Advanced Home Theater Video Terms
The purpose of this section is to provide the more advanced user a little more detail about the various video terms you will encounter (Thanks Chris). This is a work in progress so please keep checking back.
Cathode Ray Tube
The staple of older TV sets this glowing tube has projected images into our homes for many decades. With this technology the front of the tube is electrically charged by attachment to an anode. Behind the screen is the cathode, the source of a ray of electrons. In the area in the middle of the tube is kept in a vacuum. The cathode shoots a stream of electrons out which is focused by a couple of smaller anodes and sent to to illuminate a line on the front of the tube. It is these lines which build up the picture. For those in the UK using the PAL video format the picture is composed of 625 lines while those in NTSC countries have 525 lines. The actual viewed picture is often reduced down to 480 lines of picture to improve the refresh rate. For color TVs there are three phosphors used in the target screen, green, blue and red. The electron stream is focus on each to build up the color required for each picture displayed. This is achieved by directing the stream through a making grid to separate out the colors. The phosphors used in the CRT are also subject to burn. This is when a static image persists after the electron stream as left the phosphor. You can often see this with something like MUTE on the screen where the word will still be visible especially when that part of the screen is displaying a dark image.
A single video signal that contains luminance, color, and synchronization
information. NTSC, PAL, and SECAM are all examples of composite video systems. Composite video signals are connected between products with a single 75 ohm coax cable, usually with RCA connectors on each end. Composite video inputs or outputs are present on almost all contemporary video equipment. Composite video signals can also be modulated onto an RF carrier, along with an audio signal, and transmitted over-the-air or on coax cable, by broadcast stations and cable TV systems. RF video signal cables are usually 75 ohm coax terminated with screw-on F-connectors.
In producing a color picture from light, our color television system starts out with three channels of information; Red, Green, & Blue (RGB). This is certainly one form of component video. In the process of translating these channels for use in distribution, they are often first converted to Y, R-Y, and B-Y or Y Pb Pr. This is another form of component video. The term component describes a number of elements that are needed to make up the picture. It could be argued that an S video signal is also a component signal. A composite video signal on the other had contains all the information needed for the color picture in a single channel of information. Much higher program production quality is possible in the component domain because analog compression is used to place the three channels of component information into the single channel of composite information. Once that compression take place it is extremely difficult to get back the original quality of the component signal.
The type of television most of us are used to is the interlaced television. In this format the odd lines of the picture are scanned onto the screen followed by a second pass for the even lines. This format is based on the good old fashion Cathode Ray Tube (CRT), the big, bulky glass tube in most TVs. This technology is well over one hundred years old. Since the phosphors of the screen take some time to degrade, that is to lose the brightness, the picture is perceived by the eye as a single image, albeit with flicker. There are many artifacts inherent with the use of the interlaced format. Among the most popular are line crawl, where you can perceive the motion of the electron stream. Saw Tooth distortion, where the edges of the images are not sharp but take on a zig-zag distortion. The designation used by interlaced screens is usually 480i where the 'i' denotes interlaced video format.
Basically, progressive scan indicates the ability to display a picture on the Television or monitor in one pass. The old standard of Interlaced painted the screen one line at a time, alternating odd then even lines. This resulted in a lot of screen flicker. With the progressive scan the refresh rate is increased from 30 times per second to 60. There are usually 480 or 1080 lines of resolution on a progressive scan screen. The designation for this is 480p or 1080p, the 'p' referring to progressive as opposed to 480i for the normal interlaced screen. When you have a progressive source like most DVD players and a digital or high definition TV (HDTV) the result is a clearer picture with virtually no flicker or edge and motion artifacts. Since most motion pictures are filmed at 24 frames per second there is some conversion that is required to ensure a smooth picture at the 60 refreshes per second rate. What is employed is called a 3:2 pull down or 3:2 inverse pull down. Whit this method compensation is added to provide even less distortion during fast motion scenes. The connections are usually through component video cables. Most DVD players are capable of also outputting regular analog interlaced signals so you can get a progressive scan DVD player now and upgrade to a digital TV later on.
Short for Super Video. A high-quality method of transmitting video signals over cable to a television from a device such as a Camcorder VCR or game machine. S-Video separates information into two signals: Chrominance (separates color information) and Luminance (brightness). This prevents color bleeding and dot crawl, and increases clarity and sharpness. Once the information is finally delivered to the TV it is done so as a single signal over one wire. S-Video requires an S-Video input jack on the television receiving video information, support for S-Video output on the device sending signals, and a special S-Video cable.