Hertz changer

[Tympanic]

Hello there space travellers!
I'm having a problem understanding how to change hertz frequencies. Doesn't Voltage change the amlitude but how do I change the time period? current? I know I can change it with a resistor in a electric circuit but what am I actually doing with the resistor, am I voltage dividing or current dividing or something completely different?
cheers from Volcanoland!

[Steerpike]

Originally Posted by Tympanic

but how do I change the time period... I know I can change it with a resistor in a electric circuit

No you can't. The only way to change the frequency of an AC signal is by a process called heterodyning, where you multiply it by an additional AC signal. Wikipedia will explain the details.

[Tympanic]

Great! that's what I've been looking for. so when you're changing the pitch in oscillators is that based on the heterodyning principle?

[bob91343]

Well that's an oversimplification. There is a whole pile of methods to create a new frequency. Oscillator, synthesizer, heterodyne, and more. They are all inefficient and complex.

I suggest you get a motor and a generator and connect them with a variable ratio gearbox. A bit noisy and unstable but it works. Or develop what is called a quasi-sine wave (better known as some strange waveform that doesn't resemble a sinusoid) with switching transistors controlled from a crystal controlled frequency synthesizer.

[Steerpike]

Well we dont know WHAT frequency Tympanic is trying to change. Is it mains waveforms? Is is a musical instrument tone? Is it a radio frequency? Is it a DAC clock?

Strictly speaking, oscillators & synthesisers don't change the frequency - they create a new wave from scratch - which may be created in such a way as to track the original in a modified way.
Heterodyning does it by physically altering the original waves shape through sinusoidal multiplication.

Some oscillators - such as older test bench oscillators- use heterodyning because if low freqency outputs are needed, two high frequency oscillators provide a more stable and cheaper solution than to build a true low frequency oscillator.

Radios, TV sets, almost all communications receivers, use heterodyne oscillators to recover the high frequency broadcast wave.

Heterodyning is also used in organs and guitar effect pedals to create vibrato.

If you have a simple single frequency oscillator (a Hartley or Colpitts oscillator etc), they oscillate at a particular frequency because of the resonance of the LC tank circuit. A capacitor+inductor combination has a natural frequency at which it 'rings'. Altering either component in the tank changes its natural frequency.

A Wien bridge oscillator is somewhat different: Wien's bridge is a network of capacitors and resistors that has a specific phase shift depending on the freqency. If you design it right, you can create a feedback loop where you get reinforcing feedback (i.e., the phase of the feedback is right to reinforce) which promotes amplification, then oscillation, at only one frequency

[bob91343]

I have an old ring modulator unit from the early days of 'musical' synthesizers. It shifts the frequencies of the tones. Really a stupid idea but they sold them, and now I find myself with one in nice condition. Using it is out of the question for me, since I consider myself a musician.

[Steerpike]

Originally Posted by bob91343

Really a stupid idea but they sold them, and now I find myself with one in nice condition.

Not such a stupid idea I think. Ring modulators we (are) a core building block of all analogue synthesizers such as the Moogs, and even the semi digital ones like Roland's "Linear Algorithmic" series like the famous D50. Or are those the very 'instruments' you disdain, being a musician???

Oh and it is the primary tone generator in any Theremin.

What you can do with a ring modulator ON ITS OWN is a bit limited I guess... unless you have an original Moog to patch it into.

[bob91343]

Yeah I confess to not liking this electronic trend in instruments. I finally broke down and bought an electronic piano only because nobody has pianos any more. But I can't hear any musicality in the output of a modulator.

Maybe my modulator has historic value.

Anyway this is off topic to some extent.

[Tympanic]

brilliant,,, thanks allot guys. well the frequencies that I'm planing to change is both guitar tone and also I'm planing on making some oscillators and even blend it all... I'm really new to this but this is too exciting!!!! I just really wanted to know why the pitch changes when I turn the potentionmeter and understand more what I'm doing to the wave by adding/removing resistance and such and such,,,, now I know more what to look for...... I've got allot of reading to do
cheers from Iceland

[PeteMcK]

Guitar Pitch Shifter - Pitch shifting
4 Band Pitch Shifter
BossArea - Boss PS-3 Digital Pitch Shifter/Delay
Audacity will do it for already recorded signals