Squeezing through the pipe: digital compression 217 Advanced Audio Coding (AAC) A further development in MPEG-2 (Part 7) introduced Advanced Audio Coding. AAC provides a very high-quality audio coding standard for 1­48 channels at sampling rates of 8­96 kHz, with multichannel, multilingual and multiprogram capabilities. AAC works at bit-rates from 8 kbps for a monophonic speech signal to over 160 kbps/channel for very high-quality coding that permits multiple encode/decode cycles. Three profiles of AAC provide varying levels of complexity and scalability ­ Main Profile, Low Complexity Profile and Scalable Sampling Rate Profile. An AAC bit-stream is not backward-compatible, i.e. it cannot be read and interpreted by an MPEG-1 audio decoder. The MPEG-2 AAC standard provides very high audio quality at a rate of 64 kbps/channel for multi- channel operation, with the capability of up to 48 main audio channels, 16 low-frequency effects channels, 16 overdub/multilingual channels and 16 data streams. Up to 16 programs can be described, each consist- ing of any number of the audio and data elements. AAC adheres to the same basic coding paradigm as MPEG-1/2 Layer 3, but has additional coding tools and improves on details. Some of the improvements implemented by AAC are a filter bank with a higher-frequency resolution, improved unpredictability (entropy) coding and better stereo coding. Two new coding tools are an optional backward prediction (used only in the Main Profile) and noise shaping in the time domain, which mainly improves quality of encoded speech at low bit-rates. As a result, AAC is approximately 30% more bit-rate efficient than MPEG-1 Layer 3. The MPEG-4 AAC standard (Part 3) is a slightly modified version of MPEG-2 AAC, forming the basis of the MPEG-4 audio compression technology for data rates above 32 kbps per channel. Additional tools increase the effectiveness of AAC at lower bit-rates, and add scalability and error resilience charac- teristics. It has become popular for use on portable music players, particularly the Apple iPod.