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Why invert audio in modular?
MUFF WIGGLER Forum Index -> Modular Synth General Discussion  
Author Why invert audio in modular?
anotherjones
Hi all. I'm familiar with phase invertion for corrective purposes with mics etc, but what's the use of inverting the polarity of audio in modular patches? I presume polarizing VCAs invert the phase, not cut out the positive or negative half oof the signal as audio signals are bipolar. I'm sure I'm missing something ... Can inverting polarity of audio signals serve crative purposes, or is it a purely correctional function? Any insights would be much appreciated.
flo
Here's a "tremolo" patch based on using inversion to cancel out the "original" sound (with a quadrature oscillator that provides all inverted outs from the get go). The stereo is done using all four quadrature outs (0° and 180° on L, 90° and 270° on R). Also some sync and FM...

[soundcloud url="https://api.soundcloud.com/tracks/285389732&amp" params="color=ff5500&auto_play=false&hide_related=false&show_comments= true&show_user=true&show_reposts=false" width="100%" height="166" iframe="true" /]

https://soundcloud.com/contracommunemopinionem/sept-16-phase-cancellin g-oscillations

As always, patch notes on Soundcloud or in my patching thread thumbs up

Of course, there are many things you can do using inversion on audio sources. It's often helpful in feedback patches (e.g., when feeding back non-inverted versions cancels the sound). It can be used to patch EQs. You can patch other filter types (e.g., patching a highpass filter with a lowpass only filter). And so on...

Cheers! Guinness ftw!
anotherjones
Many thanks Flo. Wondefully rich texture in that snippet. I will certainly check out your soundcloud posts.

If I understand correctly, the basic principal is splitting the signal into positive and negative to process each half differently.

I'm very familiar with negative modulation signals... Need to explore audio polarization in Nord M with the level modules
davidh
in theory you can mix the inverted signal, and the same signal processed, to get a full wet version of the processed signal

you can also make a high pass filter from a lowpass filter (but a lp filter often invert the signal already.
flo
anotherjones wrote:
Many thanks Flo. Wondefully rich texture in that snippet. I will certainly check out your soundcloud posts.


Thank you very much! It is highly appreciated we're not worthy

anotherjones wrote:
If I understand correctly, the basic principal is splitting the signal into positive and negative to process each half differently.


No, that would be half-wave rectification (once for the positive, once for the negative part of the signal) and then process the two separately. That is interesting as well, but the principal with inversion is that the phase is flipped for one of the two signals (but they can still be bipolar).

Check this pic of quadrature sines:



The full and dotted lines of each color represent the "original" and "inverted" signal. Seeing it like that also shows nicely why they must cancel and result in no output if you mix them at unity. That is how the "silence" in my "tremolo" patch is created (and additionally, imagine the "green" signals for left and the "red" signals for right in terms of stereo).

Cheers Guinness ftw!
anotherjones
Thanks so much guys. Indeed I was geeting confused between rectification and invertion.

So using the Nord G2 level converter modules I just patched up a satisfying phasing effect, splitting the signal from an Osc, converting 1 to positive, 2 to negative. I then put rectifiers before each polarity converter rectifying one signal to positive, the other neg. And damn, satisying became most satisfyng! Completely in a different league to the dedicated phaser module!

As always with the G2, difficult to stop, now a 26 - module patch wiyh me thinking how the f am I going to afford implementing something like that in euro rack! eek! - (I know there ar plenty of function units which are the equivalent of a combination of many G2 logic, swicth, level and other module)

Thanks again!
dubonaire
flo wrote:
Here's a "tremolo" patch based on using inversion to cancel out the "original" sound (with a quadrature oscillator that provides all inverted outs from the get go). The stereo is done using all four quadrature outs (0° and 180° on L, 90° and 270° on R). Also some sync and FM...
Cheers! Guinness ftw!


Awesome!
flo
anotherjones wrote:
Thanks so much guys. Indeed I was geeting confused between rectification and invertion.

So using the Nord G2 level converter modules I just patched up a satisfying phasing effect, splitting the signal from an Osc, converting 1 to positive, 2 to negative. I then put rectifiers before each polarity converter rectifying one signal to positive, the other neg. And damn, satisying became most satisfyng! Completely in a different league to the dedicated phaser module!

As always with the G2, difficult to stop, now a 26 - module patch wiyh me thinking how the f am I going to afford implementing something like that in euro rack! eek! - (I know there ar plenty of function units which are the equivalent of a combination of many G2 logic, swicth, level and other module)

Thanks again!


Nice! Sounds like a lot of fun with the G2 hyper

Patching half-wave rectification is easy, if you don't have a dedicated module: Just patch an offset to a VCA, and your signal source to the VCA CV input. To get the negative signal, just invert the source before and after the VCA. Then you have the positive and negative parts of the signal separated and can treat them differently. In terms of dedicated modules, the Addac 208 is nice (positive and negative half-wave rectification are present simultaneously, as well as full-wave rectification normal and inverted, plus a comparator).

(To patch full-wave rectification from building blocks, mult your source to both inputs of a 4 quadrant multiplier / "ring mod".)

dubonaire wrote:
Awesome!


Thanks a lot! SlayerBadger!
anotherjones
To follow up, the G2 signal converter provides 6 output polarities: Bi-Pol Pos, Bi-Pol Neg, Pos Pos, Pos Neg, Neg Pos, and Neg Neg. So if I understand right, it's a signal inverted as well as polarizer.
anotherjones
flo wrote:


Nice! Sounds like a lot of fun with the G2 hyper

Patching half-wave rectification is easy, if you don't have a dedicated module: Just patch an offset to a VCA, and your signal source to the VCA CV input. To get the negative signal, just invert the source before and after the VCA. Then you have the positive and negative parts of the signal separated and can treat them differently. In terms of dedicated modules, the Addac 208 is nice (positive and negative half-wave rectification are present simultaneously, as well as full-wave rectification normal and inverted, plus a comparator).

(To patch full-wave rectification from building blocks, mult your source to both inputs of a 4 quadrant multiplier / "ring mod".)

dubonaire wrote:
Awesome!


Thanks a lot! SlayerBadger!


You read minds! I was about to ask for your recommendations for eurorack rectifyers, converters.

I don't have any eurock yet. What's great about having had the G2 is that ...well all the principles are familiar! Just a question of underatanding all the functions of a given module to see if it may suit me. But I am very undeided about which function / logic / modulation modules to get. Of course most recommend Maths which looks the biz - would combine it with Erica dual EG/LFO. At the same time, I'm very interested in Batumi. Poti, Sewastapol + EG (Such as Befaco ADSR with the gate outputs) as a more expensive, but potentially more flexible option. Anyway, I guess this dilemma is for another thread, or better still, I should try and replicate the above options in the g2
anotherjones
Listening to your Road to the Stars EP, right up my street! we're not worthy

I look forward to going through your patch workouts.
flo
oops That is very kind of you, thanks again! Glad that you like my stuff! And feel free to post any questions about the patches.

Yeah, I'd advise to create new threads for specific modules / functions once you are actually researching them / about to buy them. You will get much more replies that way. Personally, I don't have any dedicated rectifiers, but tons of VCAs and inversion, so it's easy to patch up.

In general, let me say though that indeed you will profit a lot from the knowledge acquired with the G2. The basic principles are perfectly applicable to all modular formats.

Enjoy the ride! applause SlayerBadger! Guinness ftw!
SB-SIX
It can lead to cool fx. For instance, mix a variable shape osc with an inverted version. Modulate the shape with an lfo or whatever, keep the inverted signal static. What you get is only the difference of the shaping.
All kinds of variations on this can be cool too, like putting a sine thru a wavefolder, and mix it with the inverted original sine etc
Dave Peck
Inverting an audio signal in a modular can be very useful when your patch is mixing multiple filter outputs together. When you invert one of them, you get completely different sounds since the portions of the audio which are common to both filter outputs are now cancelling (and leaving only the portions which are not common to both filter outputs) instead of summing.

Simplest example is that you can turn a resonant low pass filter into a resonant high pass filter - mix the pre-filter raw osc signal and the low pass filter output together in a 50/50 mix. Now if you invert thelow pass filter output before sending it to the mixer, everything in the filter output which is also common to the unfiltered signal (all of the lower harmonics) will cancel in the mixer instead of summing together, leaving only the signal which is NOT common to both (higher harmonics, giving you a HIGH pass filter at the mixer output).

In general, the ability to invert an audio signal becomes really useful when you are combining two versions of a signal together in a mixer module - one version processed somehow like through a filter or phaser and the other version not processed, or both versions processed somehow, but processed in different ways. Inverting one of these signals gives you lots of additional sonic possibilities.
Don T
On polarity: Polarity and phase are NOT the same thing! Phase is moving the same signal in time relative to another, while polarity inversion is literally flipping the signal over. The confusion comes when people use nothing but Sine waves and square waves as test signals. With Sines and squares, 180 degrees back in phase and inverting the polarity look the same.

Not so with the sawtooth, which most sounds are, especially acoustic sounds. Picture a signal sawtooth in slow motion, with the rising end forward. This mimics most natural sounds, but in this case, picture a bass drum being struck. The pressure wave starts first (rising edge of sawtooth), followed by a rarefaction, or drop in pressure. If mimicking this electronically, this means the speaker cone needs to move outward rapidly to generate the pressure wave. If you invert the polarity of the signal, the speaker cone is pulled IN rapidly then recoils slowly. Basically it cuts the initial attack off the bass drum attack if you invert the polarity (And pretty much most other sounds as well).

To hear the difference and/or experiment, listen to a tune you know really well on your stereo system. Then flip the speakers connections, red-to-black and black-to-red (you must do both) and listen again. Correct polarity will sound more natural and have a fuller range. Inverted polarity will cut highs and lows especially, and make spoken/sung words less intelligible.

The problem is some commercial recording are inadvertently inverted, and some are not. Also, the playback device matters as well. Turntables do not invert (unless you screw up wiring the cartridge), nor do cassette decks. EVERY CD player I have ever listened to/worked on, from the $10 kind to the $13,000 kind, inverts the polarity of the signal. Luckily some higher-end preamps and DACs have polarity-inverting switches.
anotherjones
Really informative posts. thumbs up I will patch up filtering through inversion.

The G2 signal converter has both phase nversion and polarity inversion, therefore the output switches between six wave types Pos Pos; Pos Neg (i.e. phase inverted) Bipolar Pos; Bipol Neg; Neg Pos; Neg Neg. Is it common for hardware modular polarizers / inverters to have both phase and polarity invertion?

With an expanded G2 I rarely run out of memory so most of the time I just pull up another enveloppe or LFO when I want more modualtion options. So until now I've been using the signal converter for modulation sigmals, not audio, Much exploring for me to do!
cycad73
Dave Peck wrote:
.

Simplest example is that you can turn a resonant low pass filter into a resonant high pass filter - mix the pre-filter raw osc signal and the low pass filter output together in a 50/50 mix. Now if you invert thelow pass filter output before sending it to the mixer, everything in the filter output which is also common to the unfiltered signal (all of the lower harmonics) will cancel in the mixer instead of summing together, leaving only the signal which is NOT common to both (higher harmonics, giving you a HIGH pass filter at the mixer output).


This tip works great for bandpass.

I find that that the parallel connection [BP = LP (high cutoff) -(minus) LP (low cutoff)] usually results in a more usable sound than the series connection [BP = LP (high cutoff) -> HP (low cutoff)]. Gain staging and nonlinearities are more controlled, especially when the resonant peaks become close.

which makes me wonder, why did Moog decide on the LP/HP filter coupler rather than LP+LP (where there can be two resonant peaks)?

One caveat: Sometimes the filter outputs are already inverting, which I think is standard with all state-variable topologies. With others they are not. Having an inverter allows one to try both options. This is especially true with feedback: if uninverted feedback doesn't do anything interesting, usually inverted feedback will be interesting and vice versa.
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