User`s manual

70
4.4.9: Multiple Notch Filter
Application:
The Multiple Notch filter is used to remove, or "notch out", up to 16 single-frequency noises,
such as tones or whistles, from the input audio with minimal effect to the remaining audio.
This is accomplished using a frequency-sampling-synthesized 1024-tap FIR filter which is
calculated in the PC by the PCAP II Plus Master Control program. The Multiple Notch filter,
unlike the Notch filter, is able to tolerate moderate wow and flutter variances in the frequency
of a noise source as it implements a “square” notch, not a “V” notch. If the frequency of a
noise source varies excessively, then the 1-Channel Adaptive filter is recommended.
To properly utilize the Multiple Notch filter, you will first need to identify the noise
frequencies; this is best done using the Spectrum Analyzer window. See Section 4.8.2: for
complete instructions on operating the Spectrum Analyzer window.
Once the noise frequencies have been identified, set the Notch Freq(uency) of each notch to
be used to the desired noise frequency.
Usually, the noise frequencies will not remain constant but will vary slightly due to
modulation, wow and flutter, and acoustic "beating". Therefore, you may need to increase
the Notch Width of each notch from its minimum setting to avoid having the noise move in
and out of the notch.
Once all the Notch Freqs and Notch Widths have been entered, you will need to build the
actual filter using the Build command. Unlike the Notch Filter, which responds to controls
immediately, the Multiple Notch Filter must be constructed in the computer and transferred to
the external unit. A brief delay thus occurs before the filter is implemented.
Figure 4-31 depicts the control window for notches 1 through 8. A second window for 9
through 16 is button accessed.