| Mpeg-4 Part 3 |
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Information AboutMpeg-4 Part 3 |
| CATEGORIES ABOUT MPEG-4 PART 3 | |
| audio codecs | |
| iso standards | |
| lossy compression algorithms | |
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BIFURCATION IN THE AAC TECHNICAL STANDARD The Advanced Audio Coding in MPEG-4 Part 3 was enhanced relative to what was previously specified in MPEG-2 Part 7, in order to provide better Sound Quality relative to the bit rate used for the encoding. It is assumed that any Part 3 and Part 7 differences will be ironed out by the ISO standards body in the near future to avoid the possiblity of future bitstream incompatabilites. At present there are no known player or codec incompatibilities due to the newness of the standard. AAC's mulitple codecs
AACPLUS, OR HE-AAC aacPlus is a (AAC), Spectral Band Replication (SBR), and Parametric Stereo (PS). aacPlus was standardized by the , which also employs SBR. aacPlus is supported in the and MPlayerOSX plan to soon support aacPlus, as well. External links
MPEG-4 AAC-SSR MPEG-4 AAC-SSR or '''MPEG-4 Advanced Audio Coding - Scalable Sample Rate''' was introduced by Sony to the MPEG-4 standard. The audio signal is first split into 4 bands using a 4 band Polyphase Quadrature Filter bank. Then these 4 bands are further split using MDCT s with a size ''k'' of 32 or 256 samples. This is similar to normal MPEG-4 AAC which uses MDCT s with a size ''k'' of 128 or 1024 directly on the audio signal.
MPEG-4 AAC-SSR is very similar to ATRAC and ATRAC-3 . Why AAC-SSR was introduced The idea behind AAC-SSR was not only the advantage listed above, but also the possibility of reducing the data rate by removing 1, 2 or 3 of the upper PQF bands. A very simple bitstream splitter can remove these bands and thus reduce the bitrate and sample rate. Example:
Note: although possible, the resulting quality is much worse than typical for this bitrate. So for normal 64 kbit/s AAC a bandwidth of 14-16 kHz is achieved by using intensity stereo and reduced NMRs. This degrades audible quality less than transmitting 6 kHz bandwidth with perfect quality. BIT SLICED ARITHMETIC CODING (BSAC) Bit Sliced Arithmetic Coding (BSAC) is an MPEG-4 standard (ISO/IEC 14496-3 subpart 4) for scalable audio coding. BSAC uses an alternative noiseless coding to AAC, with the rest of the processing being identical to AAC. This support for scalability allows for nearly transparent sound quality at 64 kbit/s and graceful degradation at lower bit rates. BSAC coding is best performed in the range of 40 kbit/s to 64 kbit/s, though it operates in the range of 16 kbit/s to 64 kbit/s. The AAC-BSAC codec is used in Digital Media Broadcast (DMB) applications. SEE ALSO
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