Selecting the Right Scan Converter for your application

WHAT IS A SCAN CONVERTER?

A scan converter is a device that converts a computer-video signal into a signal that is compatible with a television. Sometimes a scan converter is referred to as a "PC to TV" converter or a "PC to video" converter because it allows you to view computer-video on a TV set. Although the concept seems simple, scan converters use complex technology to achieve signal conversion because computer signals and television signals differ significantly.

WHAT DOES A SCAN CONVERTER DO?

It would seem feasible that with today's high-tech televisions you could hook up your computer to a TV as a display option, but there are several factors which make this impossible. For starters, a computer's video signal runs at a higher frequency than does a television's. This means that a computer puts out more video information at a faster rate than a television. More information is "painted" onto the screen, which in turn provides a better picture and greater resolution.

A television, however, operates within a standard that limits the video signal's frequency. It can only accept signals at a frequency of 15.75 kHz (NTSC). Compared to a computer's signal frequency, which ranges from 31.5 kHz to greater than 100 kHz, 15.75 kHz produces a less detailed, dimmer picture. Less information is painted onto the screen, and that is why a TV does not produce those intense, crisp colors and images you get from a PC. Scan converters change the scan rate of a computer signal (i.e., 31.5 kHz) to fit the scan rate of television (15.75 kHz). Therefore, in order to view higher frequency video from a computer on a television, you must use a scan converter.

Another reason why you cannot simply connect a PC to a TV is because they both receive signals differently. A computer outputs red, green, blue, and sync as separate signals. This gives you the best picture quality because each signal is isolated. Televisions can only accept lower resolution signals such as RF, composite, S-video and component. Composite video is a composition of red, green, blue and horizontal and vertical sync as a single signal. The process of combining red, green, blue and sync information is called encoding. Therefore, a scan converter must encode the computer-video signal so that the television can accept it.

When a signal is down-converted the resulting image will never look as good as the original. It is inevitable that when you decrease image resolution, the result is a less than perfect picture. This is true with all scan converters or down converters.

WHEN DO YOU NEED A SCAN CONVERTER?

Anytime you want to display a computer-generated image on an NTSC or PAL-based component, such as a TV or VCR, you must use a scan converter. Typically, scan converters are used in applications such as computer-based training video production and videotaping, videoconferencing, distance learning, broadcasting or production. All of these applications require the conversion of high frequency computer-video to a signal that is compatible with VCRs, cameras, editing systems and other video related products.

FEATURES OF A SCAN CONVERTER

The basic function of a scan converter is to convert the scan rate of computer-video and encode the signal so that it is compatible with NTSC or PAL-based components. There are, however, other functions of a scan converter that provide additional benefits.

Autoscanning is a feature that allows a scan converter to accept any incoming computer signal within a given range. A true autoscanning scan converter will recognize any incoming computer signal and lock on to its particular frequency and resolution. For example, a computer may be running at a scan rate of 70 kHz with resolutions ranging from 800 x 600 to 1280 x 1024, and true autoscanning allows it to lock at any frequency within its range.

Horizontal filtering (anti-aliasing) enhances and stabilizes the final scan rate converted video image. As the original signal's frequency is slowed down, a loss of detail is inevitable. This is due to the reduction of picture elements, or pixels, that are visible on the screen. Less information is being painted onto the screen both horizontally and vertically. Horizontal filtering prevents anti-aliasing or pixel elimination so that less detail is dropped when the television scans from left to right.

Vertical filtering (flicker filter) also enhances and stabilizes the final scan rate converted video image. Also known as a flicker filter, vertical filtering provides line averaging so that before the scan converter eliminates lines in the down conversion process it fills in missing information. By filling in averaged information, the scan converter helps to maintain vertical detail and eliminate picture flicker.

Color sampling (color depth) represents how accurate the color reproduction of the scan converter will be. In order to match digital color to analog color, a scan converter must be able to provide a high degree of color reproduction. In the process of color or color depth sampling, measurements of analog color level (intensity) are taken and then reproduced in the binary code that best represents the original analog video color. Extron's scan converters provide 8-bit sampling per color, or 16.8 million colors, to ensure that they achieve the closest color possible when compared to the analog color.

Encoding is the process of combining red, green, blue, and sync video information into one signal (such as composite video). A high quality digital encoder will allow for this composition and still provide the clean, sharp transitions between colors.

Underscan and overscan are two ways to adjust the computer image's size on a television display. When you take a computer image to TV, the image will oversize off the screen horizontally and vertically. This can pose a problem especially when vital information has sized off the screen. Underscan resizes, or shrinks, the image both horizontally and vertically, maintaining the correct aspect ratio. Overscan is just the opposite and allows you to increase the size of the image on the display screen while still maintaining the proper aspect ratio.

Zoom/pan and centering are features that also allow you to change the way the audience views an image on a TV. Zoom refers to closing in on an image and enlarging a particular detail or decreasing the overall size of the image. Zoom also refers to "stepping back" from the image, as in "zoom out." Pan is what allows you to view specific parts of the graphic while moving it around the screen. Centering is just what it sounds like; this feature allows you to horizontally and vertically center the image on the display screen.

Memory is a valuable feature on a scan converter. With a memory preset you can make adjustments to the image from a particular computer, and these settings will be stored in the scan converter's memory. When you reconnect the computer the scan converter will recognize it and pull up the saved settings from memory. Extron's scan converters provide at least ten memory presets that save automatically so you don't have to remember to save each time.

Genlocking is a necessary feature for broadcast or production environments. Genlocking is the process of synchronizing all video devices for the purpose of pixel switching and chroma keying of computer overlays. A specific type of genlocking, known as chroma lock, refers to the synchronization of the sync signals and chroma. This allows the signals to be edited and composed to make a complete segment without any kind of distortion, glitch or signal irregularity.

RS-232 control allows for control systems to control several components in a system without having to use the front panel buttons. Extron's scan converters are often integrated into a system environment, whether it is a broadcast and production setting or a professional corporate boardroom setting. When integrated, it is more convenient for the user to control the system via one control system. An additional advantage to Extron's RS-232 control function is something called Simple Instruction Set™ (SIS™). The SIS makes programming the scan converter very simple. A pre-written series of codes are available and readily accessible to the installer, making it easier to set up the RS-232 control function for the scan converter.

There are several factors to consider when purchasing a scan converter. The first is the scan converter's computer input compatibility. But there is more to finding the best scan converter than meets the eye, sometimes, the features, performance, size, or cost.

RESOLUTION

Let's start with the most important: scanning compatibility. When choosing a scan converter you need to look at the frequency rate and resolution of your computer. What is the maximum resolution you are using? All scan converters can output a good looking image with a 640 x 480 input, but for higher resolutions the better scan converters will output better pictures. Something else to think about is that the computer you have now may not be the computer you have tomorrow.

DOES YOUR APPLICATION REQUIRE GENLOCKING?

If you plan to use the scan converter in a broadcast or production studio environment you will definitely need genlocking and want a device that provides chroma lock. Chroma lock, as previously mentioned under genlocking, is the feature that provides full color burst synchronization necessary for computer overlaying and chroma keying processes. Chroma lock is not necessary, however, in most professional boardroom installations.

CONSIDER COLOR REQUIREMENTS

Consider the scan converter's color bit sampling performance. In a broadcast/ production setting color bit sampling must be very precise so that the final video image looks exactly like the computer-generated video or graphics. All of Extron's scan converters provide high quality, 24-bit, color sampling for a range of 16.8 million colors.

DIGITAL ENCODING

Another consideration for choosing a scan converter is encoding. The broadcast/production environment requires a superior quality digital encoder. Remember, that a premium encoder outputs exceptional image quality which may affect the overall price.

SERIAL DIGITAL, COMPONENT, S-VIDEO, COMPOSITE VIDEO

Which of these video formats is important for your system? Will you ever use digital video or even need component output? Component will look better than composite video, but in teleconferencing applications, for example, composite video may be all you need. Maybe you are using equipment that will only accept composite video. It's just one more thing to think about when choosing a scan converter.

PRICING

When you look at your application and consider everything, chances are the scan converter's price will be a consideration. You will pay more for more advanced features. And you will pay less for a basic model.

MANUFACTURER

The manufacturer is a critical factor to the pricing question. Look for manufacturers who support their products and those who use them. Consider the company's track record for customer support, its reputation in the industry, and its warranty program. When you pay for a product, you are also paying for the support and guarantee that it will do what it should.

A good quality scan converter is indispensable to your AV system as it will affect how the final image looks. Before you purchase a scan converter, size up your application and look at what will give you the best image quality at the best price.