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.
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?
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