The present invention relates to filter designs, and more particularly to a variable passband autoregressive moving average (ARMA) filter using a non-causal filter design.
A problem common to several signal processing applications, including radio frequency (RF) tuning, audio tone controls, variable transient filtering, sampling rate conversion, jitter/wander compensation and/or measurement and the like, is that a zero-pole based digital filter, such as an infinite impulse response (IIR) and finite impulse response (FIR) combination, needs to have a variable frequency response at a given sample rate, or vice versa—constant frequency response at a variable sampling rate. In video there is such a need for a variable bandwidth video luminance filter in order to reduce sensitivity of transients seen by a signal out-of-range detector/alarm. Such a filter needs to have a nominal lowpass response, as given by IEEE-P205, and via one control parameter continuous range from full bandwidth to some small fraction of nominal bandwidth. An additional requirement is that the filter needs to have greater computational efficiency—faster/less expensive—than existing filter methods in order to have realtime performance in a video waveform monitor, for example.
Prior digital variable bandwidth filters include the use of:
1) either only FIR or only IIR filters;
2) analog filter simulation with variable C, L, R, gyrator, etc.;
3) discrete bandwidth selection of ARMA design.
The first class satisfies the continuously variable bandwidth requirement, but generally requires more computation to approximate a pole with many zeroes and vice versa. The second class also satisfies the continuously variable bandwidth requirement, but generally does not work well when the cut-off frequency approaches Nyquist—at which point instability may take place for higher order filters or extra processing may be required to prevent it. Also in the second class mapping the passband control parameter to new filter component values is not always readily apparent or may be difficult or impractical to implement. The third class does not satisfy the continuously variable bandwidth requirement.
What is desired is a stable, continuously variable bandwidth controllable via one parameter, digital filter for processing signals from full bandwidth to a small portion of the bandwidth.