New fast algorithms for the low delay MDCT computation in the MPEG-4 AAC enhanced low delay audio coding standard

• New fast algorithms for even-length low delay MDCT (LD-MDCT) computation are proposed.
• A simple relation between the LD-MDCT and TDAC-MDCT matrices is derived.
• Exploiting this relation results in improved TDAC-MDCT-based fast LD-MDCT algorithms.

Recently, the MPEG committee has completed the development of a new audio coding standard, the MPEG-4 Advanced Audio Coding-Enhanced Low Delay (AAC-ELD). The state-of-the-art MPEG audio coding standards, such as MPEG-4 AAC Low Complexity (AAC-LC), High Efficiency AAC (HE-AAC) and AAC Low Delay (AAC-LD), utilize for the time-to-frequency transformation of an audio block and vice versa, the well known time domain aliasing cancellation modified discrete cosine transform (TDAC-MDCT). In order to achieve low algorithmic delay, the AAC-ELD has adopted a perfect reconstruction low delay filter bank, called the low delay MDCT (LD-MDCT). New fast algorithms for the LD-MDCT computation in the AAC-ELD are proposed. Further, a simple relation between the LD-MDCT and the TDAC-MDCT is derived. Exploiting this relation, as an alternative, the improved fast LD-MDCT algorithms based on the TDAC-MDCT are presented. They do not require reverse operations and sign changes with respect to both the time and frequency indices compared to existing TDAC-MDCT-based fast algorithms. Since the AAC-LC, HE-AAC and AAC-LD audio codecs use the TDAC-MDCT, the new proposed and improved TDAC-MDCT-based fast LD-MDCT algorithms provide the unified efficient implementation of LD-MDCT and TDAC-MDCT filter banks in all four codecs: AAC-ELD, AAC-LD, HE-AAC and AAC-LC.