Oscillator problem discovered and fixed.

[rocknrollkat]

Hi Gang,
As you know if you're following my posts, I found and solved an interesting problem with Flowstone's meters.
So now it's time to develop an amplitude modulator.
Simple enough, feed a gain cell with a signal and a modulating LFO signal and we're done.
But wait.
Why is my 'scope giving me a REPEAT of the meter problem when there are no meters in the hookup ?
I modulate a sine wave with a sawtooth wave and expect to see a slowly rising sine wave, then an instant drop to zero, repeat and repeat.
But what do I see ?
The sine wave rising in amplitude and falling in amplitude at the SAME RATE, but DOUBLE the frequency of the sawtooth modulating wave !
And to boot, this is NOT just happening at VLFs, but at higher frequencies as well !
The modulated signal is Double Timing !
So I went back to my meter biasing schematic and applied it to the modulating oscillator and the problem is solved !
This is important stuff, if any of you are wondering why your VCFs, VCA,s etc. are not responding correctly, this is the reason !
Apparently Flowstone converts ALL below "0" voltages (numbers) to positive values.
And in case you're wondering (I know I was), did the meters react the same way with this lashup ?
Yes they did.
At higher frequencies their internal ballistics MASKED the problem that was OBVIOUS on the scope !
I built a demonstrator for you, the "A" setting on the slide switch is the stock oscillator, the "B" setting on the switch is my biased and scaled oscillator output.
I hope this helps some of you, I know it's solving a lot of problems for me before they start !
Remember, if you're using LFO control signals for your VCFs, etc. they will sound different now that you see what's really happening.
Here's the test bench hookup, select different waveforms for the signal and modulating LFO, do as many A/B comparisons as you like, open up the box and see how I biased and scaled the oscillator output, and have fun !

ROXY

[Spogg]

If you modulate a multiplier with a standard LFO, the LFO will give an output swing between -1 and +1. This is bipolar.

On the negative half cycle of the LFO it will correctly cause the multiplier to multiply by -1, so the effect is that the phase of the audio signal is inverted every half cycle of the LFO.

You need to make the LFO unipolar instead of bipolar, so it swings only between 0 and the value you want.
Multiply the LFO by 0.5 and add 0.5 to the result. It will then swing between 0 and 1.

Hope this helps!

Cheers

Spogg

[rocknrollkat]

Hi Spogg,
Thanks for the reply, and if you think about it, you've just suggested another way of biasing the oscillator.
We're both doing the same thing, I'm adding 1 and dividing by 2.
The addition of 1 guarantees that when the oscillator dips to -1, the output will be -1 + 1 or 0, which is what we want.
Dividing by 2 guarantees that when the oscillator swings to +1, the output will be 1+ 1 = 2, divided by 2 = 1, the value we want.
I wanted to see what would happen if I increased the oscillator output beyond 1, and erratic behavior set in again, so I came up with my scaling hookup, which keeps things under control.
I can pump the oscillator to 2 volts and the results are still in order.
I just pumped the osc. to 4 volts and still no problem.
Why would I want to increase the output to 4 volts ?
To see what would happen.
It's all part of catching problems early.
Thanks again for your input !

ROXY

[rocknrollkat]

Hi Spogg,
Addendum....
I built your schematic, it works a predicted, same as mine.
However if I increase the output with your hookup, the same problems arise, and I'd have to develop a scaler for your method. My scaler doesn't work with your method.
My scaler works fine with my approach.
1 volt PtoP, no problem.
4 Volts PtoP, still no problem.

It's all part of the learning process.
Thanks again !

ROXY

[tester]

Amplitude modulation means, that both, negative part of modulating signal and positive part of modulating signal - will shape the envelope of what you modulate. It's math. If modulator is a sine or any symmetric signal - then the modulated result will sound/look as if the frequency was doubled.

At higher modulation frequencies (and at frequencies comparable to modulated signal), things start to change a little bit, depending on context.

Basically, modulated signal will be doubled, but depending on frequency relationship between carrier and modulator (actually it doesn't matter which is which, because it's multiplication) - one pitch may be higher and the other one lower and pitch-inverted, or the result can be something crossed in-between or - one pitch higher and the other nonexistent (try to modulate sine by sine multiple times, each sine with the same frequency).

[rocknrollkat]

Thanks for your input.
True, it's all math.
But here's where I'm going.
LFOs are used in music synthesis as a mainstay of signal shaping.
If you have a filter whose modulating voltage input is calling for a range of 0 to 1 volt, then anything BELOW 0 volts will at best do nothing, and at worst give some pretty erratic performance.
Since I will be designing some fairly interesting voltage controlled devices, I need to have a stable, reliable and scalable LFO source, with output swings always at 0 to 1 volt.
And let's face it, if you've ever worked with real world electronics you know that meter needles do NOT take it upon themselves to 'double time'.
So if we're emulating real world synthesis, then I need instruments that respond in kind.
I hope this sheds some light on what I'm doing.

ROXY

[rocknrollkat]

Amplitude modulation means, that both, negative part of modulating signal and positive part of modulating signal - will shape the envelope of what you modulate. It's math. If modulator is a sine or any symmetric signal - then the modulated result will sound/look as if the frequency was doubled.

The carrier wave and the modulating voltage interact to produce a modulated carrier wave and two acjacent sidebands, both upper and lower. There is no 'frequency doubling' involved unless the carrier and modulating signals have direct frequency relationships to each other.

At higher modulation frequencies (and at frequencies comparable to modulated signal), things start to change a little bit, depending on context.

Basically, modulated signal will be doubled, but depending on frequency relationship between carrier and modulator (actually it doesn't matter which is which, because it's multiplication)

The sidebands (upper and lower) are added and subtracted from the carrier frequency, not multiplied.

- one pitch may be higher and the other one lower and pitch-inverted, or the result can be something crossed in-between or - one pitch higher and the other nonexistent (try to modulate sine by sine multiple times, each sine with the same frequency).

This is called 'overmodulation', when the modulating voltage swings exceed 100%. The percent of modulation typically varies between 0% and 100% and is called the "Modulation Index".

I hope everyone takes these comments in the spirit in which they are intended, to add to the knowledge base and not be confrontational.

ROXY

[tester]

Let's not confuse things.

In amplitude modulation - physical (time domain) signals are multiplied by each other.

What is added or subtracted - are spectral representations.

This is important to remember, because with multiplication - carrier may become modulator and vice versa.

[rocknrollkat]

Tester,
In your original reply to me you stated....

It's math. If modulator is a sine or any symmetric signal - then the modulated result will sound/look as if the frequency was doubled.

Okay, let's say that's true.
Consider a tone of 1,000 Hz. as the tone we want to amplitude modulate.
Let's modulate it with a 3 Hz. tone, under the 100% modulation index figure to keep things simple.
How does the modulated result, the 1,000 Hz. wave, sound and look like it is doubled, or 2,000 Hz. by modulating it with a 3 Hz. tone, as you imply ?

We all know that what you will hear is a 1,000 Hz. tone"warbling" at a 3 Hz. rate.
No "doubling", no 2,000 Hz. tone.

ROXY

[rocknrollkat]

Hi gang,
For those of you who are following this discussion, here's a terrific explanation of what's going on with Amplitude Modulation.
And yes, the Carrier and Modulator (message) are SUMMED, not multiplied.

From the page....

"When both the waves are combined and put over a single conductor, their amplitudes are summed up and the resultant amplitude is higher than the amplitude of message signal as shown."

That is why we hear a warbling tone when 1,000 Hz. is modulated by 3 Hz.
Enjoy the video presentation, I sure did !
Here's the link....

https://www.tutorialspoint.com/communic ... lation.asp

ROXY