This month, we look at the DTV computer as a source of titles and graphic overlays. This has proven a very popular use for computers in the video studio, thanks to add-in video cards from a wide variety of manufacturers.

Digital to Analog

Overlaying computer graphics onto video requires converting the computer’s screen image into an NTSC video signal. Though a computer monitor may resemble a TV, the signals are quite different. The main difference: the speed at which they scan an image onto the screen. Unless two video signals “paint” display lines onto the screen at the same rate, they’re incompatible.


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Many character generators (CGs) attach an external box to a computer; this box scan converts the computer’s video output into analog NTSC. The term scan converter means that the computer and TV signals’ scan rate must be in sync. Scan conversion is the first half of the overlay process.

The second half of the process: encoding the RGB signal. Encoding is a process that converts the three RGB color components to a luminance–or “Y”–signal and a chroma–or “C”–signal. The encoder then combines the Y and C elements into one NTSC composite video signal.

Calling RGB to NTSC conversion encoding makes sense, when you realize that our TV sets contain decoders. The decoder converts the incoming TV signal back into the red, green and blue (RGB) signals needed for the three electron guns of our TV sets.

Overlaying the scan-converted and encoded video from these external boxes on our ordinary NTSC video requires that the two video signals be synced or “genlocked” to the same video reference. This last capability is genlock.

The Amiga benefits from a scan rate like that of NTSC. Thus Amigas can output an NTSC signal without external hardware; they also key graphics with minimal extra circuitry. This explains the Amiga’s popularity as CGs, paint boxes and graphic still stores in small TV and cable stations. Digital Creations’ SuperGen for the Amiga is probably the most common genlock/overlay in the world.

Analog to Digital

Of course we can reverse all these steps as well, mixing our video signal with computer graphics inside the computer. This requires converting our video signal into the digital RGB data stream of the computer.

To the computer industry, video overlay usually means playing video on the computer screen, as video-in-a-window. The real beginning of desktop video: when our NTSC video first entered the computer itself.

Some might think this was a time of madness, consider all those FCC warning labels on our computer products telling us to keep them away from our TV sets. What will happen to good, clean video when we actually run it inside that electromagnetic torture chamber? Not to worry. We just convert the analog video into digital data; this data is right at home in the computer.

Let’s take a look into the computer. Here we need a genlock/overlay card; this card digitizes our NTSC video, combining it with the output from the computer’s video graphics card.

On the PC side, many card makers formed the Video Electronics Standards Association (VESA) to connect these two cards. They approved a feature connector cable for VGA cards which carries 2 megabytes (MB) per second of data between cards. This is a small fraction of the data in a high-quality RGB video signal.

To better performance, some manufacturers use an external pass-through cable from the VGA card to the overlay card. Recently, VESA defined an advanced feature connector (VAFC) as well as a multiboard connector. This VESA Media Channel (VMC) can transfer multiple video signals simultaneously for video switcher/SEGs inside the PC.

These feature connectors open a PC Pandora’s Box. Each board in a PC requires the proper setup of interrupts (IRQs), I/O Base Addresses, Memory Base Addresses and DMA (direct memory address) channels. The potential for conflict is enormous; more so if you want a sound card in your computer to capture sound, and a SCSI hard disk. Some cards need memory apertures; these exclude certain memory positions in your computer, or require you to remove RAM memory, typically above 8 megabytes.

Certain video cards reviewed in Videomaker required a great deal of time resolving these conflicts during installation. Caveat videomaker!

The best video performance thus comes from single cards that integrate overlay and display functions. This is the norm on the Macintosh; its high performance overlay cards are also graphics display cards.

This is also the norm for pro PC overlay cards, like those from Truevision and Matrox.

Often called “videographic” cards, they combine the graphics overlay on video and the video overlay and screen graphics display functions.

Chroma Key or Color Key?

How do the two video signals combine inside the computer? There are two techniques: 1) chroma key; and 2) color key. Some boards do only one or the other. The best do both.

To understand the difference, ask which signal you want to lay atop the other. You can lay your video on graphics, like the classic weather report. Or you can lay graphics over video, for scrolling titles and the like. They both use a keying technique; one color of the foreground signal appears transparent, so the background signal shows through.

When you key one analog video signal over another by selecting one color (actually a small range of colors), you’re chroma keying. Remember, the color component of a video signal is chroma.

When you key graphics over video, it’s color keying. Basically, you key out just one specific color in the computer’s palette. Though you can specify a range, the overlay board usually makes out one specific RGB value transparent.

Using a single key color makes for harsh character edges; the resulting image goes from computer graphics to video signal in the space of one pixel. More sophisticated boards allow you to pick one key color that’s transparent, and another color that’s only partially transparent. Put these side by side; smoother edges line your keyed graphics.

Some systems even let you pick 16 or 256 colors of varying transparency. Now your graphic softly blends into the background video. The best systems allow you to specify the transparency of each pixel, regardless of color.

Videotape Output or Screen Display?

Most Videomaker readers produce videos for distribution in videotape form. Be aware, however, that a much greater application for DTV technology looms on the horizon: multimedia.

Multimedia originated in computer presentations combining graphics, audio, live video and animations. Current applications include computer-based training and educational programs; these boast an “interactivity” not possible with linear videotape.

Multimedia video overlay cards offer functions similar to their NTSC cousins. However, they usually lack the NTSC output that makes images available for recording on videotape. To use them in your productions, you must buy the encoder option if available, or invest in a separate encoder.

First, your video enters the computer for digitization. Then, multimedia boards display it on screen as video-in-a-window. Some let you capture still frames–called frame grabbing–to incorporate in multimedia presentations.

Video-in-a-window is cute and popular; but it may not be relevant for your video productions. This type of computer display is not as good as a standard video monitor for judging video quality. Also, why make a video that uses just a small portion of the screen?

The latest capture boards go beyond placing video in just a small portion of the computer screen. These capture and compress full-screen video to a hard drive. Once there, you can add as many layers of titles as you wish.

What to Buy

Installing cards in computers can be confusing and difficult. Producers who just need a CG for titles might invest instead in a dedicated titler like the terrific Videonics Titlemaker.

If you must have graphics too, then you need a DTV computer and a graphics overlay card. Think twice before spending as much as a couple of thousand dollars on just the graphics overlay function. In the next two DTV columns, we’ll look at affordable computer-based switcher/editors and nonlinear editors that make graphics, titles and overlays.

If you insist on an overlay card, check for chroma key, color key, soft linear keying, NTSC encoded output and video-in-a-window.

The best cards offer many other powerful features as well. Anything the computer can do in real time you can now record to videotape. Scrolling and crawling titles are typical. You can “fly” a single graphics frame around on the screen. Don’t expect animations, though. Garden-variety PCs can not write successive images to the screen at 30 frames per second. For this, you’ll need a single-frame animation recorder. Or one of the new nonlinear systems that plays compressed graphic frames as real-time animation.

One slick feature: hardware pan and zoom. You put the computer display in a special mode that allows you to zoom in on just a part of the computer graphics. Then you can pan left and right and up and down the screen. This is important; the computer screen resolution is much higher than your video resolution, so it won’t look good on VHS tape. If you enlarge just a part, it will look great.

Another important feature is flicker filter. Fine horizontal lines around the menus and dialog boxes in the typical computer interface appear on only one horizontal line of NTSC video, in an odd or even field of the frame. Consequently, they flicker at 30 Hz. Check to see that your board offers a flicker filter; it blurs thin one-pixel lines into two lines before encoding to interlaced NTSC video.

Expect a great bundle of software with your card. Good software will let you keep your graphics images in a still store; then you can call them up with a mouse click. You can even place graphics in a timeline, where you can perform transition effects like dissolves and wipes to the graphics. Some systems use a freeze frame of the last video clip to simulate A/B roll editing (called A/X editing).

Need More Info?

Manufacturers’ brochures are a great source of info. Visit a good professional video dealer; make sure the dealer assumes ultimate responsibility for installing your card.

Telephone tech support sounds great; but if you aren’t fluent in computer jargon you’ll waste your valuable and limited production time.

Next month, we’ll look at DTV products that take two or more video signals into the computer and digitize them, making your DTV computer into a switcher and special effects generator.

Stay tuned.

Videomaker contributing editor Bob Doyle directs a desktop video group and a camcorder users group.

Graphic Overlay Cards

PC and Mac
Aitech Pro VGA/TV Plus
Cardinal SNAPplus
Everex Vision VGA/Overlay
Grass Valley Products G-Lock VGA
Magni VGA Producer Pro
Matrox MGA Video Pro
Roctec RocGen Pro
Truevision VideoVGA
Visionetics VIGA Genlock
Willow VGA-TV GE/O

Digital Creations SuperGen 2000
Grass Valley Products G-Lock
Roctec RocGen Plus

Multimedia/Video-in-a-Window cards

AITech Pro PC/Video
FAST Screen Machine
Matrox Marvel II
New Media Graphics Super Video Windows
Truevision Bravado
Videologic DVA-4000

RasterOps 24XLTV, 24MxTV, 24STV

Video Encoder Cards

Genoa VGA2TV
Truevision Bravado Encoder
Willow VGA-TV 4000

Multi-function Videographic Cards

Intelligent Resources Explorer
Matrox Illuminator 16 and Pro
Truevision Targa+ and NuVISTA+

This list is a sampling of available products, and is not meant to be comprehensive.

The Videomaker Editors are dedicated to bringing you the information you need to produce and share better video.