Perfect Reconstruction Cosine/Sine-Modulated Filter Banks in the Dolby Digital (Plus) AC-3 Audio Coding Standards

Abstract

The Dolby Digital (AC-3) and the Dolby Digital Plus or Enhanced AC-3 (E-AC-3) audio coding standards developed by the Dolby Laboratories are currently the key enabling technologies for high-quality compression of digital audio signals. For the time/frequency transformation of an audio data block, and vice versa, the AC-3 and E-AC-3 have adopted the modified discrete cosine transform (MDCT) as a perfect reconstruction cosine-modulated filter bank. The AC-3 besides the MDCT defines additional two variants of cosine-modulated filter banks called the first and second short transforms. Moreover, the current AC-3 and E-AC-3 codecs for better spectral estimation and for phase angle adjustment have adopted the modified discrete sine transform (MDST) which together with the MDCT forms a complex filter bank. This chapter is devoted to the perfect reconstruction cosine/sine-modulated filter banks used in the Dolby AC-3 and E-AC-3 audio coding standards. The chapter presents: Definitions of the analysis/synthesis AC-3 filter banks, their general symmetry properties both in the time and frequency domains, and their efficient unified implementations. Matrix representations of AC-3 filter banks, [54] their properties and relations among transform (sub-)matrices provide the basis to derive relations between the frequency coefficients and the time domain aliasing data sequences of AC-3 transforms, and in particular, the basis for derivation of a fast algorithm for conversion of frequency coefficients of AC-3 transforms directly in the frequency domain. Finally, conversion methods of the MDCT to MDST frequency coefficients directly in the frequency domain are presented.