Enhanced Audio For DTV: ATSC Publishes New Digital Audio Compression Candidate Standards (From The ATSC Newsletter)

2-Jun-04

ATSCís new Enhanced AC-3 specifications will expand audio capabilities for broadcast, cable, satellite, and DVD applications. ATSC first standardized the AC-3 digital audio system in November of 1994. Since then AC-3, popularly known as ìDolby Digitalî, has become widely used in digital television systems around the world. ìEnhanced AC-3 is a fine example of how ATSC is evolving standards in response to marketplace requirements,î remarked Mark Richer, ATSC President. ATSC issued a Request for Information in December 2002 and Dolby Laboratories responded by submitting the Enhanced AC-3 (E-AC-3) specification. E-AC-3 offers new coding tools that fundamentally improve performance and new features that allow operation over a wider range of bit-rates and numbers of channels. It can also be converted into legacy AC-3 for playback compatibility on consumerís existing A/V decoders. According to Craig Todd, Dolbyís Senior Technologist, ìE-AC-3 or ëDolby Digital Plusí has been designed to meet four major requirements of a next-generation broadcast audio codec: compatibility with legacy equipment, improved spectrum efficiency, cost effectiveness, and interoperability with other future media formats.îThe enhancements to AC-3 are contained in ATSC Candidate Standard documents CS/T3-613 and CS/T3-614. Document CS/T3-613 specifies revisions to the ATSC Digital Audio Compression Standard (A/52) that can be used in a variety of media. Document CS/T3-614 describes additions to the ATSC DTV Standard (A/53) that specify use of E-AC-3 in the Enhanced VSB (E-VSB) robust mode currently under developmentin ATSC. First standardized in 1994, the AC-3 digital audio system known as ìDolby Digitalî is widely used around the world. The new enhancements to AC-3, which will be marketed as ìDolby Digital Plus,î are contained in Candidate Standards documents CS/T3-613 and 614. Enhanced AC-3 (E-AC-3) will provide the industry with expanded audio capabilities that can be used for broadcast, cable, satellite, and DVD applications.CS/T3-613 adds technical specifications to the ATSC Digital Audio Compression Standard (A/52) that can be used with a variety of media. The document details features that are relevant to ATSC television systems, and also specifies features that are likely to be used in other (non-ATSC broadcast) applications. A/52 is the fundamental source document for AC-3 and is relied upon by other (non-broadcast) industries. Inclusion of the additional features in the E-AC-3 spec-ification will enable the use of E-AC-3 in other applications, indirectly benefiting the ATSC digital television system. CS/T3-614 describes additions to the ATSC DTV Standard (A/53) that specify use of E-AC-3 in the Enhanced VSB (E-VSB) robust transmission mode currently under development in ATSC. The E-VSB mode would allow broadcasters to trade-off through-put for robustness. With an E-VSB transmission, some of the approximately 19.4 Mbps data is allocated to the robust mode and the rest is allocated to the normal 8-VSB mode. The robust mode symbol stream includes additional forward error correction bits to improve reception under weaker signal and stronger multipath (ghost) conditions.Since E-AC-3 builds upon the current version of AC-3 specified in ATSC Standard A/52A, all decoders for the enhanced version will also decode all legacy A/52 AC-3 bit streams. In addition, although the new enhanced audio format is not directly compatible with current A/52 decoders, it is feasible to perform a modest complexity conversion into a compliant A/52 bit stream syntax, thus enabling backwards compati-bility to legacy decoders that have S/PDIF bit stream inputs.Important technical capabilities of Enhanced AC-3 that relate directly to ATSC broadcast applications include the following:Expanded data rate flexibility: E-AC-3 allows the number of blocks per sync frame and the number of compressed data bits per frame to be adjusted to achieve significantly more data rate flexibility than standard AC-3, including a greater maximum theoreticaldata rate and finer data rate granularity.Spectral extension: Enhanced AC-3 decoders support a new coding technique called spectral extension. Like channel coupling, spectral extension codes the highest frequency content of the signal more efficiently. Spectral extension recreates a signalís high frequency spectrum from side data transmitted in the bit stream that characterizes the original signal, as well as from actual signal content from the lower frequency portion of the signal. Because in some circumstances it may be desirable to use channel coupling for a mid-range portion of the frequency spectrum and spectral extension for the higher-range portion of the frequency spectrum, spectral extension is fully compatible with channel coupling. Both tools can be enabled at the same time, for different portions of the frequency spectrum.Transient pre-noise processing: This is an optional decoder tool that improves audible performance through the substitution of audio segments just before transients to reduce the duration of pre-noise distortions. This technique is called time scaling synthesis, where synthesized PCM audio segments are used to eliminate the transient pre-noise, thereby improving the perceived quality standard of low-bit rate audio coded transient material. To enable the decoder to efficiently perform transient pre-noise processing with no impact on decoding latency, transient location detection andtime scaling synthesis analysis is performed by the encoder and the information is transmitted to the decoder. The encoder performs transient pre-noise processing for each full bandwidth audio channel and transmits ìhelperî information once per frame, only when necessary (for example, when transients are present that will benefit from the technique).Adaptive hybrid transform processing: In 1995, the transform employed in A/52 AC-3 - based on a modified discrete cosine transform (MDCT) of length 256 frequency samples - provided a reasonable tradeoff between audio coding gain and decoder implementation cost. With continuing advances in silicon manufacturing processes over the years, the integrated circuit complexity that constitutes a reasonable level has now increased. This increase in chip performance provides an opportunity toimprove the coding gain of AC-3, and hence perceptual audio quality at a given bit-rate, by increasing the length of the transform. This is accomplished through use of the Adaptive Hybrid Transform (AHT), which adds a second transform in cascade in order to generate a single transform with 1536 frequency samples. Enhanced coupling. This is a new tool that improves the imaging properties of coupled signals by adding phase compensation to the amplitude-based processing of conventional coupling. Prior to downmixing the coupled channels to a single composite signal, the encoder derives both amplitude and additionally interchannel phase information on a sub-band basis for each channel. The phase information includes a decorrelation scale factor as a measure of the variation of the phase within a frame. This side chain information is transmitted to the decoder once per frame. The decoder uses the information to recover the multiple output channels from the composite signal using a combination of both amplitude scaling and phase rotation. The result is an improvement in soundstage imaging over conventional coupling. This improvement allows the technique to be used at lower frequencies than conventional coupling, thus improving coding efficiency.The E-AC-3 documents are available at www.atsc.org under Candidate Standards.

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