enigma

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## Main Question or Discussion Point

Anybody know what a Hilbert transform does?

The NB4 function is looking at how the frequency of noise from a gearbox changes as a damaged tooth passes the sensor. I understand the concept, but I don't understand what the math is actually computing...NB4 uses the envelope of the signal, band-pass filtered about the mesh frequency. The envelope, s, is computed using the Hilbert transform and is given by:

[tex]s(t)=[[b(t)+i[H(b(t))]]] [/tex]

Where b(t) is the signal band-pass filtered about the mesh frequency. H(b(t)) is the Hilbert transform of b(t) and i is the sample index.

A Review of Vibration Based Techniques for Helicopter Transmission Diagnosticsby Paul D. Samuel and Darryll J. Pines, p19

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