June 18, 2003
For Advanced TV, Try Analog
What do the first standard issued by the Advanced Television Systems Committee (ATSC) and a mechanism allowing TV viewers to choose whether or not to listen to audio descriptions in surround sound have in common? Neither has anything to do with digital terrestrial television (DTT) broadcasting.
That first ATSC standard was intended to allow delivery of ghost-free pictures just as DTV can, but it did so by the inclusion of a ghost-cancellation signal in analog broadcasts. The fact that manufacturers haven’t chosen to include the necessary equalizers in analog TV sets is something to consider in regard to the performance level—ghost-equalizers included—that may be expected in the DTT reception circuitry required of them starting next year.
Then there’s that aural description for the visually impaired (VI), which describes what’s going on in TV pictures. The service is provided for in the ATSC DTT-audio standard, but there’s a catch.
ATSC audio packages may have from one to six channels each. Obviously, a six-channel surround sound package requires more bits per second than does a one-channel package. The standard, fortunately, allows packages to be combined in homes to save bits.
A single-channel video description service for the visually impaired can be added to a complete mix (CM) in six-channel surround sound inside a DTT receiver. That is, it can be added if the receiver has dual-stream audio decoding. Estimates of the number of U.S. DTT receivers with dual-stream audio decoding hover around zero.
Broadcasters may transmit two large surround sound audio packages, one CM and one VI. More likely, to save bits they will transmit a six-channel CM and a single-channel full-mix VI.
What they can’t do is transmit a descriptions-only VI to be added to CM, because non-dual-stream receivers can’t combine the two. Those listening to a less-than-full-mix VI then wouldn’t hear a show’s normal dialogue, music, and sound effects.
Meanwhile, millions of analog TV sets already offer the capability to mix a descriptive secondary audio program (SAP) with a matrixed surround sound full main mix. Analog viewers with surround sound can choose whether to add the descriptive audio. That’s a choice DTT viewers can’t make.
It’s not just video description. Consider data delivery.
At the NAB convention in April, Disney’s Michael Eisner announced Movie Beam, a mechanism for delivering Hollywood features to homes by using spare datacasting capacity. Datacasting brings DTT to mind, but it has been widely reported that Movie Beam might use Dotcast technology, which allows a large amount of data to be carried on analog television stations.
Why use analog-transmitted data instead of DTT? It’s possible Disney will use both. But, at the moment, analog signals can be received at more U.S. sites than can DTT. And the analog-transmitted data might be carried in cable and satellite retransmissions, too. DTT stations are carried on few cable systems and aren’t carried by satellite at all.
Of course, new techniques are being proposed to allow reliable indoor reception of ATSC DTT signals and perhaps, someday, even mobile applications. But rabbit ears speak to the omnipresence of indoor analog TV reception, and the antennas on the trunks of limousines indicate that mobile analog TV reception has existed for years.
How much data can be carried on an analog TV station? At last year’s NAB show, Telisar demonstrated the transmission of two different movies simultaneously on a single NTSC channel.
What about HDTV? At a suite at this year’s NAB convention, Pixonics demonstrated a technology for creating HDTV DVDs compatible with existing players. It’s conceivable that the same concepts could be applied to HDTV transmission on analog stations, compatible with the hundreds of millions of existing analog TV sets.
Strange as it may seem, analog datacasting receivers can take advantage of the latest advances in bit-rate reduction better than can digital ones. An analog-transmission set-top box using MPEG-4 Part 10, Windows Media 9, or some other new algorithm need have only that; DTT receivers must still include older MPEG-2 technology.
Technology marches on, and there’s no stopping progress. DTT, however, is not the only place to look for the latest and greatest.
Mark Schubin is an engineering consultant with a diverse range of clients, from the Metropolitan Opera to Sesame Workshop.