Rotating Stereo Field? Anyone?

[ZiggyZipgun]

I was originally looking for a good panner schematic with stereo in/out, but I'm not sure it would have the same effect.  I know I've used the "Rotate Stereo Field" option in Adobe Audition (aka Cool Edit Pro) - the manual adjustment works wonders on binaural recordings of impromptu jam sessions - but the "spinning" effect is much more dimensional than the usual panning stompbox.  Has anyone toyed around with a stereo in/out circuit that actually moves the left and right signals around and around and around?  I don't recall any dopplar-like pitch variation, but naturally, as the two signals cross, one is slightly louder, and the next time they're both crossing, the other is louder.  Any suggestions on how this could be carried out, and if there is some slight dopplar-like vibrato, how could that be added in a realistic way?

[frequencycentral]

I did something similar but with tubes a while back: http://www.diystompboxes.com/smfforum/index.php?topic=71168.0

I used a mono input, but it could easily be adapted to stereo inputs. There is no doppler - just panning.

[ZiggyZipgun]

Nice clips from the Tube Panner!  However, I would probably have gone for the intro from "Sheep" in all it's Fender Rhodes/stereo tremolo wonder (which, incidentally, doesn't pan on A Collection of Great Dance Songs! The '80s really were a crap decade).

I know there are a few tremolos/panners with stereo input and output, but there's no intermingling of the channels, though I have found that having a Mix control can be useful, even with a mono input - using the "Auto Pan" on the PODxt Live, there's a nice, subtle, spacious tremolo to be had by setting the Mix control at 50% and the speed to 3x the normal tempo.

Maybe something like an SSM2024 could be used in the rotating stereo field scheme?

[frequencycentral]

Thanks! Aah yes 'Sheep' - I'm going to have to play that right now!

It it was me, I would go for two LM13700 configured as four VCA's, two for left and two for right. Then use an LFO into with normal and inverted outputs fed into the appripriate VCA's.

[ZiggyZipgun]

I know it's been nearly a year since we talked about this, but could you elaborate on that suggestion at all?  It made sense at the time, and I haven't tried it, but reading over it now it's not clicking in my brain how that would work exactly.

[frequencycentral]

Rotating stereo field is something I do on my modular synth. Send the left signal into VCA 1, the right signal into VCA 2. Modulate VCA 1 with an LFO, modulate VCA 1 with the same LFO but via an inverter. Have a look at the LM13700 data sheet. Figure 2 shows a VCA. You can build two of these using a single LM13700. That just leaves an LFO to add, together with a single opamp inverter stage to get the mirrored control signal.

This thread may also be of interest to you: http://www.diystompboxes.com/smfforum/index.php?topic=80510.0

[Taylor]

A truly realistic "spin" is about phasing rather than simple volume adjustment. I think a single stage phaser on each channel, with separate non-synchronous LFOs would give the more complex stereo spinning sound you're looking for.

This is because, when something is to the right of you, it's louder in your right ear, but also the sound reaches your left ear slightly later than your right. There's also more complex filtering that your ears do (that is, the skin and cartilage actually create resonant frequencies depending on the directionality of sound) but this is a little too complex to do with analog circuitry.

Portishead did some cool stuff with this more complex stereo imaging on their last record, "Third", but doing it manually instead of with LFOs.

[Mark Hammer]

Completely complementary amplitude modulation can sound cool for a while, if used on a riff-wise basis in post-production, but gets irritating pretty quickly after that if one is using it in set-and-forget manner.

My suggestion would be to use a quadrature LFO instead of simply inverting.  Many quadrature oscillator designs will be able to produce 3 additional outputs that are staggered by 90, 180, and 270 degrees.  Use of 0 and 90 degree outputs will drive the control element (whether that be FET, OTA or LDR) in a manner whereby you'll never have a situation where one side is fully on while the other is fully off.  In a sense, the sound gets to "walk over" to the other side, instead of magically appearing there in a "Whoops, I'm over here.  Now I'm over there." manner.  Far more tolerable for lengthy periods.

Searching for "quadrature oscillator schematics" will get you lots of hits.

[ZiggyZipgun]

I've gathered a handful of schematics for quadrature oscillators, as well as that old article about building a quadrature function generator, but I haven't done anything on the breadboard yet.  Would the signals be panned around with that setup, or would it act more like a synched stereo phaser?  (which, actually, is one of the other things I've been looking into putting together, and the reason I had the article for the quadrature function generator already)

[Mark Hammer]

It doesn't have to be a 4-or-more output/source arrangement.  You can simply use a single 90-degree output inaddition to the 0-degree principal output.  The basic idea is that the "other channel" always have at least some signal going to it so that it sounds like the signal is moving position smoothly rather than suddenly popping up over here, now over there, now back again.

[Joe Kramer]

Hey Friends,

Here's a very simple but effective trick I've never seen mentioned anywhere else.  Take your garden variety four-stage phaser and feed the normal output to your right channel.  Now take a tap from the output of the third stage of the phaser (a coupling cap might be needed) and feed this to your left channel.  This "tapped phaser" effect is unlike conventional inverted/noninverted stereo set-ups, where the sound seems only to shift from side to side, but instead it also seems to move close-to-far as well--a sort of rotating effect.  BTW, this trick will work with phasers any number of stages long provided the two taps are odd and even respective to each other.   Give it a try!

Joe

[Mark Hammer]

Hi Joe!

Excellent idea.  I have a little Fender Sidekick SK-30 amp that does stereo chorussing but has a one-out-two-return effects loop before the chorus/power section.  On of thwe things I can do with it is to feed a wet-only phaser signal to the "dry" side and let the wet delay channel do its thing.  Makes for a delightfully animated-sounding aperiodic shimmer.

When it comes to autopan, shimmer and aperiodicity are where it's at AFAIC.

[Joe Kramer]

Hey Mark!  Thanks.  Your chorus/phaser patch opens a whole 'nother wonderful world of swirly, rotary-type sounds.  Amazing things can also be done with a phaser/univibe in combination with a Boss DC-2, CE-300, and even the RCE-10, which is the digital half-rack version of the "Dimension" chorus effect (an inexpensive sleeper, IMO).

BTW, I'll append my tapped phaser idea with the suggestion that the tap should probably be mixed with the dry signal, using either a simple passive or active mixer stage on its output.  Or, the wet-only phase signals could be mixed with your dry signal using an outboard mixer, etc.  The tap might also work totally wet, but it's been a while since I've experimented with this trick. . . .

Joe