STM32F4 FFT spectrum

[matas]

Posted on November 13, 2015 at 09:13

Hey, I am going to use FFT in one of my projects, and I have stumbled upon a question. The output buffer from the fft is the spectrum only in the first half of the buffer or the whole buffer is the spectrum. I found a tutorial which states that is only the first half:

http://stm32f4-discovery.com/2014/10/stm32f4-fft-example/

qoute:''

We will get back an array, basically 256 length, but results from 0 – 127 are valid, results from 128 – 255 are the same results as first one, but in reverse order.

''Yet in this forums discussion:

/public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/Flat.aspx?RootFolder=https://my.st.com/public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/STM32F407%20%2b%20DSP%20from%20STM32Cube_FW_F4_V1.7.0&FolderCTID=0x01200200770978C69A1141439FE559EB459D7580009C4E14902C3CDE46A77F0FFD06506F5B&currentviews=86

They use FFT length of 1024 and in the picture you can see that the spectrum goes right to the 1024.Can someone clearify this for me? #!stm32-!fft

[AvaTar]

Posted on November 13, 2015 at 12:03

These ''second half results'' relate to ''negative frequencies'', which are an inherent property of the Fourier functional transformation (can also be done analytically, not only numerically).

The magnitude and phase values for these ''negative frequencies'' are identical to the corresponding positive one's - a mirror image, stated differently. BTW, our math lecturer at university had a hard time to explain the relevance/correspondence of those negative frequencies to the real world ...

[Tesla DeLorean]

Posted on November 13, 2015 at 12:23

Signals in the negative frequency domain rotate backward