Why Cable Carries HDTV with "QAM"

[Kevin Arnold]

I found this useful explanation of how and why cable companies convert the transmitted HDTV signal to "QAM". Thought I'd share it:

Cable Prepares
to Carry HDTV
by Ken Freed.
.
The difference is VSB or QAM, and the fact cable must carry digital ATSC broadcasts.
 
The national broadcast networks are initiating ATSC feeds as a growing list of local broadcasters start transmitting high-definition TV (HDTV) or multiplex standard-definition digital TV (SDTV). Is cable ready to deliver broadcast digital channels to cable customers?

When it comes to HDTV, said Bill Wall, technical director of subscriber networks for Scientific-Atlanta. The critical difference between cable and terrestrial broadcast is signal modulation.

Cable uses 256 QAM (quadrature amplitude modulation) to form the digital bitstream in the cable. Broadcast HDTV uses 8VSB (vestigial side band) to form the waves in the air. Each method works perfectly for its transmission environment (e.g., error correction), but the two modulation schemes are incompatible.

"It's been a real issue inside cable on how to handle the HDTV situation." he said. Neither broadcasters are not about to alter their infrastructure for cable. To deliver HDTV content, therefore, cable must either pass through the VSB signal or else convert it to QAM.

Converting VSB to QAM makes the most sense for cable, said Denton Kanouff, marketing VP for digital network systems at General Instrument, noting that GI also builds transcoders for broadcasters. If the VSB is passed through untouched, the HDTV content cannot easily be merged into the electronic program guide (EPG), which might churn out the cable customers with HDTV sets.

That's the risk being taken by Time-Warner Cable in Manhattan, "To satisfy early consumer demand," said Wall, "they signed a deal with CBS to carry a daily HD news feed at 8VSB over cable. The pass-though is not efficient, but it works. They only have a few hundred customers with digital sets, but it may help keep them customers."

In support of the alternative solution, Wall said, "S-A is now building a special Explorer 2000 set-top with full HDTV decoding." Converting VSB to QAM at the headend [S-A builds the converters], the QAM is returned to VSB in the box. He explained, "We figure the people who do have HDTV sets today can afford to get a special box designed for them. We're shipping the device now."

General Instrument introduced an HDTV solution for cable at the 1998 NCTA show, said Kanouff, noting that the DTC-5000-plus integrates HDTV decoding into the box. GI also can sell VSB to QAM transcoders to any interested broadcasters.

A source at AT&T Broadband said that GI is "very close" to completing a customized HDTV version of the DTC-5000+ with a Apples' 1394 "firewire" interface for deployment on the former TCI cable systems "within a year."

Today's expensive digital TV sets need a digital set-top box with a QAM modulator inside to receive HDTV signals through the cable rather than off-air. Essentially, this means a cable modem with a robust graphics chip. The second generation of HDTV sets, he said, will not only be more affordable, but the sets will be cable-ready.

"This first generation of HDTV sets now in the stores," said Wall, "are designed only for terrestrial broadcast reception. The goal is to get QAM converters built into cable-ready HDTV. Therefore, NCTA and SCTE (Society of Cable Television Engineers) are talking with CEMA (Consumer Electronics Manufacturers Association).

For the time being, he said, "a special HDTV set-top seems to be the direction most of the MSOs are going to supply HD content to those with digital TV sets." He's not willing to make any predictions.

"The main thing," Wall said, "is that HDTV works today on a number of cable systems. Nationally, he noted. HBO signed a deal to carry an HDTV feed from Viacom, and similar deals are happening among all the big players, including HITS. "I think cable wants to sign up as much HD footage as it can get."