MPEG-4 ALS is an extension of the MPEG-4 audio coding family for the lossless compression of audio data. The ALS compression scheme assures the perfect reconstruction of the input signal at the decoder. It is based on forward-adaptive linear prediction and variable length coding of the prediction residual signal. Fundamental structure of the encoder and the decoder is shown in Figure 6-18. Due to some additional tools on top of this simple fundamental structure, ALS offers remarkable compression performance with relatively low complexity. Additional tools include hierarchical block switching, inter-channel processing such as joint stereo and multi-channel coding, long term prediction, and progressive order prediction. ALS also offers much flexibility in terms of the compression-complexity tradeoff, ranging from very low-complexity implementations to maximum compression modes, and thus adaptability to different requirements.
Figure 6-18: Fundamental structure of MPEG-4 ALS lossless encoder and decoder
MPEG-4 ALS offers many features:
General support for virtually any uncompressed digital audio format including Sony WAVE 64 file format and Broadcast Wave Format (BWF).
Support for linear PCM resolutions of up to 32-bit at arbitrary sampling rates.
Multi-channel / multi-track support for up to 65536 channels including 5.1-, 7.1-, and 22.2 channels surround.
Support for audio data in IEEE 754 32-bit floating-point audio format
Quick random access to the encoded data
Support MP4 file format which allows the multiplex and synchronization with video.
Examples for the use of lossless audio coding in general and MPEG-4 ALS in particular include both professional and consumer applications:
Network distribution of audio files (IPTV, broadcasting, streaming, online music store, download)
Archival systems (broadcasters, studios, record labels, digital transfer)
Studio operations (storage, collaborative working, digital back-up, digital transfer)
High-resolution disc format
Portable music players
Compression performance is highly dependent on the nature of original signals. Compressed data size varies from 15 to 70 % of the original file size. As a result of comparison experiment, MPEG-4 ALS provides better compression performance and faster encoding and decoding time than software tools such as [b_FLAC], [b_OptimFrog], and [b_Monkey's]. The CPU load of real-time encoding and decoding is approximately 1 to 2% of a popular CPU. Real-time decoding of 48 kHz stereo signal requires 20 MHz ARM9 processor. Those results read that MPEG-4 ALS is one of the most efficient lossless audio compression schemes in terms of the compression performance and its processing time.
The specification of MPEG-4 ALS (ISO/IEC 14496-3) is associated with the conformance testing (ISO/IEC 14496-4) and the reference software (ISO/IEC 14496-5). By default, the bit stream of MPEG-4 ALS is recorded and transported with the ISO media file format. For broadcasting applications, MPEG-4 ALS stream can be transported over MPEG-2 TS, which is specified in [ITU-T H.222.0 Amd.1]. For consumer appliance applications, MPEG-4 ALS stream can be transported over S/P DIF channels, which is specified in [IEC 61937-10]. MPEG-4 ALS can be conveniently used for archiving applications if it is combined with the MPEG Professional Archival Application Format (PA-AF) in [ISO/IEC 23000-6].
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