Inverted signal

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How does inverting a signal effect the sound..

I've read much about inverting signals and phase but I'm not sure just how relavent this is to the actual sound ie if I invert a saw wave should I hear something different to the normal saw wave ? Same thing with phase from the 902's ...I don't really get it ...but seeing so much written about it I'm feeling a bit of a dunce.

[analogmonster]

Well, there are different scenarios:

Looking at a LFO signal it is obvious. A rising ramp modulates in a different way compared to a falling ramp, and a non symmetrical square wave modulation differs too if inverted, as the ON OFF times are exchanged.

In audio frequency range there is no audible difference between a saw signal with rising or a falling ramp, as the distribution of harmonics within the spectrum is the same. The same looking at a square wave (symmetrical or not) which is inverted.

It is more interesting if you MIX different signals. The voltages of all points in time are added to create a new output signal which changes its structure if you change the ramp from falling to rising (= invert) or invert the ON OFF relationship within the square. These different output signals can have different spectrums, and this is audible.

The same if you bring a signal out of phase and mix it to another one. The result is an audible difference (all phasers do that). In some cases 180 degrees out of phase and inverting it means the same, like for sine or triangle signals.

[Rarecomponentadsr]

Many thanks C.

And I appreciate the scope images you placed ..thanks for your time.

I understand what you described regarding mixing the signals and the various inversions etc.

So let's say I have two triangle waves ...I invert one ..then mix together to produce a different timbre ? I think that's what your saying.? Is this what you mean about "out of phase "?

But I'm still not sure about the input and outputs of the 902... The top and bottoms. Are the two different sets one for audio and one for CV ?

Lastly, 992... I'm assuming that the variable control + or - input is, as you described, to invert LFO etc to the 904a?

Thanks for your patience.

[analogmonster]

Many thanks C.

And I appreciate the scope images you placed ..thanks for your time.

I understand what you described regarding mixing the signals and the various inversions etc.

So let's say I have two triangle waves ...I invert one ..then mix together to produce a different timbre ? I think that's what your saying.? Is this what you mean about "out of phase "?

Quite close. If you invert a signal and just mix it to the original, the result is a flat line. This is what the term "invert" expresses; doing the opposite in every point of time. If you mix your original (lets say a saw) to OTHER signals and then mix the inverted signal (inverted saw) to the other signals WITHOUT your original saw you might hear a difference, as the mixed result differs in structure.

This is what I meant with "inverting a signal". Putting a signal "out of phase" is something different. You can use a phaser to put a signal out of phase for an adjustable amount of degrees compared to the original. Up to 180 degrees out of phase the resulting sound changes if you mix the phased signal to the original.

But in some cases the waveform of an inverted signal is identical to the same signal put out of phase for 180 degrees, e. g. if you look at a sine or a triangle. This is what I saw when I tested my 902 clone with a sine and a triangle signal and compared the outputs. Perhaps it was a bit confusing talking about phase differences, because indeed what the output amplifer of a 902 does is to INVERT a signal and not putting it 180 degrees out of phase. Only for symetrical waveforms like sine or triangle it is the same, a ramp for instance changes only if inverted but does NOT change if put 180 degrees out of phase.

Outputs of a 902 processing a sine: the inverted side looks like a sine beeing 180 degrees out of phase:

But I'm still not sure about the input and outputs of the 902... The top and bottoms. Are the two different sets one for audio and one for CV ?

Ah no, all the inputs and outputs can be used for DC and audio as they are DC coupled. They are differential stages with connectors on both sides which work in a contrary way. Examples:

DC / CV:
Top in: 1V
Top out: -1V
Bottom out: 1V

Bottom in: 1V
Top out: 1V
Bottom out: -1V

AC / audio:
Top in: non inverted
Top out: inverted
Bottom out: non inverted

Bottom in: non inverted
Top out: non inverted
Bottom out: inverted

Lastly, 992... I'm assuming that the variable control + or - input is, as you described, to invert LFO etc to the 904a?

This is one of an infinite amount of possibilities of using the inverting inputs and outputs of the differential amplifier stages. Just be aware of the fact that the inputs are processed directly on one side and inverted on the other side related to one dedicated output and vice versa. This enables functions and effects which are far beyond the possibilities of other VCAs

Thanks for your patience.

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Thanks once again C

This input of yours must be the most definitive explanation of phase and inverted signals on the Moog systems
That's currently on the web.

Whats intuitive on a hands on and ears way makes a little more sense now ...it's good to know the maths as well.

Anyhow it's made me a lot more aware of potentially more sounds and mixes available.

[GregAE]

Thanks once again C

This input of yours must be the most definitive explanation of phase and inverted signals on the Moog systems
That's currently on the web.

Whats intuitive on a hands on and ears way makes a little more sense now ...it's good to know the maths as well.

Anyhow it's made me a lot more aware of potentially more sounds and mixes available.

If you can get your hands on one, an oscilloscope will allow you to see exactly what's going on with waveforms. You can learn a lot from observing changes as you turn the knobs & change patches. It's an invaluable tool with a modular system, IMHO