A delayless subband adaptive filter architecture

Some adaptive signal processing applications, such as wideband active noise control and acoustic echo cancellation, involve adaptive filters with hundreds of taps. The computational burden associated with these long adaptive filters precludes their use for many low-cost applications. In addition, adaptive filters with many taps may also suffer from slow convergence, especially if the reference signal spectrum has a large dynamic range. Subband techniques have been recently developed for adaptive filters to solve these problems. However, the conventional approach is ruled out for many applications because delay is introduced into the signal path. This paper presents a new type of subband adaptive filter architecture in which the adaptive weights are computed in subbands, but collectively transformed into an equivalent set of wideband filter coefficients. In this manner, signal path delay is avoided while retaining the computational and convergence speed advantages of subband processing. An additional benefit accrues through a signifwant reduction of aliasing effects. An example of the general technique is presented for a 32-subband design using a polyphase FF'T implementation. For this example, the number of multiplies required are only about one-third that of a conventional fullband design with zero delay, and only slightly greater than that of a conventional subband design with 16 ms delay.