11/2/07
You’ve
just brought home a new unit, and you’ve discovered that one unit requires a
VGA cable while the other requires component video cables. Some manufacturers have anticipated your
situation and have designed their sets to sometimes break the rules. That is, some VGA ports might allow syncs on
the green wire. (For VGA, Hsync and Vsync are the 4th and 5th wires. In
component video these syncs are multiplexed onto the green wire (Y in a YPrPb system).) For
example, the author’s TV set will accept component video force-fed into its VGA
port. There was nothing in the set's instructions about this. But a menu item
was discovered that allowed the set to accept green-wire syncs.
If the unit literature says nothing about this, ask the “expert” at the store
where you bought it. If he doesn’t know,
you can experiment. Adapters that might
work are available via the Internet. An
alternative to the adapter is to get a VGA cable that has 5 BNC connectors on
one end, and then get 3 BNC-to-RCA adapters from Radio Shack. The cable will
probably cost $40, but if you can return it, this would be the quickest way for
you to try it out.
If
unsuccessful, your only recourse is a transcoder.
Lets say the FCC has given the NTSC
station on channel 3 permission to use channel 41 as a DTV channel. But when you tune to channel 41, your new
receiver says you are now on channel 3-1, and you have also discovered there is
a 3-2. What are these channels and how
did you get there?
Welcome
to the world of virtual channels. Every
virtual channel has a physical channel.
The physical channel is the actual RF spectrum being used. The virtual channel could be called almost
anything. In this example, 3-1 and 3-2
are virtual channels, and are also referred to as sub-channels of virtual
channel 3, which is physical channel 41, and thus not the same as the analog signal
on physical channel 3, even though they look about the same.
The
data stream of DTV channel 41 has data blocks called PSIP data. The PSIP data tells the receiver that channel
41 has two sub-channels: 3-1 and 3-2.
The channel 3 people chose these sub-channel names to remind you whom
you are watching. Not every station
follows the example of this hypothetical channel 3. A different management might have chosen 41-1
and 41-2 for physical channel 41’s sub-channels.
Your
remote controller will let you key in either the physical or the virtual
channel number, but there are some differences between manufacturers. Some receivers will assume 3-0 means analog
channel 3.
Your
first days with your new HDTV can be a very confusing and frustrating time,
particularly where OTA stations are concerned.
You can’t tell the receiver that a channel is digital. The receiver has to figure out for itself whether
a physical channel is analog or digital.
If the antenna is marginal or misaimed, the
receiver can guess wrong. Then you can’t
aim the antenna because the receiver thinks the channel is analog, and you
can’t convince the receiver to switch because the antenna is misaimed. In strong signal areas the receiver might
eventually right itself. Otherwise you
might have to figure out how to make the receiver unlearn a channel. But that still doesn’t solve much since the
antenna is still misaimed.
Nearly
all DTV receivers have a signal strength “meter” of some type. But most of these meters read zero until the
signal is good enough (or almost good enough) for reception. In weak signal areas these meters will not
tell you much about whether you need to aim more to the right or to the left. When you get no reception, you are left not
knowing whether your antenna is just misaimed or is inadequate, and if
inadequate, by how much. Maybe the
receiver is locked up on analog. Maybe
the station isn’t there today or is at lower power. Maybe an antenna connector is poor or
open. Maybe the pre-amp isn’t
working. Maybe you missed something in
the instruction booklet. Will it ever
work?
Once
the receiver has learned all the channels correctly, these problems go
away. People in strong signal areas will
never see most of these problems.
All
receivers have a “Channel Learn” sequence, in which the receiver will search
for and learn all the channels at once.
When you initiate the learn sequence, some receivers will forget
everything they learned previously, which creates problems for users who use a
rotor or who switch between two antennas.
These users have to learn how to add channels manually. Other receivers never forget anything, which
creates other problems.
The
image quality is not affected at all by a low to moderate level of noise in the
signal. This is true for both satellite
and OTA DTV. Yet some people can’t
resist wondering “could I improve the image by improving the signal
strength?” The answer is NO!
When
the signal becomes too weak, you will see “macro-block errors” (parts of the
screen that are shifted or obviously wrong), sound dropouts lasting a few
seconds, or image freezes lasting a few seconds. All of these errors are crude, unsubtle
errors. If these are not present, your
image is perfect.
If
your image is perfect, there is still one reason you might want to improve the
signal: It would make dropouts less
likely in bad conditions, such as heavy rain.
Rain can affect DBS and UHF reception, but not VHF. In some places, wind can affect UHF.
Reception errors
If
you are experiencing mild reception problems, you will see video errors and
audio dropouts. In all cases you will
see a ratio of about 5 to 10 video errors for 1 audio dropout. If you are seeing video errors and/or audio
dropouts but not in this ratio then your reception is perfect and the fault is something
else. Usually the station or the network
is at fault, but occasionally it is the STB.
This
can be hard to tell. When a TV station
decides to provide an HD sub-channel, that sub-channel is normally 1080i (or
720p) all the time at the transmitter, even if some (or most) of the
programming originates at NTSC cameras. There is no technical requirement for
this, but it seems to be nearly universal practice. Thus your receiver’s “HD
detector” is not a reliable indicator of whether the program is actually HD.
NTSC 4:3 images will have black bars on the side that you probably cannot
eliminate because they are part of the 16:9 transmitted images.
The
most reliable way to tell if you are seeing HD:
If the image is 16:9 and it is not stretched and there are no black bars
on the sides and nothing is clipped off the top or bottom then the image is
720p (ABC, FOX, and ESPN) or 1080i (everyone else).
Why won’t satellite companies provide HD network feeds?
The
National Association of Broadcasters lobbies effectively for the local
stations. As a result, Congress has
legislated that these stations continue to enjoy their monopolies. Contracts between the networks and the local
stations vary a lot. The result is a
patchwork of rules on who can receive what. In most cases the satellites
operators are forbidden to offer viewers feeds or stations that would compete
with the locals.
If
you are out of range of the ABC, NBC, CBS, or FOX digital stations then the
satellite company might be allowed to provide you with the New York or Los
Angeles equivalents. The rules are too
complicated to state here, but if the local station is owned by the national
network then the odds are in your favor.
If you are told that a waiver is required from the local station then
you can apply for the waiver, but you will probably be turned down.
This
varies a lot. If a lot of cooking occurs
in the house, once per year is typical.
Fireplace usage can make cleaning necessary in only a couple
months. But the set might go for many
years if there is electrostatic air cleaning and no smoke sources.
This
is usually unwise. Refer to the owner’s
manual.
Refer
to the owner’s manual. The only thing in
the top half of an RPTV is a flat mirror.
The only issue is whether the weight of the STBs will deform the RPTV
case over time.
Extremely unlikely.
Any top and side speakers are quite far from the CRTs or DLPs, which are
in the bottom of the cabinet.
In
image processing, “artifact” refers to any predictable flaw in the image
resulting from shortcuts or shortcomings in the processing technology. In HDTV, most artifacts result from
compromises that have to be made when the picture changes too rapidly and
requires more than the allowed bandwidth (data rate). Sometimes the choice is to delete frames. There are a number of common artifacts that
result from converting 24 frames/sec films to 30 frames/sec
TV. Your particular TV model
might introduce some more artifacts.
(Snow and interference are not generally called artifacts.)
The
phosphors in CRT and plasma sets dim with usage. Making the screen age evenly is the user’s
responsibility.
These
sets age more slowly if the brightness is not set high, and are thus less prone
to uneven wear. “Brightness” here means
the brightness of a spot in the image.
Actually the contrast control has more effect on the brightest
spot than the brightness control does.
So adjust both of these no higher than you can get by with. Serious burn-in is most likely to occur in
the few hours after adjustment of the contrast and brightness controls. Note: If you notice a decided decrease in
brightness or contrast, you should clean the mirror rather than increase the
brightness. Otherwise you are tempting
fate.
Network
logos and scrolling “ticker tapes” on news channels are suspects to watch. But the black sidebars on 4:3 programs are
the biggest threat if you watch mainly 4:3 programs.
If
you do notice a very slight burn-in and you turn down the contrast, it will go
away slowly over time. (Some people say
turning the contrast down and the brightness up will make burn-in go away
faster, but the author is not sure of the wisdom in that.)
There
has been a significant improvement in the average quality of NTSC broadcasts
over the last several years. There are
two reasons:
First,
640x480 used to be a target that TV production crews believed they had to aim
for but didn’t really have to meet.
Their product was often well below 640x480, for a variety of
reasons. But now, with large high-def
sets becoming common, these production people are seeing how bad their product
can look, and are paying more attention to the details (camera focus, circuit
noise, cable reflections, filter circuit selection, etc).
Second,
many NTSC broadcasts are of shows that were shot with high-def equipment. These always reach the limit of what NTSC can
do.
Probably
all TV production people have the pride that makes them want to put out the
most attractive product possible. But
some crews are prevented by their circumstances. For example, the equipment upkeep budget
might be slashed because it is known that the equipment will soon be
junked. Whatever the reason, it is
always somehow economic.
I
sometimes hear complaints that HD broadcasts aren’t quite as good as they could
be. But this will always be true. At least now there is a higher target.
This page is part of “An HDTV Primer”, which
starts at www.hdtvprimer.com