Broadcast Engineering - February 2009 - (Page 22)

TransiTion To digiTal Digital hanDbook region and the unified requirement from the Technical Group of the HighDefinition Television Forum in Spain. The European Information & Communications Technology Industry Association (EICTIA) has developed an “HD TV” logo that qualifies products as meeting a minimum requirement for receivers. The audio specification includes the following decoders: • two-channel, MPEG-1 Layer 2; • for satellite and cable, at least one of AC-3, enhanced AC-3 or HE-AAC; and • for terrestrial, at least one of enhanced AC-3 or HE-AAC. Note that enhanced AC-3 is also called Dolby Digital Plus. The majority of the national specifications are converging on these same codecs. Audio description Audio description (AD) is additional commentary that explains points like body language, expressions and movements for those with impaired sight. Existing systems use a separate low bit-rate MPEG-1 audio stream, which is mixed with the main program in the receiver. Receivermixed AD is described in an annex to the ETSI encoding standard. As some users may not have acute hearing, the main program audio can be dipped during descriptive passages. The broadcaster transmits metadata to control the program level, and also to pan the AD to a suitable position on the sound stage. The receiver allows the listener to adjust the AD level independently from the overall volume to get an optimum mix to suit the user. Dolby Digital Plus also allows for the mixing of secondary audio services like AD and commentaries within the main stream. (See Figure 1 on page 20.) Audio description is not used in many countries. Instead, closed captions have proved far more popular with service providers and have the advantage of needing a low data rate. Multiple languages MPEG-1 Layer 2 can encode two channels as stereo or two mono language tracks. The ETSI standard does not recommend the latter. Instead each language is encoded as a separate stream, which could be stereo or surround. Many European broadcasters transmit more than one language. This could be the local language and the original soundtrack, or a number of regional languages. For example, in Spain, receivers should be able to display menus in Spanish (Castilian), Catalan, Galician, Valencian and Basque, although one service may only carry two of the languages, depending on the region where it is being transmitted. A broadcaster must compromise on the number of languages. Each stream consumes more bandwidth, so more languages means fewer channels per multiplex. A common practice is to transmit the primary language as 5.1 at 384kb/s and a secondary language as stereo at 128kb/s to 192kb/s. Ancillary audio data The ETSI encoding standard allows the audio stream to carry metadata to control consumer-side dynamic range processing, downmix and dialog normalization. The EBU Tech 3307 also specifies that audio/video sync should be within 5ms early (audio) and -15ms late with respect to decoded video. Audio interfaces Although the new standards provide for surround sound, most viewers still watch in stereo or even mono. As a consequence, the standards and recommendations describe alternative outputs from STBs or receivers to feed additional equipment in the home. Analog stereo or stereo downmix is still a necessity, and for digital interfaces like HDMI, uncompressed stereo PCM as well as Dolby and DTS bit streams must be provided. The day is approaching when receiver manufacturers can sell a generic product that is suitable for many national markets, and for receivers to be equally suited to terrestrial, satellite and cable HDTV services. These economies of scale will benefit the consumer, with lower prices. For operators, the choice of codec is narrowing down to Dolby Digital Plus (E-AC-3) or MPEG-4 High Efficiency Advanced Audio Coding (HE-AAC). BE 22 broadcastengineeringworld.com | February 2009 http://www.broadcastengineeringworld.com

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Broadcast Engineering - February 2009 Contents Viewing the Slowdown in 3-D Examine Workflows Tapeless Technology Digital Audio 24p and 25p Judder Video Routing: A Look at What's Next Managing AFD The Right Connections Solid State Logic's AWS 900+ SE and More... Advertisers Index

Broadcast Engineering - February 2009

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