Multicasting. The wrath of KRON..............................
tvbroadcast.com
KRON Begins Multicasting
San Francisco Station At Forefront Of DTV Era
By Joseph Maar
(June 18, 1999)San Francisco -- With little fanfare but major industry implications, on May 19, Bay Area NBC affiliate KRON-TV began multiplexing two different signals on their digital spectrum. The station is broadcasting one signal at full 1080i HDTV and the other at 480i DTV. On television sets they appear as digital channels 57.1 and 57.2.
The decision to multicast offers a "real world" look into the creative ways that stations might handle future bandwidth issues in order to satisfy both HDTV and NTSC-legacy viewers. KRON is using General Instruments (GI) HDTV and SDTV encoders to multiplex the signals together at the station and send it on a single DS3 line to their transmitter. By adjusting the bit rate of each channel, KRON is disproving the often-repeated belief that 1080-line HD signals leave no room within the 6 MHz band for "other things."
"Everybody said it's theoretically possible--now were doing it," remarked Craig Porter, KRON's chief engineer. "We just crank up the compression a little bit [on the 1080i], then you've got space. The General Instruments SD chassis can take multiple channels, [while] the SD chassis feeds into the HD chassis." The GI encoder, based on a C-Cube chipset and "MicroCode" software, allows a user to adjust the bit rate of each service within a single channel; basically enabling the allocation of specific bandwidth space for individual signals. The software-based system also allows Porter to seamlessly switch from HD to SD modes without experiencing any down time while reconfiguring the encoder. KRON engineers simply stop encoding the HD channels and allow multiple SD channels to pass through.
"You assign a percentage of the total payload to each one of the services you want to transmit," Porter said. "It's then the encoder's job to make sure that it adds enough compression to ensure that the signal fits within the allocated bits. Compression is an ever-advancing science, so each [version] of software makes the encoders work that much more efficiently. Since the system we have is software-based, there's no reason we can't multicast whatever signals we want to."
Using a Larcan transmitter, the HDTV signal carries an upconverted, re-broadcast of KRON's standard definition channel, while the second signal displays a KRON traffic camera. Although the station does operate a regional cable news channel, Bay TV, Porter stated that due to agreements with local cable operators it cannot be broadcast over-the-air.
"I'm sort of waiting for them to come up with a programming concept," Porter remarked when asked what other content might be shown on the second DTV channel. "Our primary interest in doing this is that the SD [digital transmission] is a backup to the HD," added Porter. "In a worst case scenario, if I lost my HD channel, I could use the SD [signal]."
Porter also said that because in MPEG compression is much more difficult than decompression, there's a lot of opportunities for broadcasters to be more efficient with their allocated channel as time goes by.
"For example, we broadcast The Tonight Show, which is fairly static," he explained. "There's not a lot of motion going on inside the picture. There is probably a lot of available bandwidth there. If you get into sports or segments with a lot of fast motion and fine detail, then it becomes more difficult.
"The other big issue is how clean is the input signal that you're feeding the encoder," Porter added. "That's another way that you can increase the efficiency of the coding."
The station was very happy with how quickly they were able to begin multicasting. "In my wildest imagination I didn't think we'd see the kind of reliability we've seen with this system," said Porter. "That's what's been great about it... our success with GI is that it comes out of the box, you install it, and it works."
Porter also stated that GI is planning a new version of its encoder that will include statistical capability so that it can automatically look at what's going on in the picture and adjust encoders to provide the best signal on all channels. This will also allow KRON to transmit data if and when then they want to.
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