9/22/09
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 many 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 “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 every 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.
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.)
This page is part of “An HDTV Primer”, which
starts at www.hdtvprimer.com