The Federal Communications Commission (FCC) of the United States has already adopted a standard for digital television, based on the ATSC (Advance Television Systems Committee). The FCC specifies Dolby Digital (AC-3) as the encoding standard for DTV (Digital Television) signal audio.
Dolby Digital was developed to provide 5.1 channels of audio (five full-band and one subwoofer) for movies and HDTV. This dolby can be decoded from mono to stereo and up to 5.1 full channels of surround depending on the system that the viewer has.
This system consists of an information stream of 384 Kbps per second, which is specified in the DVD standard. It provides uniform sound levels and a dynamic control range.
Although it seems to be a widely accepted choice for DTV, which currently produces a stream encoded in the complex system of production and post-production studios, and television stations, the AC-3 presents some problems and questions. This poses difficulties given the FCC's recent announcement to some affiliate stations in the United States, which are due to begin broadcasting DTV in 1999.
Production process
The questions begin at the moment the production process is considered. Although large-scale Hollywood productions have for years used multiple audio channels to eventually use six in theaters, television production has been primarily stereo with two channels. With the advent of DTV and AC-3, television production houses, and also broadcasting houses, must not only now consider working with six audio channels, but must adapt to work with digital audio. At this point no one knows exactly how this system is going to be implemented, who is going to manufacture the equipment and, in some cases, who will do the work.
Production line
Looking at the first stage of the chain, an initial doubt arises in this regard that except for the D-6 HDTV machines and the Sony HDD-1000, there is no longer a VTR with enough channels to record the six audio channels that Dolby Digital uses. These machines could be manufactured if they become much needed, but it won't be done before the FCC deadline. One possibility to work with six audio channels could be to work with a dual system, for example using a DAT in parallel with a VTR.
Routing audio into the studio
After the audio is recorded, another problem arises around the routing of six digital channels on the premises of a television production company. Almost all its aspects are unknown: range of information, lack of consoles capable of working with 6 channels, cables, and devices for playback and editing, switchers, routers and more.
At this point in production AC-3 encoded audio is not appropriate for several reasons. For example, there is no practical way to edit an AC-3 bit stream. Once the AC-3 is encoded it is necessary to decompress it to edit it and the system is not designed to have multiple encodings and decodings.
At present there is no equipment in the facilities of a television production company to work six audio channels. An essential question would be how six channels are put into a four-channel VTR. One solution could be after doing the mixing of the six channels would be put into two VTRs.
A DTV signal can have one HDTV signal or multiple regular television signals, or use some band to transmit information. Stations are still deciding whether they will use high-definition TELEVISION or multiple regular channels. Only the future will have the last word.
