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
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].