Six in one

Digital television offers broadcasters a number of interesting possibilities. One of them is the ability to transmit multiple standard-definition channels
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Digital television offers broadcasters a number of interesting possibilities. One of them is the ability to transmit multiple standard-definition channels within 6MHz. Depending on the quality of service the broadcaster is willing to settle for on each channel, they can run as many channels as they wish.

This is accomplished through a technology known as statistical multiplexing (stat-muxing). The two more common forms of multiplexing are variable bit rate (VBR) and constant bit rate (CBR).

VBR is utilizing whatever bandwidth is required, up to the limits, as the complexity of the transmitted material demands, with nothing else added.

The CBR works the same way except those portions of the bandwidth that are not being utilized by the transmitted material are stuffed with filler bits that literally do nothing except occupy space and time.

Stat-muxing looks at all the digital information to be sent and fills the entire bandwidth, according to statistical analysis of what is required and what is being sent. Relying heavily on buffers at both ends of the transport stream, all the digital information usually gets sent down the transport stream. This entire process relies very heavily on the controlling feature known as PSIP, or Program and System Information Protocol.

Several stations, including one in Detroit and one in San Francisco, are transmitting one high-definition program channel and an additional channel or two of standard-definition material, all within their 19.34Mb/s.

In early June of this year, Paxson announced its Chicago station would be the first digital television station to multicast six network feeds on its digital television station. WCPX-DT, operating on Channel 46, will multicast six channels 24 hours a day. The itinerary will be three channels of the PAX network, consisting of the local, central time zone on the primary feed with the East Coast and West Coast feeds on Channels 2 and 3 respectively. The other three channels will consist of the Worship Network, Praise Television Network and the Total Living Network (TLN), a Chicago-based TV network, on Channels 4, 5 and 6.

Through the use of multiple output servers and other program sources, the program material is fed into DiviCom/Harmonic encoders, the outputs of which are fed into the statistical multiplexer along with the output of the Thomcast Pearl PSIP generator. The output of the multiplexer is a fully compliant ATSC transport stream at 19.34Mb/s.

This 19.34Mb/s stream is then fed into two Evertz distribution amplifiers (DA) that drive a Teleview DTV receiver for monitoring of both the feed to the transmitter and the off-air signal. They also drive a Thomcast Turquoise ASI to SMPTE 310M stream converter. The SMPTE 310M is then carried via the Studio to Transmitter (STL) radio link to the transmitter where it is fed into the exciter of the Thomcast DTV transmitter. The accompanying diagram shows how all this is laid out.

Paxson has entered a strategic relationship with NBC. It will be interesting to see how much of the 2000 Olympics from Sydney will be carried.