These notes describe some concepts on the conversion of computer video to TV video signals, generally performed by a device called ‘encoder’. We apologize to the experts for some oversimplification here and there. But is is important, we believe, to have some basic information on the process (of conversion) and its ingredients to select and use the equipment properly.

First of all some agreement on terms. When we use the word "video" we tend to think of a signal displayed on a "monitor". By far, the most common monitor is the television monitor, hence by deduction, we associate "video" with "television" and tend to think of the two concept or words as equivalent. This is NOT so. There are in fact two classes of video signals:

• Computer Video signals
• Television Video signals

Converting any one computer video signal to any one television video signal is the task of a class of products referred to as scan converters or "encoders".

For accuracy the term "encoding" refers (and used to refer exclusively) to the process of transferring (i.e. encoding) color information on to an (otherwise) monochrome video signal (whether computer or tv). Usage makes the law also with words and now ‘encoder’ refers loosely to those devices that adapt or convert a computer video to a television video signal. The accurate term for an ‘encoder’ is - and should be - ‘scan converter’, because the key to conversion is to adapt the ‘scanning’ frequency of the computer video signal to the ‘scanning’ frequency of a television signal. Scan conversion can be done through software, hardware or a combination of both. A true scan converter does the conversion all in hardware, the other devices (and very common) do it by combining hardware and software. We should describe those other devices as "converters that modify somewhat the computer video scanning frequency so as to make it compatible with the television scanning frequency". For brevity we will refer to these other encoders as "hybrid scan converters" (this is an unofficial Computer Friends definition).

A true scan converter is totally independent of the computer or of the computer operating system. It can also ‘convert’ displays of higher resolutions than the standard 640 x 480 pixels, for example 800 x 600, 1024 x 768 etc. And it can work with computers whose vertical scanning frequency is different from the standard 60 HZ,for example the Macintosh, which has a vertical scanning frequency of 72 Hz - we will return later to the concepts of horizontal and vertical scanning frequency. A hybrid scan converter can only work on PC compatibles.

We just spoke of different screen resolutions. To avoid misunderstandings, even a 25,000 $ scan converter will give you, on output, a television signal which will yield a screen image of (roughly) 640 x 480 screen resolution. What is the point, you will ask, of converting a higher resolution computer video signal if I cannot see an equivalent resolution in the output TV signal? The point is that YOU CAN SEE IT. If you select a 1024 x 768 resolution on a a hybrid encoder the tv output signal is a blank screen. Why bother then? Because some programs, for example CAD/CAM and others make use of the higher computer video screen resolution. For teaching or demonstrating CAD/CAM on large TV screen or for recording related training sessions to a VCR you need a true scan converter (if you use the higher resolution display mode on your computer monitor).

The main trade-offs between a true and a hybrid scan converters are:

• price (hybrid scan converters are really economical)
• screen resolution (as above, a hybrid scan converter is limited to converting 640 x 480 (in NTSC, American standard) and 800 x 600 (in PAL, the European standard). This (640 x 480) is by far the most used and popular resolution
• occasional system dependence (a problem now almost totally overcome). A hybrid encoder relies for activation on TRS or terminal resident software, provided with the unit.
• Broadcast quality video signal (for TV transmission, including closed circuit TV) requires a signal with certain characteristics. Of the (4) encoders we offer, the Hyperconverter has a broadcast quality video signal.

We will offer some additional information on the characteristics of computer video versus TV video signals, on flicker reduction etc. If you feel you have adequate information for a selection here are the converters (encoders) we offer:

LifeView II (hybrid scan converter), external. Good, all round hybrid converter, excellent color reproduction, good flicker reduction. Can be used with a laptop, very handy.

Elite (hybrid scan converter, available as internal or external). Good, all round hybrid converter, good flicker reduction comes with appealing bundles. A bit more expensive and for those who love the bundles.

• TV SuperScan - true scan converter, reaches resolutions (in NTSC) of 800 x 600, good flicker reduction. More expensive but still below $500.00
• Hyperconverter, internal and external, reaches resolutions (in NTSC) of 1024 x 768 and now 1260 x 1080. Good flicker reduction - true broadcast quality signal, More expensive, but still a fraction of what TV stations pay for equipment doing the same thing.

Television Standards

Here is a table showing the characteristics of different Television Systems in the world.

• NTSC stands for (National Television Standards Committee)
• PAL stands for (Phase Alternate Line)
• SECAM stands for ("Sequentiell Couleur A’ Memoire)

A bit of historical levity. The first convention introduced in the world was the NTSC - each system has trade offs, the trade off in the NTSC system resulted (at the beginning) in color alterations under different conditions of transmission. The French came up with their own system and the British thought of reaching the best trade-off with theirs. In the internal lingo of TV folks, NTSC stands for Never Twice The Same Color, SECAM stands for Something Extremely Complicated and Anti-American and PAL stands for Peace At Last.

Scanning Rates

A video monitor displays an image by scanning an electronic gun (3 guns in a color monitor) from left to right and from top to bottom. This gradually builds an image on the screen and must be done at least 50 times per second for the human eye to perceive an unbroken image. TV transmission is now an ‘old’ technology. When it was introduced the standards were established as a compromise between picture resolution (ability to represent small detail) and flicker. Note that while it is easy to visualize the vertical frequency (a beam going from left to right and from top to bottom 50 times a second), the horizontal frequency is not so intuitive. The horizontal signal is a modulated wave where the modulation carries information about the picture and its color. By increasing the vertical frequency we increase resolution - however, at the same time the horizontal frequency must also be increased otherwise there would be flicker. In fact an SVGA signal has a vertical frequency varying from 45 to 72 Hz and a corresponding horizontal frequency of 31.5 KHz. Why should not the TV signal have the same characteristics of a computer video signal? Because by increasing these frequencies, we increase the bandwidth (i.e the portion of the wave spectrum) of the TV signal. There is only a limited number of frequencies available for TV transmission, if we wish to transmit more than (1) channel - hence the compromise and the limitations and the need of converters (encoders). With the advent of Digital TV in a few years time these limitations will be gone, but technology, like nature, advances gradually (see the Latin proverb, Natura non facit saltus", "Nature does not advance by jumps").

Interlaced and Non Interlaced Displays

An image can be displayed in two modes, interlaced and non-interlaced. This refers to the way an electron beam that creates the image is moved horizontally and vertically across the screen. A non interlaced image is completely displayed during one vertical scan of the video monitor. An interlaced image requires (2) vertical scans. During the first scan, the electron beam writes the first line, jumps a line and write the next etc. At the end of the first scan the beam retraces back to the top and writes the lines that were ‘skipped’ during the first vertical scan. This is a way to double the resolution without increasing the scanning rate. TELEVISION IMAGES ARE ALWAYS INTERLACED. Interlacing works with images of soft edges, without fine detail, that would cause flicker. COMPUTER VIDEO IMAGES are rarely interlaced.

The unusual shape of color video waveforms.

As indicated earlier, ‘encoding’ refers more properly to the process that makes a waveform carry color information. Color is reproduced by combining in different amounts (for each represented point) the individual components of RED, GREEN and BLUE (RGB). Let’s first consider a TV signal. In a monochrome TV the screen needs only one signal, that is, brightness or LUMINANCE to convey the black and white information. In color the (3) components, red, green and blue are needed. But TV signals are not transmitted using (3) separate RED, GREEN and BLUE signals. This is because:

• The TV transmission color system has to be compatible with existing black/white set - that is you should be able to view a TV signal in black/white on a monochrome monitor and in color on a color TV monitor
• The need to reduce the frequency band used by each channel so as to have more channels available
• The human eye is less sensitive to color clarity (chrominance) than to intensity (LUMINANCE). Hence the color differences can be transmitted in less detail

To transmit color over the air waves, TV uses a single signal, formed by adding together:

• the luminance value (Y)
• the color differences, referred to as (U) and (V)
• a synchronizing pulse indicating the beginning of each line and field.

We refer to this TV signal as a COMPOSITE signal, because it is ‘composed’ of individual, superimposed signals. As color information is conveyed with less detail, there is some color blending on smudging. Also the superimposed signals must be filtered at the receiver. These are some of the reasons why a TV video signal looks inferior to a computer video signal.

In S-video two signal are used, one to carry the Luminance value (Y) and the other (C) to carry the (2) color difference signals (chrominance). S-VHS does not require filtering and picture quality is significantly improved.

YUV professional video, uses (3) signals, Y (luminance) and U and V, the color difference signals. No color decoding or filtering is required and this results in excellent picture quality.

In RGB (the signal you typically see on a computer video monitor) (4) signals, Red, Green, Blue and a sync signal are used to display the image. This is an almost direct connection to the TV picture tube and picture quality is excellent.

More on Converting a Computer Video to a TV Video Signal

We said we offer two types of converters, one type (hybrid) requiring TRS terminal resident software and one where the conversion is all in hardware and requires no software.

The hybrid converter type uses a memory buffer (FIFO first in first out) to convert one scanning rate to another. Data is sampled at the input and read back at exactly half the sampling rate. This works quite well because the VGA horizontal scanning frequency is approximately double the TV horizontal scanning rate or frequency, 31.5 kHz versus 15.734 kHz. With 800 x 600 displays the scan rate is higher and this hybrid system does not work.

The full Scan Converters (TV SuperScan and Hyperconverter) convert both the scan rates, horizontal and vertical in hardware. They include a large memory buffer (more memory, more money). The buffer stores a complete computer image received at one scanning rate and outputs it at another TV-compatible scanning rate. Both operations occur simultaneously.

Overscan and Underscan

A computer image is underscanned - there is always a black b order around the edge. A TV picture is always overscanned, i.e. no black border around the edge. Why? Because in a computer image the borders are used to display menus, scroll bars etc. which are critical functions or commands for running the computer. Underscanning ensures that they are not off the edge of the screen. TV images are made as big as possible so as to fill the entire screen. When converting computer video images to computer TV images there will be occasions when the image must be underscanned (for example a Windows program) and when it should be overscanned (picture of a person, landscape etc.). With the hybrid converters under/over scanning is achieved by a software command. In hardware scan converters the function is performed by actual switches.

Flicker Reduction

Flicker is inherent to a TV display due to the limitations in the signals involved and the effects due to interlacing. If you have one white, pixel-wide, horizontal line, it will be displayed 25 times per second (refer to the interlacing diagram). The white line will appear to pulse on and off. Remember that any interlaced video image (frame) is composed of (2) fields. One field refers to the odd scan lines that make up the first half of the frame, the other refers to the even scan lines that make up the other half of the frame. To reduce flicker the software (and the hardware) slightly soften the image vertically, thus avoiding a bright horizontal line to be next to a dark horizontal line, which is the original cause of the flicker. Flicker reduction works, but we advise some practice to optimize the use of the flicker reduction function.

Why not use a video card that has built in a TV signal output?

The reason is simple - computer video cards evolve more rapidly than does TV broadcast television. If you lock yourself into a VGA card because of its TV signal output capabilities, you will probably end up not being able to use it because you need an upgraded VGA card thus also losing the conversion to TV function.

Genlock and Overlay The following notes refer to the use of our Video Editing Cards.

‘Genlock’ is a function required when mixing two video signals or overlaying one video signal on top of another. The terms refers to ‘locking’ a video signal to a ‘generated’ reference synchronization signal. The word is an import from TV studios where all the various cameras are in synch (‘locked’) with each other via a common ‘generator’ synchronizing signal. It is easier to visualize overlays (e.g. titles on a video image) than a synch signal - besides, ‘genlock’ is required for overlay or for mixing two video signals. Consequently ‘genlock’ and ‘overlay’ are often used as synonyms though technically they are not.

Overlay is the function of mixing two synchronized video signals together so that a section of one signal (i.e. a title) can be superimposed onto the other signal.

The most common method of overlaying is ‘chroma key’. A ‘key’ color is selected and the background (i.e. live TV signal) shows through only when that color is selected. For example let’s say we have on the computer screen the picture of a house, in Photostyler or Corel Draw or whatever. Let’s make one of the windows blue and let’s select that value of blue (from the palette) as the chromakey color. The other video signal, typically the ‘live’ video, will now show in the window. Chromakey is typically used in titling - in TV stations it is used, for example, to show the weather map behind the weatherman. He stands in front of a color sheet - additional equipment replaces the color with a weather map. At home you get the impression that the weather man stands in front of the weather map. He will never wear a suit of the same color or only part of him would show with probably somewhat comical results.

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