Gerhard Stoll

Method of transmitting or storing masked sub-band coded audio signals

In the transmission of audio signals, the audio signal is digitally represented by use of quadrature mirror filtering in the form a plurality of spectral sub-band signals. The quantizing of the sample values in the sub-bands, e.g. 24 sub-bands, is controlled to the extent that the quantizing noise levels of the individual sub-band signals are at approximately the same level difference from the masking threshold of the human auditory system resulting from the individual sub-band signals. The differences of the quantizing noise levels of the sub-band signals with respect to the resulting masking threshold are set by the difference between the total information flow required for coding and the total information flow available for coding. The available total information flow is set and may then fluctuate as a function of the signal.

ISO/MPEG-Audio: A Generic Concept for High Quality Audio Coding of Two-channel and 5+1 Multichannel Sound

Summary Since 1989 the ISO MPEG committee is developing a coding standard for video and audio applications in the field of multimedia and broadcast, such as cable and on-air. The first milestone in MPEG 1 Audio has been achieved with the ISO/IEC 11172-3 in November 1992. It follows a three layered structure in order to fullfil the requirements of various applications. Transparent audio quality at a bit rate of about 100 kbit/s per monophonic channel is achieved. Providing a significant reduction in the bit rate of a high quality audio, this generic coding system is applicable to a wide range of consumer and professional electronics seryices, such as storage on inexpensive storage media or transmission over channels with limited capacity (e.g. Digital Audio Broadcasting or ISDN audio transmission). Because of the excellent performance of this standard, C.C.I.R. (Comitte Consultatif International des Radio-communications) is producing recommendations to use Layer II worldwide for contribution, distribution and emission, and ventually Layer III for commentary applications. ISO/IEC MPEG 2 Audio is the consequent extension from two to five audio channels providing backward compatibility to MPEG 1. The main aspects are high quality of five (+1)_audio channels, low bitrate and backwards compatibility - the key to ensure that existing 2-channel decoders will still be able to decode compatible stereo information from five (+1) multichannel signals. The envisaged applications are High Definition Television systems such as HDTV-T, HD-SAT, ATV, HD-MAC, as well as digital storage media. MPEG 2 Audio is based on standards and recommendations from international organisations such as BR (formerly CCIR), SMPTE and EBU. International standardizing bodies will ensure highest audio signal quality by extensive testing. For audio reproduction the loudspeaker positions left, center, right, left and right surround are used - according to the 3/2-standard. This paper describes the efforts and present status of the ISO MPEG Audio group in defining standards based on high quality coding. Techniques of redundancy reduction and perceptual-based irrelevance reduction have been employed to design efficient coding algorithms which can achieve substantially more compression than was thought possible only a few years ago. In particular, the knowledge of auditory perception has lead to hearing-specific coders which perfom remarkably well if sophisticated forms of adaptive noise shaping controlled by psychoacoustic criteria are employed.