Your Questions about DTV
I frequently receive letters from readers, addressed to
me either here or at other publications. Some I answer in the letters section of this
site, but sometimes a group with a common theme will arrive, and it seems reasonable to
deal with them together. A hot topic is digital television (DTV), and here are some of my
correspondents' questions.
My cable company has been advertising "digital-quality
picture and sound" for ages now, but although I've called their service people, they
are very vague as to the quality of the picture. I am at the point of upgrading my TV to
HDTV and I'm trying to decide between cable and satellite. I've never had satellite and am
not sure about their programming, cost, and the quality/service of the providers.
It's definitely very confusing. Video images can be
delivered to your set by several digital methods, and they are not necessarily compatible
with one another. What's generally called DTV is the system adopted in the United States
several years ago, and which is being implemented in that country for terrestrial
television. It uses a data-reduction scheme called 8VSB, which allows a high-definition
signal -- or several standard-definition signals -- to fit into the space taken by one
analog signal now. Eventually all television will be digital.
Satellite services are all digital, at least when it
comes to signal delivery -- a lot of the original material is in fact analog, converted to
digital for transmission -- but they use a different compression scheme, the same MPEG2
system that's used in DVDs. The exceptions are the HDTV signals that satellite companies
carry, which use 8VSB.
Digital cable is really a hybrid. The major cable
companies offer a number of their premium channels in digital form, and these are
excellent, but the majority of cable signals are still analog. Signals that are digital to
begin with -- true DTV -- will be digital on cable, too.
I am about to make a decision on a new high-end TV and if I
understand correctly, what I'm considering in Canada would be an "HDTV ready"
set. So at some point down the line will I need to add an external set-top box?
Digital television is going to do wonders for the
set-top-box business. At present, I'm not aware of any sets available in Canada that
contain a digital tuner -- there's not much point, as there are no terrestrial digital
signals on the air, and the satellite services have their own boxes. That means if you buy
a set now, it will require an outboard tuner sometime in the future if you want to receive
over-the-air signals.
By the same token, if you decide to stick with an
analog-only set, the day will come when digital signals will be the only ones on the air,
and you'll have to buy a box that will down-convert them so you can see them on your
regular set. When this is likely to happen is open to question. In the United States, the
plan is definitely to pull the plug on analog television as soon as it's practical, and
use the frequencies for other purposes. In Canada, the two types of service could co-exist
for quite a while.
One thing is puzzling to me: what is S-video? I have a
satellite dish, and the company keeps asking us to plug in the S-video cable to get the
best picture possible. I've done so but I really haven't noticed a change in anything. I
do have a really great picture all the time so maybe that's as good as it can be.
The most common way of connecting a satellite receiver
to your TV set is by running video cables from the receiver's RCA outputs to the
corresponding inputs on the set. What you get is called a composite signal, in which the
color and brightness information are combined. Theoretically better is a system where the
two aspects of the picture signal are separate, and one way to do that is by using the
S-video connections, when they are provided.
This can result in some visual improvement, but I admit
the difference can be tiny, especially if you are starting out with a very good signal, as
satellite pictures tend to be. Still, since you have the connections and the cable, you
might as well derive whatever benefit you can, however small.
The latest receivers, like many DVD players, offer component-video
outputs, which use three separate connectors. This system offers even higher video
quality.
I live downtown, and by using a combination UHF/VHF antenna
in my attic, I receive a number of VHF stations, but no UHF stations. I prefer not having
an antenna outside. In my previous house, just around the corner from where I live now, I
received many more, including several UHF. How is it possible that I can't do better than
that now? Does my antenna need a better sightline to the transmitter tower? Is it possible
that my neighbor's brick wall is blocking my signals?
Having a clearer shot at the tower might reduce
multipath (ghosting), but shouldn't affect the number of channels you are able to receive.
Nor is your neighbor's wall likely to make that much difference. All else being equal, you
should get pretty much the same reception as when you lived around the corner.
So all else is obviously not equal. Without more
specific information, I can only hazard a guess, but I encountered a similar situation,
and my solution might work for you. In my case, it occurred with the same antenna in the
same place -- I had removed my own set in order to review several others, and when I
hooked it up again several weeks later, I lost all my UHF signals.
The reason became clear enough, but only after I had
waded through numerous levels of onscreen menus. The trouble is that, although both cable
and TV broadcasters use the same frequencies for channels 2 through 13, from 14 up, they
are entirely different. They may use the same numbers, but that's just a matter of
convenience.
Cable is not very good at passing frequencies as high as
the UHF band, so the higher channels are actually at frequencies much closer to the VHF
band, which cable handles well. Most modern sets have provision for operating with either
an antenna or cable, and switch the tuner accordingly. For the most part, this is
determined in the setup menu when you first get your TV, and then you can forget it. In my
case -- and perhaps yours -- cable operation is the default setting. I had set it for
off-air viewing years before; when I actually unplugged the TV (as opposed to turning it
off with the remote), all my customizing disappeared, and the set reverted to cable mode
and I could no longer receive UHF. Possibly, in your move, the same thing happened.
...Ian G. Masters
ian@mastersonaudio.com
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