Subject: (Fist) HDTV and Digital Television (fwd)
Date: Fri, 5 Mar 1999 021037 -0600 (CST)
From: "Roy L. Beavers" <rbeavers@llion.org>
To: emfguru <rbeavers@llion.org>
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---------- Forwarded message ----------
Date: Fri, 05 Mar 1999 14:02:54 +1100
From: Stewart Fist
To: "Roy L. Beavers"
Subject: HDTV and Digital Television
There appears to be some confusion about High Definition TV and digital TV.
The most common suggestions appears to be:
1. that digital and HDTV TV transmission will necessarily be stronger in
power-output than existing analog TV.
2. That digital signals are 'pulsed" and therefore possibly more dangerous.
These are both false assumptions, although the answer needs to be more complex
that just that.
First of all digital television uses the same basic frequencies and, indeed,
the same type of transmissions as analog TV. Digital TV is transmitted in the
same part of the spectrum using the same analog radio wave-forms and the same
fundamental channel divisions.
Your digital modem also uses the same analog telephone connection as voice.
Both of these digital technologies (TV and modems) use analog transmissions,
but they superimpose changes on the analog carrier waves which can be
interpreted unambiguously as Yes or No -- one thing or the other. This is
what allows the digital information to be carried on what is basically an
analog system.
There is no 'pulsing' of digital television transmissions. This is a common
misunderstanding arising from the confusion of digital cellular mobile phone
handsets (not towers) with so-called time-division techniques. Time-division
is a way of switching power on and off in fractions of a second, to allow
between three and eight users to share a single channel. The pulsing comes
from this sharing (TDMA) not from the digital nature of the signal.
Time-sharing can also be used with analog transmissions.
And because it deals with unambiguous (Yes-No) changes, high quality digital
TV can usually be detected at a lower power level than analog. In fact,
achieving the same quality results with lower power is one of the main
advantages of digital transmission in general.
On a per-carrier basis (6MHz in US, 7MHz in Australia, 8MHz in Europe), the
television programmers can usually insert one HDTV image or, say, four
conventional images into the carrier bandwidth which currently carries only a
single existing analog channel. In the latter case, all four 'standard
digital sub-channels' only have in total, the power-density of one of the old
analog TV channels. This doesn't help much, because all four will be
transmitted anyway -- even if a couple of them only run ancient movies.
The main threat to local exposures comes from the fact that, during the
change-over period, television companies will continue to transmit their
analog signals, and supplement these with a channel (more properly now called
a multiplex) which will be used for either one HDTV signal or a few standard
images. Therefore for the change-over decade, we can expect the R/F exposure
experienced by everyone within range of each transmitter location to double.
For most people it will still be very low, but it will be twice what it was
previously -- equivalent roughly to the exposures you would experience
roughly, if you moved your house one-third the distance closer to the transmitter.
Also, television channel owners, appear to be using the opportunity to try to
raise the power level of their transmissions, primarily to achieve greater
area coverage. The regulators are inclined to permit this, because digital
signals are also more easily to discriminate and identify in any areas of
confused reception. This makes digital reception better at the outer limits of
transmitter range, where a householder sits roughly half-way between two
transmitters using the same frequency (co-located transmitters).
So the television stations therefore believe (rightly) that they can increase
their coverage area with greater power, and this will also help force signals
into shadow areas where reception might be poor.
The point is that there is nothing about digital transmission that requires
this higher power (the opposite in fact), but the TV owners figure they can
use the higher power permitted by digital (through reduced interference) to
extend their coverage. Not all regulators will allow this.
--
Stewart Fist - writer and columnist
See http://technology.news.com.au/opinion/
http://www.abc.net.au/http/sfist/ (some archives)
http://www.electric-words.com (main archives)
70 Middle Harbour Road, Lindfield, 2070, N.S.W, Australia
Phone +61 2 9416 7458 Fax +61 2 9416 4582
Archive provided courtesy of WaveGuide, http://www.wave-guide.org
Reprinted with permission of Roy Beavers, http://www.feb.se/EMF-L/EMF-L.html