Digital audio for DTV
Feb 1, 2009 12:00 PM, By Aldo Cugnini
There are various technologies available to broadcasters and consumers to augment video and audio programming. Digital technology allows the use of additional services and features, but doing so effectively means understanding the underlying requirements and limitations. Among the useful features of ATSC are associated audio services and audio metadata.
Associated audio services
Frame Grab
Demand for Internet video increasing
45 percent would likely download an Internet browser to next TV.
Click to enlarge
Typically, ATSC broadcasters transmit audio using the complete main audio service (CM), which contains a total audio program with dialog, music and effects. This service can include one to 5.1 channels. One way to support multiple languages is by transmitting multiple CM services, each in a different language. In addition to the main service, the ATSC Standard A/52 includes support for several other specific audio services: music and effects (ME), dialog (D), visually impaired (VI), hearing impaired (HI), commentary (C), emergency (E) and voice-over (VO). While these are mostly self-explanatory, a few usage issues are pertinent.
The ME service is similar to the CM service, but it does not contain the dialog for the program, which can be transmitted separately using one or more D services. In this manner, a more efficient use of bandwidth can be achieved, such as when transmitting multiple languages, each on its own D service. Supporting this type of transmission requires receivers to simultaneously decode one ME service and one associated D service in order to form a complete audio program, i.e., the receiver would require multiple audio decoders. The D service audio is then mixed with the main audio, usually with the center channel, if present. (See Figure 1.)
Figure 1. An associated audio service can be sent separately, requiring a separate decoder. Alternately, the second audio program can be sent on a second CM service.
Click to enlarge
The VI associated service provides a narrative description of the visual program content, a feature that grew from the need for audible descriptions of program action for visually impaired viewers. Special audio-only receivers could also be designed this way for viewers not needing the picture. Currently, many broadcasters are using the VI audio service for video description, according to the WGBH National Center for Accessible Media (NCAM) in Boston. VI is used throughout the Public Broadcasting System, as well as some of the other networks' stations, providing the video description produced by NCAM to local affiliates.
But there is currently no requirement for the transmission of the VI service. In 1998, NCAM sent a letter to the FCC, recommending that all DTV receivers should be equipped with the appropriate audio decoding and mixing capability (i.e., dual-stream decoding) to access and deliver the VI associated service and that all DTV receivers should be capable of selecting video description when available. In 2000, the FCC adopted rules mandating that a certain amount of programming contain video description. These rules took effect in April 2002, but were struck down by a federal court in November 2002. As a result, video description is not required, and its use is voluntary.
While the FCC requires broadcasters to comply with the ATSC standard, it does not require broadcasters to transmit specific elements of the standard other than PSIP, closed captions, and ratings information and descriptors. In addition, the FCC has stated that digital-to-analog converter boxes are not required to process all associated audio services. (The FCC nonetheless advises consumers to check with manufacturers and retailers to understand whether specific products provide video descriptions.)
The requirement in the FCC's rules that emergency information provided on television should be accessible to individuals who have vision disabilities was updated recently, but the specific means were not addressed. Emergency audio could be supplied using the special E service, ostensibly by automatic receiver switching, but this mechanism is not mandated by the FCC, and neither is the transmission of an E associated service. Instead, broadcasters typically replace regular program audio with an emergency message.
| Want to use this article? Click here for options! |  |
