DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

I and Q refer to in-phase and
quadrature; these are the same
concepts as in NTSC chroma
modulation, but applied to
completely different ends.
ATSC defines a cable mode using
16-VSB without trellis coding, but
this mode hasn’t been deployed.
Digital cable television is detailed
in Ciciora, Walter, and James Farmer
and David Large, Modern Cable Television
Technology (San Francisco:
Morgan Kaufmann, 1999).
zation information comprising segment and field syncs
is added after interleaving and coding. A low-level pilot
carrier is inserted 310 kHz above the lower band edge
to aid in carrier recovery. Analog techniques are used to
upconvert to the UHF broadcast channel.
At a receiver, analog techniques downconvert the UHF
broadcast to intermediate frequency (IF), typically
44 MHz. Demodulation is then accomplished digitally.
Typically, an analog frequency and phase-locked loop
(FPLL) recovers the carrier frequency based upon the
pilot carrier. A quadrature demodulator then recovers I
and Q components. The I component is converted from
analog to digital at 10.76 MHz to recover the bitstream;
the Q component is processed to effect phase control.
The bitstream is then subject to trellis decoding,
deinterleaving, R-S decoding, and MPEG-2 demultiplexing.
It is a challenge to design a demodulator that is
immune to transmission impairments such as multipath
distortion and co-channel interference from NTSC transmitters.
An interference rejection filter – a variation of
a comb filter – is built into the demodulator; it attenuates
the video, chroma, and audio carriers of
a potentially interfering NTSC signal. An adaptive
equalizer built into the demodulator alleviates the
effects of multipath distortion; the field sync component
of the signal serves as its reference signal. An
adaptive equalizer is typically implemented as an FIR
filter whose coefficients are updated dynamically as
a function of estimated channel parameters.
Cable television has very different channel characteristics
than terrestrial broadcast. DTV over cable typically
does not use 8-VSB modulation: Quadrature amplitude
modulation (QAM) is used instead, with either 64 or
256 levels (64-QAM or 256-QAM).
For direct broadcast from satellite (DBS), quadrature
phase-shift keying (QPSK) is generally used.
Consumer receivers in the United States must accept
the diversity of frame rates and raster standards in
590 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES