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."
.
