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Mpeg-4 Part 3MPEG-4 Part 3 (formally ISO/IEC 14496-3) is, as the name suggests, the third part of the ISO/IEC MPEG-4 international standard. It specifies audio coding methods. 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. One codec defined in MPEG-4 Part 3 is known as Low Complexity Advanced Audio Coding (LC-AAC). Another one is known as High-Efficiency Advanced Audio Coding (HE-AAC) or "aacPlus". Another one is known as Scalable Sample Rate Advanced Audio Coding (AAC-SSR). aacPlus, or HE-AAC aacPlus is a lossy data compression scheme for audio streams. It is based on MPEG-4, and combines three techniques: Advanced Audio Coding (AAC), Spectral Band Replication (SBR), and parametric stereo (PS). aacPlus was standardized by the MPEG under the High Efficiency AAC (HE-AAC) name. The codec can operate at very low bitrates and is good for Internet radio streaming. A 48 kilobit-per-second stream is considered to have higher quality than 128 kbit/s MP3. aacPlus is supported in the free FAAD2 decoding library, and hence in many free players, and in Winamp and foobar2000. 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 MDCTs with a size k of 32 or 256 samples. This is similar to normal MPEG-4 AAC which uses MDCTs with a size k of 128 or 1024 directly on the audio signal. The advantage of this technique is that short block switching can be done separately for every PQF band. So high frequencies can be encoded using a short block to enhance temporal resolution, low frequencies can be still encoded with high spectral resolution. However, due to aliasing between the 4 PQF bands coding efficencies around (1,2,3) * fs/8 is worse than normal MPEG-4 AAC. 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: 4 subbands: bitrate = 128 kbit/s, sample rate = 48 kHz, f_lowpass = 20 kHz 3 subbands: bitrate ~ 120 kbit/s, sample rate = 48 kHz, f_lowpass = 18 kHz 2 subbands: bitrate ~ 100 kbit/s, sample rate = 24 kHz, f_lowpass = 12 kHz 1 subband: bitrate ~ 65 kbit/s, sample rate = 12 kHz, f_lowpass = 6 kHz 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. *.m4a *.m4a is the file extension attached to names of files using the MPEG-4 Audio standard. The preferred extension for any MPEG-4 file is *.mp4, which is also mentioned in the MPEG-4 standard. Apple started using the *.m4a extension in their iTunes software to distinguish between MPEG-4 Video and Audio files. Currently, most software that supports MPEG-4 Audio (Advanced Audio Coding) also supports the *.m4a extension. Protected MPEG-4 Audio When these files have digital rights management (DRM) restrictions applied to them, their extension are changed to *.m4p, the p standing for protected. Music files purchased from the iTunes Music Store have this extension. External links See also
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