# Change AC signal frequency without changing the voltage

I have a non constant -/+1 Volts to -/+7 Volts AC audio signal. I want to design a device that takes this input signal, doubles the frequency, without changing the voltage.

So, the output voltage is the same as the input voltage, but suppose the input signal's frequency is 82.4 Hz. The output should be 164.8 Hz. I want this device to work with other frequencies.

In other words, it shifts the pitch of the audio by one octave.

• What should it do with an audio signal that has many frequencies (as per normal audio signals)? Commented Feb 24, 2015 at 12:11
• I think this problem is a bit harder than you expect... Commented Feb 24, 2015 at 12:15
• What shape (waveform) is the input signal?
– user16324
Commented Feb 24, 2015 at 13:19
• Does it have to work in real time? Or could there be some delay? You can shift a tone using Fourier techniques. Obviously some higher tones will be shifted so high that they are no longer audible. Commented Feb 26, 2015 at 3:58

If you can find a way to control the the voltage output and power, a tried and tested way of frequency doubling is to multiply the signal by itself using an analogue multiplier. It does depend on a few other details you have not provided: Is it a pure sine wave, or does the source contain signals at more than one frequency?

Since

sin(t) * sin(t) = 1/2 - 1/2 cos(2t)

it is clear that multiplying a pure sine wave (i.e. of a single frequency) by itself, results in a wave with a frequency of twice the original. i.e. the factor 2 in cos(2t). Since the frequency is low, I'm sure a standard analogue multiplier will work well.

You may find the following to be useful: http://en.wikipedia.org/wiki/Analog_multiplier

Some actual chips and application notes are mentioned here: http://www.analog.com/en/search.html?q=analog%20multiplier

• I just want to comment that the multiplication could also be done in the digital domain. In some cases, this might be simpler. Commented Feb 26, 2015 at 6:07

1.) Digitial, sample and then replay at twice the speed.. (twice the clock frequency)

2.) Analog would be harder, maybe start with an analog multiplier. Keeping the amplitude constant would be tricky.