[opendtv] Re: VHF vs UHF coverage
- From: Richard Hollandsworth <holl_ands@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sun, 24 May 2009 09:56:01 -0700 (PDT)
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/HarrisAnalogDigitalTransmitters.pdf?CFID=35361629&CFTOKEN=bf05eb97e3b664ab-732981B3-C644-F0C4-A7C30554EDFCE82C
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).
=================================================
Many ATSC Lab/Field tests were conducted using both visual observations and a
Bit Error Rate tester--using a 3x10-6 BER Pass/Fail criteria.
That works out to several bit errors per second, but since errors occur in
bursts, is observed as
perhaps several dozen seconds between glitches....although "Error Hiding" can
extend this period.
Although you probably wouldn't want to watch with glitches this frequent, it's
handy when
conducting tests--no sittting around for minutes--hours--days(???) at a time
for each datapoint.
But with such a sharp BER curve, only another 1-2 dB of SNR will push the
interval from
seconds to many minutes.
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
==================================================
--- On Sat, 5/23/09, Albert Manfredi <albert.e.manfredi@xxxxxxxxxx> wrote:
From: Albert Manfredi <albert.e.manfredi@xxxxxxxxxx>
Subject: [opendtv] Re: VHF vs UHF coverage
To: opendtv@xxxxxxxxxxxxx
Date: Saturday, May 23, 2009, 3:01 PM
Richard Hollandsworth wrote:
> NTSC power has always been measured and specified
> as PEAK power, whereas DTV power is always
> measured and specified as AVERAGE power. Peak
> power for DTV is about 7 dB higher than average
> power.
I think you mean NTSC peak is 7 dB higher than NTSC average, yes? With
scrambling, DTV average and peak power should be virtually identical, I think.
> DTV also has a performance advantage of about 5
> dB for a total of 12 dB.
I guess some people are skeptical about this extra 5 dB. Also, because analog
viewers can put up with a lot of visual "abuse," where digital viewers have to
contend with a cliff, I would err on the side of being more forgiving with DTV
signal margin.
I do totally agree with your point about UHF receive antenna gain vs VHF
antenna gain, however. It's just that propagation models do show a lot more UHF
falloff at the fringes, with low altitude receive antennas, etc.
Just 3 weeks and we'll know just how accurate those FCC predictions were for
VHF. My fingers are crossed.
Bert
Other related posts: