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Buenos Notches - The Filter Wizard versus the vuvuzela

July 12, 2010 | Kendall Castor-Perry | 222900894
The Filter Wizard gets his kicks looking at ways to filter out the drone of the vuvuzela from World Cup broadcasts
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When the insistent drone of massed vuvuzela first imposed itself on the world during televised world Cup matches, I thought Huh! Well, theyll soon filter that out. But soon it became clear that removing this narrow-band noise was presenting challenges to broadcast engineers.

Figure 1 shows an FFT of a 1.024-second chunk of vuvuzela-dominated crowd noise. Pronounced peaks in the spectrum are at frequencies consistent with whats reported elsewhere: fundamental at ~230 Hz, with plenty of harmonics. The third harmonic at ~700 Hz is the most prominent component. Thats consistent with the buzzy quality of the sound.

A single vuvuzela produces a fundamental and harmonics with a narrow occupied bandwidth. Hundreds of them, each played in an unique on-off pattern, will give a broadened spectrum equivalent to a single carrier modulated by a random signal with Gaussian shape. So the elimination problem is more than removing a single tone and its harmonics; we have to suppress several bands of noise.

(Click on image to enlarge)

Figure 1: FFT of a sample of vuvuzela

The starting choice for notching multiple related frequencies out is often a lowpass comb filter. It is a two-tap FIR filter where you add the input signal to a delayed version of itself. For any input frequency at which the delayed term causes a multiple of 180 degrees phase shift, the input and delayed signals cancel out. A deficiency of this method here is that the resultant notches only occur at odd multiples of the first notch frequency, shown in
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