Why does autopowerspectrum of noise appear to have a type of symmetry at half Nyquist frequency?

[JWMooney]

Approximately APS(i) = APS(n/2-i) + 10^(5-20*i/n)
Signals aren't "mirrored" but noise is. I first noticed this using a sampling frequency of 8K, and sample size of 512, running the data through Hanning, AutoPowerSpectrum and plotting out 10*log(APS). The plot looks fine on a logarithmic frequency scale but when I switched to a linear frequency scale, it was obvious that all of the small noise peaks were mirrored at 2KHz. I tried this again with different noise files, windows, higher sampling frequencies and larger sample sizes but they all have a "noise mirror" at exactly half the Nyquist frequency or one quarter of the sampling frequency. I'm thinking that I must have
made a stupid error somewhere.

[JWMooney]

In NMR, this phenomena would be called Quadrature Ghosts. These are artifacts created when the intensities of the sin and cos detected signal are not equal. Since my problem stems not from mismatched two channel data but from one channel data, I'm not sure what that means.

[bilalD]

Hello

What kind of noise signal are you generating to get this data? If yopu have some sample code, I could try to play around with here. Because at this point, Im not sure whether what you are seeing makes sense or not.

Thanks

Bilal Durrani
NI

[JWMooney]

Here is a wav file that I'm analyzing.