[opendtv] Re: VHF vs UHF coverage
- From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sun, 24 May 2009 12:59:23 -0700
Response interlineated:
-----Original Message-----
From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx]On
Behalf Of Richard Hollandsworth
Sent: Sunday, May 24, 2009 9:56 AM
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] Re: VHF vs UHF coverage
NTSC's Peak-to-Average Ratio (PAR) fluctuates as much as 5.32 dB,
depending on
whether broadcasting all white (PAR = 7.55) or all black (PAR = 2.23)
or in-between.
A "standard" test pattern is used when measuring average power
(actually there are several).
Since NTSC peak power is very constant due to repeating Sync Pulses,
all FCC
allocations are stipulated as PEAK power:
http://www.broadcastpapers.com/whitepapers/HarrisAnalogDigitalTransmit
ters.pdf?CFID=35361629&CFTOKEN=bf05eb97e3b664ab-732981B3-C644-F0C4-A7C30554E
DFCE82C
This is fundamental and a given
ATSC allocations are stipulated as AVERAGE power, since PAR is a
statistical curve that
approaches 8 dB for 100% of peaks and is "typically" about 7 dB for
99.99% of peaks:
See pg75: http://www.atsc.org/standards/a_54a_with_corr_1.pdf
Hence an ATSC transmitter allocation of 1 MW (average) has the same
peak voltage
as an NTSC transmitter allocation of 5 MW (peak).
[>]
The average power of a 5 MW analog transmitter (minus the sync energy)
is somewhat comparable to the average power of an 1 MW 8VSB transmitter.
The ATSC MPEG stream, when presented to the 8VSB transmitters
modulator/exciter , is processed through a Data Randomizer to spread the
energy virtually even across the channels spectrum. If you were to examine a
DTV transmitters RF output using a Spectrum analyzer, it would appear to be
flat except for a very short burst of RF pilot energy. See the following
info from Tektronix and Sencor :
DATA RANDOMIZER
With the exception of the segment and field syncs (to be discussed
later), the 8-VSB bit stream
must have a completely random, noise-like nature. This is because the
transmitted signal
frequency response must have a flat noise-like spectrum in order to
use the allotted RF channel
space with maximum efficiency. If the data contained repetitious
patterns, the recurring rhythm of
these patterns would cause the RF energy content of the transmitted
signal to ?lump? together at
certain discrete points in the frequency spectrum, thereby leaving
holes at other frequencies. This
implies that certain parts of the six MHz channel would be overused,
while other parts would be
underused. Moreover, the large concentrations of RF energy at certain
modulating frequencies
would be more likely to create discernible beat patterns in an NTSC
television set, if DTV-to-
NTSC interference were experienced.
In the data randomizer, each byte value is changed according to known
pattern of pseudo-random
number generation. This process is reversed in the DTV receiver to
recover the proper data
values.
www.crwww.com/PDF/UNDERSTANDING%208VSB.pdf
=================================================
User Satisfaction is probably the ultimate acceptance
criteria....rather than trying to compare
grainy, fuzzy, EMI ridden NTSC fringe reception to glitchy ATSC. But
FWIW, my wife is still
cursing digital TV every time it glitches---and she mostly watches
HGTV on CABLE!!!!!!!
[>]
holl_ands
We engineers often mistakenly identify picture quality as the most
important aspect of the television viewers experience when it is actually
the program quality.
Watching good programming on a less than perfect analog signal still
provides viewer satisfaction, given that is all that is available. As we
know from years of experience, folks will spend hours in front of that
grainy, fuzzy analog TV if the programming is compelling (and the sound is
useable).
However, a glitchy and freeze prone DTV signal will drive them away.
[>] Dale
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