Patent Review: Adaptive noise reduction filter with continously variable sliding bandwidth

Patent no 6,154,547

The patent provides method for dynamic noise reduction of an audio system. It compares average audio signal entering with that of a leaving signal. Adaptation rate is decided by requirement of filter bandwidth. The patent shows drawbacks of another patent (patent no 3,889,108) which reduces the noise using predefined known or estimated power value. The drawback of this is thatthe phase difference gives larger error signal. The low pass filter used has adjustable coefficients which produces variable cut off frequency. A comparator and multiplexer are used to provide required signal which then used in LMS adaptation block where adaptation parameter gets decided. This adaptation delta generates adaptive filter coefficients. Thus if audio signal is of low frequency range then the cut off frequency reduces and hence for high frequencies. Delay and attack signal are used for reducing and increasing the bandwidth. It also sets higher cut off frequency which is the limit of adaptation delta. The patent claims dynamic reduction of noise, adapting steps, adaptive filter coefficients, attack and decay time constant, upper and lower limit of cut off frequencies. Added to these it also claims about monitoring of quality, averaging of input and output signals, error calculation using multiplied average value. Thus the overall patent gives experimental design for dynamic noise reduction in audio reproduction and gives the advantage of each aspect of the design which improves the quality of audio at minimum requirement of energy.