NBC,CBS To "Compress" (Not Downconvert) HDTV
By Michael Grotticelli
new york, ny
--When NBC announced at NAB that it was pursuing a DTV strategy that includes 1080i for HDTV in prime time, joining CBS as a proponent of "full bandwidth, pristine images," many were left wondering how they were going to distribute such data-intensive signals to their affiliates and still leave room for individual station creativity.
But representatives of both networks were emphatic that 1080i was "true HDTV" and provided the best pictures. The distribution details, they said, would be worked out later to everyone's satisfaction. Some 1080i proponents at the NAB show said that starting with a 1080i signal would allow affiliates to "downconvert" the signal for standard definition use, "if they so choose," and still have a better image than if they started with 480i equipment.
In making the announcement during a press conference, the network explained how it was going to originate HDTV signals from its 30 Rockefeller Center headquarters in New York City and distribute them to its affiliates nationwide.
Taking a closer look at NBC headquarters and its recently completed "GEnesis Project" (that represented a complete upgrade to a serial digital infrastructure but lacks the capacity for 1.5 Gbps signal processing), some in the industry had speculated that NBC would have to downconvert the signal to 480i before sending it along to its affiliates.
Not so, said Charles Jablonski, NBC vice president of Broadcast and Network Engineering, "We absolutely are not downconverting the signal. We believe we've found a way to transcode it so that it will pass through 300 megabits worth of plumbing. But we're not going to downconvert it to 480i. I can't tell you much more right now, for competitive reasons."
Jublonski said the signal from New York will be "compressed and processed, because it's got to fit through 36 MHz transponders [on the satellite]. We're still working out the details and considering some proprietary technology to distribute the signal through the GEnesis system. But, again, it's not being downconverted."
CBS, for its part, will pursue a similar distribution strategy, incorporating an as-yet undisclosed amount of compression. "CBS will be delivering to the affiliates a 45 MB DS-3 compressed 1080i signal," explained Robert Seidel, vice president of engineering at CBS, "which will be at 1080p (at 24 fps) during film programs. However, we will decode the [1080p signal for them--in the satellite receive rack--back to a baseband video [SMPTE 292, 1.5 Gb standard] 1080i signal for further mixing, distribution, keying, and processing within their plant."
Seidel said that CBS will be provide its affiliates with this rack of satellite electronics, which will interface with the two existing satellite receive dishes--a 7 meter and 4.5 meter unit that are already installed at each station.
"This electronics rack will work in parallel with [affiliates'] existing analog rack, so that they will be able to receive two simultaneous streams; the analog NTSC network, as well as the HDTV network at 1.5 Gbps," Seidel stated.
This "compressed" strategy was formulated to address concern among some affiliates that having to deal with a full 1080i signal would use up the entire 36MHz bandwidth and limit their ability to multicast and add original material (such a station ID) during the prime-time hours when the networks plan to transmit their HDTV programming--such as NBC's The Tonight Show with Jay Leno, slated for sometime next year.
NBC, though, is sensitive to its affiliates' revenue-generating potential, said Jublonski, and has taken this into account as they design their distribution system of the future.
"We are not planning on sending out a [full] ATSC bit stream to the affiliates, whatever the format," he stated, "because the affiliate needs the ability to do a key, a voice-over, and that sort of stuff. I would never put our affiliate partners in a position where they couldn't do that.
"If you send out a pure ATSC signal, tear it apart and put it back together again, it's going to look [less than desirable], even if you could do it," he added. "So, we will have some mezzanine level of compression... we're still working out the number and the extent of compression at this point."
This means, for example, that The Tonight Show will be shot with a 1080i Sony camera, as NBC has announced it will be, "compressed" and sent directly to the affiliates from Burbank via satellite, using the same time-shift schedule it currently runs on. Other programming (such as feature films transferred to D-5 videotape) will be sent from 30 Rockefeller Center, in New York City.
"The picture will be true 1080i HDTV when it's acquired in the studio and when it's displayed in the home," Jublonski said, "and don't let anyone tell you different. We said we're going to do 1080i HDTV and we're sticking with our decision because we think it makes the best pictures and will look the best to consumers."
It is believed that DTV sets will be designed to "decompress" the signal and possibly add motion compensation in order for viewers to see it displayed in 1080i resolution at 60 fps. However, due to the shadow masks on these new receivers (that help keep the color pixels in the set sharp and well-defined), "early adopter" viewers will only be able to see about 700-800 lines, which has many questioning the value of acquiring with 1080-line cameras and could be one of the reasons why ABC and Fox have chosen 720 lines of resolution as their HDTV format.
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