Re: [Discuss-gnuradio] FFT --> IFFT

[Robert James]

On 11/18/13, Marcus Müller <address@hidden> 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.

>
> Greetings
> Marcus
>
> On 11/18/2013 03:40 AM, Robert James wrote:
>> When I take a signal, stream->vector-->FFT-->IFFT-->vector->stream,
>> I'd expect to get the original signal back.  Instead, I'm getting
>> something like the original signal, but following a very low frequency
>> carrier.
>>
>> I thought this was due to the limits in vector size, but making this
>> very high (many times the sample size and signal freq) and the problem
>> continues,
>>
>> What's weirdest is that a low pass filter or band pass filter on the
>> output don't seem to solve the problem!
>>
>> What is my mistake?
>>
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