HDTV & The Grand Alliance
Where the US is going from here & the
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understand where the US is headed on its own and with respect to
the rest of the world.
In 1987, the FCC issued a ruling indicating that
the HDTV standards to be issued would be compatible with existing
NTSC service, and would be confined to the existing VHF and UHF
By the end of 1988, the FCC had received 23 different
proposals for HDTV or EDTV standards. These were all analog (or
mixed analog/digital systems like MUSE) and explored a variety of
different options for resolution, interlace and bandwidth.
In 1990, the FCC announced that HDTV would be
simultaneously broadcast (rather than augmented) and that its preference
would be for a full HDTV standard (rather than the reduced resolution
On May 31, 1990 General Instrument Corp. submitted
the first proposal for an all digital HDTV system. By December 1990,
ATRC announced its digital entry, followed quickly by Zenith and
AT&T, then MIT. Thus there were four serious contenders for
digital HDTV, as well as a modified "narrow" MUSE and an EDTV proposal.
During the following year, these systems were tested.
In February 1993, the FCC made the key decision
for an all digital technology -- but could not decide among the
four contenders. Therefore, after some fuss, a recommendation was
made to form a "Grand Alliance" composed of AT&T, GI, MIT, Philips,
Sarnoff, Thomson and Zenith. This Grand Alliance would take the
best features of the four systems and develop them into an HDTV
standard. Most of the remainder of 1993 was devoted to establishing
the features of this new standard.
During 1994, the system was constructed and 1995
is slated for testing. If all goes well, the FCC may be setting
this standard by the end of 1995.
The Grand Alliance standard differs from all existing
TV standards in three major ways. First, it is all digital standard
-- to be broadcast with a packet transmission. Second, it supports
multiple formats. Third, it is designed to be primarily compatible
with computers rather than existing NTSC televisions.
The compression algorithms use both a motion compensated
and discrete cosine transform (DCT) algorithm. The motion compensation
exploits temporal redundancy. The DCT exploits spatial redundancy.
MPEG-2 syntax will be used -- because it is already well established,
will aid in world-wide acceptance, and will smooth the road to computer
and multimedia compatibility. Audio will be supported by Dolby AC-3
digital audio compression. This will include full surround sound.
The core of the Grand Alliance concept is a switched
packet system. Each packet contains a 4-byte header, and a 184 byte
data word. Each packet contains either video, audio, or auxiliary
information. For synchronization, the program clock reference in
the transport stream contains a common time base. For lip sync between
audio and video, the streams carry presentation time stamps that
instruct the decoder when the information occurs relative to the
The terrestrial transmission system is a 8-level
vestigial sideband (VSB) technique. The 8-level signal is derived
from a 4-level AM VSB and then trellis coding is used to turn the
4-level signals into 8-level signals. Additionally, the input data
is modified by a pseudo-random scrambling sequence which flattens
the overall spectrum. Cable transmission is by a 16-level VSB technique
without trellis coding.
Finally, a small pilot carrier is added (rather
than the totally suppressed carrier as is usual in VSB). This pilot
carrier is placed so as to minimize interference with existing NSTC
The Grand Alliance system is clearly designed
with future computer and multimedia applications in mind. The use
of MPEG-2 will permit HDTV to interact with computer multimedia
applications directly. For example, HDTV could be recorded on a
multimedia computer, and CD/ROM applications could be played on