On 11/18/13, Marcus M. [email protected] wrote:

Hi Robert!

This is strange – but could be explained by the fact that numerical

inaccuracy don’t allow us to *exactly* recreate all values during fft-ifft

operation.

Also, make sure you use a rectangular window.

Wow, switching to a rectangular window of fft_size solved it! I’m

baffled: I know windows are a way of pretransforming the wave prior to

FFT, to eliminate artifacts. I just used the default window. Why did

I need a rectangular window here? In what other cases do I need it?

Now, the only discrepancy I see is that the FFT->IFFT ended up

*increasing* the amplitutde by a constant (not sure why or what the

constant is).

How do you know there is this very low frequency carrier? how low is it? How

much power does it have?

On the osilloscope, I see the input signal as a constant wave, which

it is, and I see the output signal a wave of the same signal, but with

amplitude visually oscillating between very high and nothing. On the

audio, I hear the signal coming in and out, in and out, or, depending

on how I set it, I hear a ringing over it (which I assume is the beat

freq). On the FFT display, I see the carrier as a peak, and a long

thick band trailing off to the left.

I don’t know how to measure the power - according to the FFT display,

it’s not just at one freq, but trails off very far to the left.