What is a Scan Converter?

A scan converter is a device that converts computer video to television video. Although this is a very simplified explanation, scan conversion is a very complex procedure which requires the use of analog and digital circuitry to transform nonstandard computer signals into standard consumer type video. Television sets, VCRs, video cameras and other TV products operate at a frequency, or horizontal scan rate, of 15.75 kHz. In North America, this is known as the National Television Standards Committee (NTSC) standard. Any VCR is compatible with any television set in terms of frequency.

Computers, on the other hand can also operate at 15.75 kHz, but with today's technology, and the demand for high-resolution graphics, most operate at a minimum frequency of 31.5 kHz (IBM VGA frequency). That's twice the frequency of television, and that's just the beginning. Super VGA and Apple Macintosh computers, which are commonly used in the home, can operate at frequencies exceeding 60 kHz. The higher the frequency, the higher the resolution of the image. Workstations used in business and industry may have scan rates that exceed 100 kHz. So there is obviously a frequency difference between computers and televisions. This is where scan converters come in.

Here are some of the uses for a scan converter:

• display computer video onto a (15.75 kHz) monitor
• use computer video in teleconferencing
• display a computer image on a TV set
• video tape a computer video output

Do You Really Want a Scan Converter?

One of the inherent disadvantages of any scan converter is that it will decrease the resolution and frequency of the original computer video signal. A prospective buyer must recognize that the end result of scan conversion will produce an image of lesser quality than the original. So, the decision should depend on the application in which the computer video is to be used.

Other options may be available for displaying computer-video with large screen projectors. Large monitors and multi-scan projectors are available that are compatible with computer video signals. These devices do not reduce the resolution - what you see is what you'll get. LCD panels and projectors have also become an alternative to scan conversion for display purposes. They can be fairly expensive, but these devices will allow the signal to be displayed from the computer with minimal loss of picture quality.

For video taping of computer-video signals, or for displaying on a standard TV set (or monitor or projector that only handles 15.75 kHz signals), scan conversion is necessary because these devices won't accept the computer signal directly. Computer -video in teleconferencing is another growing technology that uses scan conversion to transmit computer signals over telephone lines.

The requirement for the computer signal is what determines whether scan conversion is a necessity or simply one of the options. Once we have determined the necessity, the confusing part begins because there are so many types and models of scan converters available.

Three Type of Scan Converters

There are three technologies used in scan conversion, which will be discussed here. Choosing which type is the first step in selecting a specific scan converter.

Consumer Grade

The typical retail costs for consumer grade products in this category could be from $200 - $800 (U.S.). This technology often uses software that resides on the video card to change the non-interlaced, 31.5 kHz or 35 kHz signal to a 15.75 kHz, interlaced signal. This 15.75 kHz, RGB signal must then be sent to another device to make it an NTSC or PAL video signal. This is done by eliminating every alternate line from the computer output; thus, this method actually throws away lines of picture information.

The one advantage a software driven converter has over most others is the low price, while the major disadvantage is the lower picture quality. Not only do these units produce a very poor quality signal, but they also have many limitations in graphics card compatibility, limited features, and usually have only composite NTSC or PAL output. Some do not allow the use of a local monitor. One word of warning - know and see what you're getting before you buy it!

Professional Grade

The price range for this medium level scan converter can be between $1295 and $2400 (U.S.). The technology used in this professional grade scan converter is much more complex. This uses real-time interpolation to sample the pixels in the non-interlaced computer signal. It then calculates new pixels and combines them with the originals to create an interlaced NTSC or PAL signal. This resultant video image closely resembles the computer image because lines and pixels are not arbitrarily thrown out. The resolution is still reduced but there is more control over which important pixels are kept. This conversion is done entirely within one external box and no software drivers are needed.

These professional quality converters will generally do an adequate job for a reasonable price. For recording, the S-video (S-VHS) output is the better quality while the composite video output is acceptable. A third output in RGB format is common on some of these units and usually provides the best quality output for viewing. Some manufacturers of professional grade scan converters have lowered the prices by making the RGB output an extra cost option.

Most units will have enhancement features to allow easy conversion to standard television sets, VCRs and other display and recording devices. Other features found in this type of converter include: overscan/underscan, image sizing, picture positioning, Genlock, anti-flicker control and freeze frame. Professional quality scan converters are not limited by software, and therefore are able to handle a wider range of inputs and computer signal timings. Most of these models work with all modes of VGA and VESA and some also work with Macintosh. Also, because the actual scan conversion is done outside the PC, the local monitor is not affected.

Despite the capabilities and features of this group of products, be aware of what to accept as "reasonable"; results. These units do not provide broadcast-quality outputs and are subject to some flicker because the picture is an interlaced NTSC or PAL signal. Some sharpness of the PC screen will be lost due to the lower resolution, but this type of converter should provide an acceptable solution for most professional applications.

Professional Broadcast - Quality

These are the top of the line VGA, Mac and workstation scan converters and can cost upwards of $8,000 - $30,000 (U.S.). That's the only disadvantage! If you can afford it, this is the best way to go. The technology used here is very similar to the professional quality units described earlier in that it uses digital pixel sampling. However, the sampling is done much faster and with greater accuracy. Picture quality is not sacrificed on the video output. The sync and video output timings match broadcast video quality specifications exactly, making recording and broadcasting top notch. Other features may include: zoom, pan, chroma key as well as horizontal and vertical picture manipulation.

Making the Decision

Before an educated decision can be made on the purchase of a scan converter, some important considerations should be taken. Here are some questions you might ask yourself.

• How much money do I want to spend? Since scan converters can vary so much in price, you need to know how much you're willing to spend.
• What types of computers will be used? Many models of converters are limited to VGA only while others can accommodate SVGA, Mac and workstations.
• Which output formats do I need? Scan converted outputs include NTSC, PAL, Composite, S-VHS, RGB Sync, and component video (y, r-y, b-y). Which format does my display device accept?
• What features do I need? Features commonly found on scan converters include Genlock, freeze frame, picture centering and sizing, overscan/underscan, interlaced/non-interlaced output, and flicker reduction.
• Do I need to view my local computer monitor while using the scan converter? Many models (including most of the software driven units) have no provision for use of the local monitor with the scan converter. You may miss that monitor once you can't use it any more.
• Does the scan converter use software? This is a tip-off that the quality of video output is probably inferior to that of the non-software units.
• Who manufactures the units I am considering? A lot of companies come and go in this business and the reputation of the manufacturer tells a lot about the products it makes. The warranty and service offered by the manufacturer is always very important.
• Have I seen the unit in operation, and do I know for sure it will work in my application? This is perhaps the most important consideration. A unit can be within your price range with all the features you want, but if you don't like the picture or it doesn't work in your application, what good is it?