High definition is a term that describes any resolution (there are many) above standard definition. High definition comes in 480p, 720i, 720p, 1080i, and 1080p. Standard definition is just 480i. The "i" means interlaced and the "p" means progressive scan. Progressive scan pictures are sharper and crisper than interlaced even at 480. The number itself refers to the number of horizontal lines of resolution.
It should be noted that even as 1080p is a 16x9 picture, "full screen" on a 16x9 wide screen tv does NOT mean high definition. A cable box connected to a 16x9 tv using coax generally produces a full screen but is also (by nature of this kind of connection) standard definition only. Even when Component or HDMI is used, sometimes a 4x3 picture will appear. This produces blank bars on the left and right of the screen. This deals with 'aspect ratio'. The settings of the box and of the tv can be changed to "stretch" or "wide screen" to make up for this. On high definition channels, the broadcaster controls this and you might see changes between a program and commercials. Sometimes they will use fillers on the left and right. You might even see a broadcast with a combination of standard and high definition on the screen simultaneously. These things are controlled by the broadcaster. You are not likely to see full screen on everything all the time unless you have a tv that automaticly adjusts itself or you are using the standard definition coax input. High definition cable boxes are able to self adjust to send to the tv whatever level of high def. is being sent to it. You might notice a short flicker while it changes resolution levels (this is normal).
I would advise anyone thinking about buying any kind of HD tv to do some research BEFORE actually putting the money down. This will help insure a more satisfactory experience with the end result. A tv with a 1,000,000:1 contrast ratio and a 3ms refresh rate (also called response time) will produce a better picture than a tv with a 10,000:1 contrast ratio and a 5ms refresh rate. A good rule of thumb would be to buy the tv with the highest contrast ratio and the fastest refresh rate (measured in milliseconds) that you can afford.
Some devices such as DVD players will have component output connections. It is important to keep in mind that even if component connections are used with equiptment like this, it does not mean that a high definition picture will appear on the screen. You will only see a high definition picture if your using high definition equiptment. A standard definition DVD played in a standard definition player will produce a standard definition picture. Upconverting can often be done but, upconversion does NOT actually produce a high definition puicture. Upconversion is an electronic "bandaid" designed to fool you. You have to remember that you can't get something from nothing. You can't get high definition from a standard definition DVD. This is also true of standard definition tv's that have component inputs. Upconversion is what some satellite companies use to advertise 1080p broadcasts. Broadcasters are NOT sending out 1080p signal yet...due to bandwidth issues.
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There are certain ways to connect a cable box to a tv while also using a surround sound amplifier. Wether component cables or HDMI is used. This way usually works better with the fewest problems. This method also allows the user to have the surround sound system on (with all the loud audio) only when it's desired and NOT at all times. This would likely be better when watching the early morning news over that first cup of coffee.
Connecting these devices this way is often practiced by installers and tends to create problems. Some of the problems that come up include picture with no sound or sound and picture out of sync with eachother.
Then there is this method. This should only be done when the tv is not designed with internal speakers or when a monitor is used.
If you are using either of the second two methods and you are not having problems, that's great...leave it. But, the fix to alot of problems is to use the first method. If the surround sound system is not HDCP compliant, then using HDMI "through" it will result in either no picture or a flickering picture on the tv. It never made any sense to me to send a video signal to a sound system anyway. Sound systems do not have a video display. Having the video signal go through a sound system does not actually do anything but add more electronic switching which can degrade the video signal itself (resulting in an out-of-sync condition). This out-of-sync condition can also occur if a digital audio signal is used and the surround sound system either can't handle it or handles it differently that the tv would handle it.