Audio Signal Processing and Coding

8.4 ADAPTED NONUNIFORM FILTER BANKS

The most popular method for realizing nonuniform frequency subbands is to cascade uniform filters in an unbalanced tree structure, as with, for example, the DWPT. For a given impulse response length, however, cascade structures in general produce poor channel isolation. Recent advances in modulated filter bank design methodologies (e.g., [Prin94]) have made tractable direct form near perfect reconstruction nonuniform designs that are critically sampled. This section is concerned with subband coders that employ signal-adaptive nonuniform modulated filter banks to approximate the time-frequency analysis properties of the auditory system more effectively than the other subband coders. Two examples are given. Beyond the pair of algorithms addressed below, we note that other investigators have proposed nonuniform filter bank coding techniques that address redundancy reduction utilizing lattice [Mont94] and bidimensional VQ schemes [Main96].

8.4.1 Switched Nonuniform Filter Bank Cascade

Princen and Johnston developed a CD-quality coder based upon a signal-adaptive filter bank [Prin95] for which they reported quality better than the sophisticated MPEG-1 layer III algorithm at both 48 and 64 kb/s. The analysis filter bank for this coder consists of a two-stage cascade. The first stage is a 48-band nonuniform modulated filter bank split into four uniform-bandwidth sections. There are 8 uniform subbands from 0 to 750 Hz, 4 uniform subbands from 750 to ...