posted 09 July 2001 14:14            

whitenoiseremoval.zip

The "spook" world knows this technique as spectral whitening, but it is otherwise known as broadband filtering, and it can work magic on really noisy stuff.

A spectral whitener computes a long term mean background signal and produces the inverse of this background to apply against incoming signals. The net result is a general whitening of the output spectrum.

The attached sound shows an example of a recording from the 7.335 MHz CHU station and an oscillator mixed together. These both go through a graphic equalizer with a negative spectral slope.

Run the sound and wait a few seconds for the output to rise. The spectrum analyzer on top shows the input signal (you can listen to it by unclicking the NR button). The bottom display shows the output processed signal. Notice how the oscillator is neatly removed, and the overall spectral shape is more or less flattened in the main bandpass.

The advantage of spectral whitening, aside from removing bothersome constant background signals, both narrowband, and broadband like buzz, is that it provides a base spectrum from which noise removal systems like the multiband gain expander can work. No need to bother with manually shaping the noise threshold here.

Of course, I am talking about removing noise from extremely noisy situations, and Hi-Fi is the least of my concerns. The recording of the CHU broadcast gives a typically mild case in point. I don't know how a technique like this would work when the noise is many dB beneath a high quality audio recording. Probably not very well...

The attached Zip file contains the new spectral whitener along with the previous white noise removal Sounds. I had to remove the latter to make space for the new file.

- DM