Scan Converters: Computer Output to Your TV Screen

By Kinnaman, Daniel E. | Technology & Learning, May-June 1993 | Go to article overview

Scan Converters: Computer Output to Your TV Screen


Kinnaman, Daniel E., Technology & Learning


Scan converters turn your computer's output into standard TV video. And that makes for a number of interesting possibilities in today's multimedia classrooms, not the least of which is large-screen display. Here's a clear look at how scan converters work and how they stack up against other options.

The greatest challenge to using today's technology in group instruction--especially when it comes to presenting multimedia programs in the classroom--is finding the right large-screen display solution. It's not just a matter of finding a way to get the job done; it's finding a solution that's powerful and flexible enough to meet your needs without forcing you beyond the limits of your budget.

So far, the liquid crystal display (LCD) panel--which projects the output from a computer onto a large screen through a standard overhead projector--has been the technology of choice for computer projection in schools. But most of the low-cost panels found in classrooms today are not capable of displaying the variety of data types (video clips, animation, high-resolution color) used in multimedia applications. Nor can they display output from commonly used devices other than personal computers (e.g., VCRs and videodisc players).

LCD panel manufacturers do offer a solution to this problem. It's called the active-matrix LCD panel and it's designed for large-screen presentation of software plus video. (See Technology & Learning, March 1993, for a review of such panels.) But at a price point of $3,000 and up, this technology has not made strong inroads into schools.

Recently, a new kind of device called a scan converter has come to market that appears to be an attractive alternative to high-cost active matrix LCD panels. But before you run out and buy a scan converter, it's important to know how this technology differs from other display technologies.

What Are Scan Converters?

A scan converter takes the display output from your computer (even when it includes multimedia features such as video clips) and converts it to a signal that can be shown on a standard television monitor--including the large-screen TV monitor currently attached to your school's VCR. While that may seem easy, technically it is not: Television monitors and computer monitors normally require entirely different video signals.

The most popular standard for television video signals in the U.S. is NTSC, which was developed more than 30 years ago by the now defunct National Television Standards Committee. NTSC is a composite video signal (more on that later). Most European countries use a different composite video signal known as PAL. France and Russia have standardized on yet another composite video signal called SECAM. And if all this is not confusing enough, it turns out that composite video is just one way to transmit television signals. There is currently another standard in the industry known as S-Video (or Super VHS).

The personal computer industry has embraced a totally different kind of video standard--Called analog RGB. This video standard comes in two varieties-MS-DOS and Macintosh. In the MS-DOS world, the standard is known as VGA (Video Graphics Array), but because of the large number of MS-DOS computer manufacturers, the VGA standard has numerous variations. In the Macintosh world, the standard is equally fragmented: Different models of the Macintosh currently support different video standards.

Manufacturers of scan converters must contend with this fragmented market--that is, they must decide which analog RGB standards to support and create products that will convert those signals to NTSC (or sometimes S-Video). As you might guess, one of the main ways that manufacturers differentiate themselves from their competitors is through the video standards they support.

The Question of Quality

But you can't choose a scan converter exclusively by this single criterion. …

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