Phil Kane wrote on Sun, 27 May 2007
23:00:15 EDT:
On Wed, 9 May 2007 15:23:30 EDT, AF6AY wrote:

On a more consumer-oriented
basis, the broadcasting industry is generally wondering
what will become of all those old analog TV transmitters
after the transition to HDTV in the USA. There's no easy
answer for that, either.

Do not confuse digital television (DTV) with high definition TV (HDTV)
which is a subset of DTV. All TV stations will be required to use DTV
but the use of HDTV is optional.

I made an "aside" for comparison and haven't confused anything.
Firstly, DTV isn't about using HF spectrum...I am very aware of that.
The transmitters for television represent a sizeable capital
investment by broadcasters. I am aware of the complaints of the
broadcast industry (pro and con) during the entire time of the "Grand
Alliance" testing that led to the eventual DTV broadcasting format
that went into broadcasting regulations. It's been years since I
"rode gain" at a control console or "took" a particular camera or tape
unit, but I do
know something of the MPEG video transport format even though it
generally gives me considerable confusion every time I've studied
it. :-)

The MPEG video transport format can accommodate four different pixel
arrangements and DTV receivers are supposed to be built to decode all
of them automatically. That video format includes the old-style
analog format (converted from analog to digital) on up to the High-
Definition TV which is considered by most consumers as "wide screen
TV." The DTV transmitters themselves handle the entirety of the video
format sent up from the studios,including the quadraphonic sound, text
for hearing impaired, and whatever else the studio central control
sticks in there.

In the old TV transmitter arrangements of any appreciable power, they
were almost always TWO, one for video (AM sorta SSB called "vestigal
sideband"), one for audio (FM) with a Diplexer (passive filter) to
connect
both to the same wideband antenna. The DTV transmitter is a single
one since ALL of the modulation information is conveyed by it, no
"extra" one for sound, seldom any need for an external filter, let
alone
a diplexer. The internal design of the DTV transmitter HAS to be
different than either the AM video or FM aural transmitters by nature
of the modulation mode. But, that DTV transmitter can handle ANY of
the video transport formats automatically. Whether the TV station
central engineering sends old NTSC video converted to "low grade"
digital format or has gone all-out to run everything in "high
definition"
doesn't bother the DTV transmitter. The end result for conversion to
DTV was a replacement of the transmitters (old) by one new one.
Draconian for the broadcasters but visual and audible pleasure for
millions of consumers receiving the DTV.

As far as the transmitters go, in general they are being junked
because most of them are near the end of their useful lives and are
held together by duct tape and baling wire in anticipation of the
transition.

I disagree with that "junk" figurative phrasing. Here in Los Angeles,
there is one central TV (and most FM) broadcast transmitter site,
Mount Wilson. It's been a few years since I was up there but all the
stations (7 on VHF, 4 on UHF) had good, long-lasting transmitters.
At the time KTLA (ch. 5) was beginning to convert to DTV; they were
a pioneer broadcaster in L.A. and this year is their 60th anniversary
here. Both studio and transmitter sites have excellent equipment and
their DTV signal is absolutely HDTV. Their morning show content
is, in my view, JUNK, but that is CONTENT, having nothing to do with
equipment or signal quality...a personal critique.

What used to be a TV "leader" in quality of equipment, NBC (ch. 4)
under the 'general's" eye of RCA, hasn't fully converted to HDTV in
their studios. NBC evening news is HDTV but local news is still
narrow TV. NBC is owned by General Electric and "RCA" exists solely
as a brand name now. I think...haven't kept up on the mergers and
acquisitions of large corporations lately. :-)

TV is above 30 MHz, definitely not in the HF spectrum. However the
type and kind of modulation carried by any transmitter will determine
whether or not it will be "junked" for a new replacement, NOT it's
supposed "held together by tape and bailing wire." If hams could run
4 KW PEP SSB on HF (not in the USA) then the ages-old Western
Electric LD-T2 SSB transmitter would be a great surplus bargain for
them...Class A stages up to the PA which is AB_2, push-button QSY
to any one of 10 pre-tuned frequencies. Problem is, the SSB format is
12 KHz wide and has internal frequency multiplexing for four 3 KHz
audio inputs combined for the output SSB modulation. A no-no for
amateur radio use, even if well-designed after WW2 by WE. Too much
modification required to fit the "tranditional" and technical specs
even
if a superb design for its intended use. That was the general point
I
was making, not something about the video format of DTV.

73 de K2ASP - Phil Kane
Life Member - Society of Broadcast Engineers

73, Len AF6AY
Life Member - Institute of Electrical and Electronic Engineers


No pixels were harmed in the generation of this message but billions
of electrons were rudely shoved around.