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Line Doublers & Quadruplers
How Do They Work?
How do line doublers and quadruplers perform the conversion from TV video to computer video?
Doublers and quadruplers generate a computer video signal that is “a function of” or “related to” the original TV signal. By performing certain manipulations to the interlaced TV signal, they are able to modify it in such a way that it can be accepted by equipment that receives only non-interlaced, computer video signals. However, the type of signal you put into the doubler or quadrupler determines the type of signal you can get out of it.
As discussed in the previous section, there are 483 visible lines in an NTSC signal; 576 in PAL and SECAM. A line doubler takes these numbers of interlaced lines and converts them into a non-interlaced format. However, there are still the same number of total lines – 483 and 576 respectively; they just occur twice as often. Quadruplers differ from line doublers in that they create an additional line for each original line, converting an NTSC signal into a non-interlaced signal with 966 visible lines, and a PAL/SECAM signal to 1152 visible lines.
The refresh rate of the converted signal generated by line doublers and quadruplers is also a function of the incoming signal format. The NTSC signal has a vertical refresh rate of 30 Hz. In other words, a full “screen” of information is painted 30 times each second. Line doublers and quadruplers simply double this 30 Hz refresh rate, creating an output with a 60 Hz refresh rate. When starting with a PAL or SECAM signal with a vertical refresh rate of 25 Hz, doublers and quadruplers generate a converted signal with a refresh rate of 50 Hz.
What's In A Name?
Many people mistakenly think that line doublers output twice as many lines as the original video picture, and quadrupler, four times. This is not exactly true. Line doublers double the number of lines painted on the screen in each individual top-to-bottom pass, and quadruplers quadruple it. However, the total number of lines comprising the picture remains the same as the original when using a doubler and is doubled when using a quadupler. (Yes, this is a bit confusing!)
“Doubling” and “quadrupling” may also refer to the change in the horizontal refresh rate of the output video in relation to the original TV video. In other words, each individual horizontal line in the converted output from a line doubler is “painted,” left to right, twice as quickly as each individual line in the original TV video. And using a quadrupler, each individual line is generated four times a fast.
An Important Word About Motion Compensation
Different line doublers, quadruplers and video scalers (yet to be discussed) offer different sets of features. The purpose of this guide is not to review these various features and their respective functions. However, motion compensation is one feature that merits discussion within the context of this guide. To help you understand what it is, it is easiest to explain the problem it is designed to address.
As we’ve already discussed, one of the jobs of line doublers, quadruplers and video scalers is to convert an interlaced signal into a non-interlaced format. One way that these products accomplish this is by storing the first field of an incoming interlaced picture and “holding” it for a fraction of a second, while the second field, containing the remaining lines of the picture, are taken in. The odd and even fields are then combined into a single frame that can be output in a non-interlaced format at twice the original frame rate.
Now, imagine that the TV video you are converting to computer video is of a football game. In many of the frames, there would be a football moving across the screen. In the original NTSC TV video, the odd fields of the picture would be generated 1/60th of a second apart from the even frames. The football, if moving quickly, would appear at a slightly different in the odd and even fields. So, when the fields are combined into a single, non-interlaced frame, the football may appear to have jagged edges, as its location in the even lines is slightly different than in the odd lines. The not-so-technical term for this condition is called “the jaggies.” (The technical term is called temporal distortion.)
There are a variety of techniques applied by line doublers, quadruplers and video scalers to eliminate, or at least reduce, the appearance of “jaggies.” The effectiveness, as well as the associated price, of these techniques vary, and not all line doublers, quadruplers and video scalers offer a motion compensation feature.
Limitations of Doublers & Quadruplers
The limitations of line doublers and quadruplers are due to the very nature of how they work. Because the format of the input affects key aspects of the converted output, namely the number of lines and the vertical refresh rate, line doublers and quadruplers are limited in the types of output they can provide.
As mentioned earlier, when starting with an NTSC signal, line doublers always produce a signal with 483 non-interlaced visible lines; quadruplers generate a picture with 966. When using a PAL or SECAM signal, line doublers create a picture with 576 non-interlaced visible lines; quadruplers create 1152.
If you are familiar with standard computer resolutions, you know that there are not any standard computer resolutions that have these number of lines. Therefore, incompatibilities arise when trying to input a picture with a different number of lines than the projector or display is designed to accept. One of several things happens: 1) the picture gets cropped; 2) the picture gets stretched and/or distorted; or 3) the projector or display performs additional processing on the incoming image to make it compatible with its display format. This additional processing almost always further degrades the quality of the picture.
Another limitation is the set vertical refresh rate generated by doublers and quadruplers.
The converted output from a doubler or quadrupler always has either a 60 Hz or 50 Hz vertical refresh rate, depending on whether the original signal was in NTSC or PAL/SECAM. Most display devices in the US are capable of taking in a picture with a 60 Hz refresh rate, and in countries using a PAL or SECAM standard, equipment is designed to take in signals at 50 Hz. However, many projectors have the ability to take in and display higher refresh rates, therefore offering a better picture. When using a line doubler or quadrupler, you are limited to using the lower 60 or 50 Hz rate.
Finally, many line doublers and quadruplers can only generate a picture with a 4:3 aspect ratio. This does not allow for the ability to display letterbox formatted video on new 16:9 plasma display panels or CRT monitors. And in the future, as we see more and more devices, such as LCD panels, offering 16:9 displays, line doublers and quadruplers will not provide a means to tap into this capability.
Having reviewed the technology of line doubling and quadrupling, along with the associated limitations, we will now explore a new alternative to these techniques: intelligent video scaling.
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Ну вот...хоть о деле заговорили...МАХ, а(+) -- Yegi -- 09.08.01@15:01 (Чит.: 152)