User`s manual

7. FILTER CONTROL WINDOWS

7.1 One channel adaptive filter

Application:

The 1CH Adaptive filter is used to automatically cancel predictable and convolutional noises from

the input audio. Predictable noises include tones, hum, buzz, engine/motor noise, and, to some

degree, music. Convolutional noises include echoes, reverberations, and room acoustics.

Figure 7-1 1CH Adaptive Filter Control Window

Description of controls is as follows:

Filter Size: Used to set the number of FIR filter taps (filter order) in the adaptive filter. Filter

size is indicated both in taps and in milliseconds. Minimum Filter Size is 16 taps,

but can be set to as high as 3600 taps (if in Mono Single mode, See Section A-8.

Small filters are most effective with simple noises such as tones and music.

Larger filters should be used with complex noises such as severe reverberations

and raspy power hums. A nominal filter size of 256 taps is a good overall

general recommendation.

Adapt Rate: Used to set the rate at which the filter adapts to changing signal conditions

(Mathematically known as "Mu"). A Mu of 1 x 2

-14

provides very slow adaptation,

while a Mu of 32767 x 2

-14

provides fastest adaptation.

Larger adapt rates should be used with changing noises such as music;

whereas, smaller adapt rates are acceptable for stable tones and reverberations.

Larger adapt rates sometimes affect voice quality, as the filter may attack

sustained vowel sounds.

Prediction Sets the number of samples in the prediction

Span: span delay line. Prediction span is indicated both in samples and in milliseconds,

and can be adjusted from 1 to 999 samples. Shorter prediction spans allow

maximum noise removal, while longer prediction spans preserve voice

naturalness and quality. A prediction span of 2 or 3 samples is normally

recommended.

Adapt Mode: Selects Auto(matic) or Fixed adaptation rate. Auto is recommended. When

Fixed is selected, the specified Adapt Rate Mu is applied to the filter at all times.

However, when Auto is selected, the specified Adapt Rate is continuously