as being in the vicinity of 1hz), a simple linear triangle sweep is perfectly fine, and may even be preferable at faster speeds or with greater sweep width settings.  So, the modulation circuit for a chorus can be different for a chorus and flanger, not just different in terms of the speeds covered.

In the case of the CE-2 and BF-2.  The clock circuits are very similar, with nothing that screams out at me "Different type of sweep" (except that the BF-2 circut can attain slower speeds than the CE-2 circuit).  One should, in principle, be able to adapt the variable range function of the BF-2 to the CE-2 sweep/clock circuit.

Note that the Depth control in the preponderance of such LFO+Manual-Sweep circuits is really more of a Source-Blend control.  That is it combines two voltage sources, one fixed and one varying, and simply adjusts how much of each is combined to form the final control votlage for the clock.  It does so in reciprocal fashion: having more of one necessarily means less of the other.  It does this so that the resulting control voltage is always within the range that the clock circuitry anticipates.  The down-side, of course, is that you cannot simply keep the same sweep width and shift the whole thing to a shorter delay range (e.g., moving a 2-16msec sweep to 0.5-4msec).

If one had intentions of using a single BBD for an assortment of flange, rotating speaker, chorus, and double-tracking functions, you'd probably want to include:

a) defeatable recirculation
b) adjustable, or at least switchable lowpass filtering
c) sweep range adjustment
d) clock range adjustment via switch in addition to variable offset.

[Steben]

Excellent explanation Mark.

And here some superficial conclusions in the box:
- Chorus vs Flanger is not so much a "circuit" enigma, yet a tweaking and bias enigma.
- A flanger (if analog preferably with a 1024-stage BBD) can always sound a bit as a chorus if the feedback (recirculation) is set to zero.
- A chorus can easily sound "flangy" with a feedback loop.
- All chorus AND all flanger timbres in one analog design is difficult. It's like trying to ride 60 and 90 at the same time in one car, while the car in fact can do both.

I always prefered stock Flangers (instead of chorus), since they can reach acceptable chorus timbres as well quite easily.
There is always the Dimension C (by Boss) of course, which is an analog ROLLS ROYCE-chorus. It's quite funny to know exactly this CHORUS-pedal can be modded into trough-zero-flanging. 

And once we are talking about the 18V electric mistress (E-H),... different ballpark ... almost chorus (feedback low, speed high, depth low) to TZF (feedback zero, speed low, depth high) with that thing.

[Mark Hammer]

The role of feedback in flanging is to focus attention on where the notches and adjacent peaks are located, or more to the point, where they begin.  This is why the sound appears to sound a bit like a descending and ascending pitch - at the top of the sweep, most of the signal has few or no notches in it, so the resonance seems to be focussed "up there".

In contrast, with chorus circuits, the minimum delay is often in the range of several milliseconds, which already situates a number of notches spread across much of the mids and highs.  No "focus".

As a sort of exception that proves the rule, you will note that there is never any feedback/recirculation in a Uni-Vobe either, even though it is basically a phaser and phasers nearly always include at least some recirculation. So, what gives?  Simple.  The notches produced by a Uni-Vibe are very broad and shallow, so there is no opportunity for audible "focus" when regeneration is applied.  You can try the experiment for yourself; regen on a Uni-Vibe does nothing.

So, to summarize, the regeneration is helpful and audible when it affects something which is very localized and specific.  At least when it comes to filtering effects.  Of course, as the delay time increases even further, our ears don't care about notches or pitch, and shift their attention to mere audible repetition and decay time.

Think of it like what your eyes are drawn to in an individual portrait, versus a family photo, versus a class photo, versus a crowd phot.  It's all people, and they're all sporting hair, clothes, facial expressions, etc., but the features you notice most will depend on the context in which they are presented to you.  I'll bet when you looked for Waldo 20 years ago, you never looked at facial expression.

[earthtonesaudio]

I have a Fab Echo with the delay time adjustable by a pot, and the simple experiment of waggling the delay time pot while playing was a real eye-opener.  At short delays (near the max that the PT2399 chip can do) with a good amount of feedback, it gets very flanger-like.  A little longer delays, less feedback, and it's more chorus-ey.  Even longer delays and it starts to do vibrato. 

I bet the Echo Base delay could cop pretty convincing flange, chorus, phase, and vibrato tones, with proper tweaking of the delay time and LFO sections.

[Cardboard Tube Samurai]

So, one could mod a chorus to sound a bit like a flanger by simply adding a feedback loop?

[Mark Hammer]

Emphasis on "a bit".

If you have a chorus with very little lowpass filtering, like the Zombie, it might work out well, although you need a place to mix the regeneration signal back to AFTER the split, and the Zombie lacks such a node.

[earthtonesaudio]

Emphasis on "a bit".

+1.  Depending on the settings, you can probably get a 50% accurate flange sound, a 75% accurate chorus, or a 90% accurate vibrato.
Your mileage may vary, as always.

[Steben]

Funny actualy, since iirc we all have different tastes which leads to different "grails".
I for one tend to find a 100% vibrato just plain ugly, it only gets nice if it's blended.