On 11/18/13, Marcus Müller <mar...@hostalia.de> 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? >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> Discuss-gnuradio@gnu.org >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
