The following is a basic discussion about computer graphics as it 
relates to video and specifically as an introduction to the 
capabilities and requirements for Truevision products.

What is the difference between my computer monitor and my 
television?

There is a somewhat complex distinction between what appear to be 
similar technologies. In fact, about the only thing in common 
between a television and a computer monitor is the use of an 
electron beam used to excite the phosphors on the inside surface 
of a cathode ray tube. For the sake of simplicity, I have broken 
down this question into several related areas.

VIDEO:

A history of video begins with the advent of television. In the 
United States, the National Television System Committee (NTSC) 
set the standard for video transmission in the black and white 
past and we are still using the same narrow signal bandwidth to 
carry all video information. When color television was developed 
in the fifties, the industry wanted to maintain backwards 
compatibility so that the older black and white sets would still 
be able to read the signal. This resulted in an additional color 
channel modulated over the black and white original. Other 
countries have other standards, such as PAL and SECAM, but 
essentially all broadcast video in the world today consists of a 
medium resolution black and white image combined with minimal 
color information. It is our perceptions that fill in the missing 
information as to how things are supposed to look. A video signal 
contains color, brightness and timing information which is 
interpreted by the receiver to create an image. This strict 
standard is needed to ensure compatibility between the 
broadcasting source and the television receiver. A television 
signal is analog, meaning that it consists of a continuous, 
varying voltage to produce a theoretically infinite number of 
shades. Actually, due to the low resolution nature of video 
signals and technology, you never get to see any subtle hues or 
shading. The television industry has become very sophisticated in 
using the medium to look as good as possible. The composition, 
lighting and color choices are made with the limitations of video 
in mind and only when someone shows up on the Tonight Show 
wearing a herringbone jacket, which appears to vibrate, do the 
weaknesses of video become glaringly apparent. This is due to a 
trait shared among all video standards that describes how the 
electron beam "paints" the image on the screen called 
interlacing. That means that the beam scans down the screen 
hitting only every other line and then returns back to the top of 
the screen and fills in the missing lines. Thin horizontal lines 
will appear to jump as the beam hits and misses them as it scans 
the image. Each scan down the image is called a field, with the 
combined two fields making up one frame of video. When you hit 
the pause button on a 2-head VCR and the image is jumping around, 
it's because the play head of the VCR is stuck between fields. 
Video, like film, is actually made out of still frames passing by 
so fast that our eyes are fooled into thinking the motion is 
continuous. NTSC video displays 60 fields, or 30 frames, every 
second. An important consideration in the video market is the 
relative low cost required to build an interlaced monitor like 
your home television set.

What is the difference between analog RGB and composite video?

Consumer grade video cameras have a single tube or chip that 
receives and must separate all the colors presented to it. 
Industrial and professional cameras have separate elements for 
each shade of red, green and blue. This results in more 
information available for each color component and therefore a 
higher quality signal. Computer displays also have separate red, 
green, blue and sync wires contained in their cables. While RGB 
is the preferred method of creating and viewing images, because 
the separate components remain distinct resulting in a cleaner 
look, most video equipment works only with composite. Composite 
video, called RS170A by the NTSC, is a combined signal that holds 
luminance (brightness), chrominance (color) and timing 
(synchronization or sync) all in one signal. When all these 
components are combined, through a process called encoding, into 
a single signal, it is composite video. Remember, if you are 
working with computer video, the work you are creating that looks 
so good on your RGB display may look completely different once 
the signals have been encoded into composite. Another difference 
between most computer displays and NTSC video is the subject of 
aspect ratios that will be discussed later. One improvement in 
video standards is the relatively new standard S-VHS. It provides 
separate signals for color and brightness, and so provides a 
better quality than RS170A composite, which rolls everything into 
one.

COMPUTER DISPLAYS:

Since computer displays are not tied to any 50 year old 
specification as to how fast and frequently they refresh the 
screen, they can exceed the video standard of 60 fields per 
second (60Hz) and 525 lines for the sharp presentation of text 
and graphics. The only standards in the PC industry are those 
that have been traditionally passed down from IBM and Apple. Most 
importantly, computer displays are non-interlaced, meaning that 
each pass down the cathode ray tube in the monitor, they hit 
every line and start the process again from the top. The faster 
the scanning, the more stable the image appears, and the more 
lines to be scanned, the sharper the image. Non-interlaced 
displays are more expensive to produce but have a much sharper 
appearance. The increased screen resolution of new standards 
means different screen descriptions and pushed monitor technology 
to accept higher and higher frequencies. The state of evolving 
computer display technology makes buying a multiscanning monitor 
a necessity to keep up with the latest video cards.

When the IBM PC was first introduced, it was a monochrome system, 
and a low resolution one at that. It was a digital display, 
meaning that each picture element, or pixel, was either on or 
off. Strictly speaking, monochrome systems are only capable of 
displaying text. The display card manufacturer Hercules was the 
first to introduce a black and white graphics adapter. In 1984, 
IBM introduced their first color display called CGA (Color 
Graphics Adapter). Like the monochrome display it was digital and 
non-interlaced, but it was capable of 320 vertical lines by 200 
horizontal resolution with 4 colors. When on, the pixels could be 
several different colors meaning that the video card's memory had 
to be large enough to "remember" and display the additional 
information. As we will see later, more colors and higher 
resolutions means that much more data the card's memory must 
contain and process. EGA (Extended Graphics Adapter), their next 
color standard ran at 640 x 350 with 16 possible colors. The next 
step up, VGA (Video Graphics Array), originally meant 16 colors 
at 640 x 480, although now with SuperVGA cards from a variety of 
manufacturers and IBM's own 8514/A, VGA now usually means at 
least 256 colors. The VGA resolution of 640 x 480 has also been 
pushed to 1024 x 768 and beyond, territory in the past that was 
reserved for high resolution CAD work stations. As the amount of 
on-screen information increases the display board must run at a 
faster speed to paint all the rows quickly enough to maintain 
stability. Many of the fastest boards available have an 
accelerator or graphics co-processing chip on board to keep this 
refresh level up and free the computer's CPU (Central Processing 
Unit) from performing such a mundane task.

TRUEVISION:

Truevision, formerly known as EPIC (Electronic Photography and 
Imaging Center), started producing graphics adapters for the 
young PC market in 1984. They began as an entrepreneurial subsidy 
of AT&T and, although they became a privately owned company in 
1987, they still have access to AT&T technological resources. 
Their first two products were the ICB (Image Capture Board) and 
the VDA (Video Display Adapter), introduced when CGA and EGA were 
the only available color boards for the PC. The VDA was capable 
of displaying 256 colors at a resolution of 256 x 240. The ICB 
went further because, as it's name implies, it could do a real 
time capture of a video signal at the same low resolution, but 
with a palette of more than 30,000 colors. Here was an exciting 
product that allowed an interlaced signal to pass though the 
computer for capture or overlay of computer images. Since they 
were not compatible with any of the digital standards such as CGA 
or EGA, or existing software, Truevision stations took on their 
now familiar two monitor configuration. One display is the 
standard DOS system monitor, whether monochrome or color, and the 
other monitor was usually a NTSC frequency RGB (Red, Green, Blue) 
display cabled to the Truevision board. As you might imagine, 
although there wasn't even any specific software available yet 
for these products, the sight of thousands of colors on a PC 
display when 16 was the maximum was pretty impressive. The real 
problem was one of resolution and that would be addressed by 
their future products that will be discussed shortly.


So even now we have a fixed, interlaced standard for video and an 
ever changing game of one upmanship among the manufacturers of 
displays cards that involve higher and higher non-interlaced 
frequencies to accommodate better resolution and image quality. 
The worldwide video community is still trying to decide upon a 
high definition video standard, but this will require a larger 
bandwidth to pass the additional information and specialized 
televisions to receive the new signal. Each country and company 
has what it thinks should be the new standard, while still 
providing backwards compatibility with the millions of existing 
television sets. Chances are that the adopted high definition 
standard will require some form of image compression to move the 
images.

VGA/NTSC SIDEBAR:

The term "multimedia" is defined in a thousand different ways. In 
my opinion, Apple's advertising people created a future market 
for something that didn't even exist at the time. On the PC side 
we have seen two converging technologies: VGA to NTSC devices and 
"high color" VGA cards. Ever since the dawn of computer displays, 
people wanted the ability to take what they saw on their monitor 
and dump it somehow to video. The proliferation of many animation 
programs available for the PC, such as Paul Mace's GRASP and 
Autodesk's Animator, has made this seemingly simple process even 
more attractive. In a simple sense, this technology has been with 
us for some time and the only device required was called a scan 
converter. This is the type of technology required to convert a 
European PAL video, which has more lines (625) and a slower 
refresh (50Hz) than NTSC. The drawback to scan converters is 
their price. A typical professional unit, capable of producing 
broadcast quality video, costs well in excess of $ 10,000, more 
than the price of most entire PC systems. Several hardware 
developers, such as Willow, USVideo and Jovian, produced VGA 
boards, add-in boards or external devices that would take the 
non-interlaced computer signal, slow it down and turn it into a 
RS170A compatible signal. Now, there are literally dozens of 
products like this on the market, providing different features 
and capabilities. Some offer a genlock ability, meaning that it 
can sync to an external video source for a more stable image and 
perhaps even combining some form of live video pass through or 
overlay features. Some can even do frame grabbing and digitizing 
although, unlike the Targa technology, they must do their 
capturing in a VGA compatible format, meaning a variable 
resolution, usually 320 x 200, with a maximum of 256 colors per 
"grab". I have seen several of these devices in action and have 
been underwhelmed. They may have some limited appeal, but the 
quality of the affordable units will not give most people the 
quality they expect. One thing to consider when attempting to 
convert computer graphics to video are the different aspect 
ratios of the different formats that I touched on before. 
Computer displays work in an underscan mode, meaning that they 
will show all the available information on your monitor. 
Television standard video works in overscan, in that it is 
designed to "bleed" off the edges of the television set, filling 
the entire screen edge to edge. More exciting for the VGA artist, 
in my opinion, is the new generation of "high color" VGA display 
boards. These sophisticated VGA cards have special circuitry, 
called RAMDAC, that greatly expands the number of displayable 
color from 256 to more than 60,000. This allows the computer to 
display nearly photorealistic images on a standard VGA monitor. 
Since VGA is an analog signal, like video, the monitor can 
display an infinite spectrum of colors, it is the card that 
determines the number of shades displayed. Since they do not yet 
effectively allow the images created to be sent out directly to 
video, I will return to the development of Truevision's product 
line:

TRUEVISION TODAY:

In 1985, only one year after introducing the ICB and VDA, 
Truevision came out with their Targa series of video graphics 
adapters. With four times the displayable resolution and up to 
512 more available colors than the earlier boards, the Targa 
boards were the state of the art in color boards for the PC. The 
first Targa boards came in both underscan (512 x 400 displayable 
resolution) and overscan (512 x 482) versions. Since then, all 
new Truevision boards come with built in overscan capabilities. 
Two years later they followed with their ATVista series for true 
broadcast quality video. The company still aggressively develops 
new products that add more features, at lower prices, for all 
levels of PC computer artists. They have set the standard for 
others to follow with their NuVista Plus boards for the Apple 
Macintosh and the new family of Targa, the Plus series, which can 
operate at different resolutions and offers VGA pass through for 
a potential one monitor solution. They also have recently 
introduced their own VGA to NTSC board, the VideoVGA, and the 
1024/32 color board for high end 32-bit desktop graphics. The 
Targa Plus and NuVista Plus boards produce RGB, composite and 
S-VHS signals internally. The ATVista series only outputs RGB and 
so requires an encoder.

A quick course on color depth: I will be referring to color by 
describing the amount of color information that the particular 
Truevision frame buffer can contain and process. The terminology 
is to describe a file as either 16 or 32 bits deep. Actually, 
some of the bits are used for video imaging purposes only, so let 
me start by describing 15 and 24 bit color before addressing 16 
and 32. A 15-bit image contains 5 bits of information for red, 
green and blue. A bit is either on or off, meaning that there are 
2 to the fifth power of information possible for each color 
component. In other words, you can define 32 shades of red, green 
and blue for each individual pixel in your 15-bit image. More 
math will tell us that means a total of 32,768 possible 
combinations (32 x 32 x 32) or shades available, from a value of 
0 red, 0 blue and 0 green, which is displayed as pure white, to 
32 red, 32 green and 32 blue, which is completely black. 
Interestingly, 15-bit color can describe only 32 shades of gray. 
Since gray is composed of equal amounts of red, green and blue, 
there are only 32 available combinations with that ratio. 32,000 
colors sounds like more than enough, but it's not: The human eye 
is capable of distinguishing more than 40,000 shades and so a 
15-bit image appears slightly blotchy upon close inspection. When 
you have subtle shading, the colors tend to band or dither from 
one to the next. For low resolution video output this may be 
acceptable, usually it is not. This brings me to 24-bit color: 
With this expanded file type, we can now have 8 bits of 
information, or 256 possible shades, each for red, green and 
blue. That means each pixel could be any one of 16,777,216 
distinct colors (256 x 256 x 256). Now we have a file that can be 
called "truecolor", meaning photographic quality. What about 
those additional bits that make a file 16 or 32-bits? Those are 
called alpha channel information and are only used with video. 
The alpha channel tells each pixel if it is on or off, meaning 
can live video pass through the frame buffer transparently or 
opaquely. The 16-bit image has 15 color bits and a 1-bit alpha 
channel, meaning that either that bit is on or off, live or not. 
The 8-bit alpha channel of the 32-bit image has 256 "shades" of 
on and off, giving the live video pass through many potential 
levels of transparency.

Resolution: All the Truevision video boards can work in different 
resolutions, both interlaced and non-interlaced, but all are 
limited by the amount of VRAM they have on board. As a general 
rule, with a fixed video memory, it becomes a trade off between 
the amount of color information and physical resolution that the 
board can support. Obviously a 32-bit image carries more 
information per pixel than does a 16-bit image. Similarly, an 
entire image at high resolution contains more video information 
than a low resolution image. Time for more math: The equation for 
figuring out the file size at a specific resolution is the 
following: (Hs x Vs x Bp)/8 = file size, where Hs is horizontal 
size, Vs is vertical size and Bp is bits per pixel. We divide by 
eight to turn bits into bytes. Using that formula, we can see 
that a Targa 16 image at 512 x 486 NTSC resolution is ((512 x 486 
x 16)/8) 497,664 bytes, or almost a half megabyte, in size. This 
fits neatly in the half meg of video memory on the Targa Plus 16. 
At the high end of the spectrum, a Vista board will go as high as 
1024 x 768 x 32-bits for an image size of greater than three meg. 
The only board capable of displaying a file that large would be 
the 4Mb Vista. Of course a scanner can create a Targa file much 
larger than 3Mb, but the entire image wouldn't be viewable 
without specialized software.

Before investing in a Truevision system, you should have a good 
idea of both your input and final output options. For the most 
part, inputing images into the Targa environment ranges in 
quality from scanning to video grabbing. With high resolution 
color scanners dropping in price, you could create images many 
megabytes in size, much larger than the frame buffer size of most 
Truevision boards. When grabbing from a video source, you are at 
the mercy of both the input quality from either tape or live 
camera and the capture resolution of your Truevision board. There 
are professional quality video cameras that will output a 
relatively high quality RGB signal for the frame buffer. This 
results in a better source input than sending in a composite 
signal that must be decoded (the opposite of encoding, decoding 
turns RS170A composite video back into RGB and sync information) 
for the frame buffer to accept. When outputing your images you 
can either create high resolution slides, variable resolution 
color or black and white prints or relatively low resolution 
video. Since we are used to the poor quality of television (most 
of the better TV sets are only capable of displaying 350 to 400 
lines of resolution), people are not bothered by the low quality 
of video so long as it moves and looks good. Obviously a 512 or 
756 line video image can't compete in quality with a 4,000 or 
8,000 line 35mm slide for still images.

Once you have decided upon your output requirements you can look 
at the software available for the Truevision file format. Since 
Truevision pioneered it in the early eighties, the Truevision 
.TGA Targa file format has become an industry standard, supported 
by hundreds of software titles for a variety of purposes, and 
compatible with almost any possible output device. The most 
common program would be "Paint" software, that would allow you to 
touch up acquired images or create your own artwork. These 
packages range in price and power from less than $ 500.00 to more 
than $ 2,000.00.

TRUEVISION BOARDS:

Targa Plus series: All share some features such as VGA pass 
through, RGB and composite output and input. Note that, while the 
16/32 and 64 boards can display higher resolutions, they have an 
upper NTSC limit of 512 x 486 at 32-bits. They must be ordered as 
NTSC or PAL and offer a MCA (Microchannel) bus model.

Targa Plus 16: has a .5Mb frame buffer of high speed video 
memory, or VRAM. Capable of grabbing and displaying a 16-bit 
image up to 512 x 486 NTSC resolution. This is a fairly limited 
board that replaces the original Targa 16 at a lower price.

Targa Plus 16/32: With 1Mb VRAM, it can go up to 32-bits at 512 x 
486.

Targa Plus 64: 2Mb VRAM, capable again of 512 x 486 at 32-bits 
for video display. The 64 board allows for dual buffering, 
meaning that you can fade from two images that are in the board's 
memory at one time for some excellent video effects. This would 
be the premium Targa Plus board if you are planning to work 
extensively in video production.

ATVista series also differ in the amount of on board video 
memory, either 1, 2 or 4Mb versions are available. The ATVista 
series differ from their Targa counterparts in that they do not 
have composite input or output. They also do not offer VGA pass 
through and are available only in ISA or Mac bus versions. They 
do not share the 512 line limit of the Targa series, meaning 
that, if their memory allows, they can go as high as 756 x 486 
broadcast video resolution or even to 1024 x 768 if you aren't 
going out to video. The Vista is a programmable board with an on 
board video processor. Vista software tends to be higher priced 
than their Targa based counterparts and Vista systems are 
typically more hardware intensive as well.

The NuVista Plus boards share the same programmability as their 
AT bus  counterparts, except they have built-in encoding and 
decoding, like the  Targas, so that they can directly accept 
composite video in and out.

The VidI/O box is Truevision's standalone encoder/decoder box. It 
provides for RGB input, output, looping, composite and S-VHS in 
and out. Decoding, the reverse of encoding, turns a composite 
video source into separate RGB and Sync components to run into an 
ATVista for image grabbing.

VideoVGA: A VGA to NTSC board that allows for overlaying VGA 
images or animations over a live video pass through. It can 
simultaneously output non-interlaced VGA and interlaced composite 
NTSC signals.

1024/32 board is an advanced, co-processed board for Windows and 
CAD applications. It works at 32-bits at resolutions up to 1024 x 
768 at up to 76Hz for a stable, high resolution display. It also 
has on-board VGA and can run in standard NTSC and PAL modes.

If you are looking to do true broadcast quality, the Vista is the 
board for you. Keep in mind that what separates true broadcast 
quality isn't always apparent to the untrained eye; sometimes the 
signals must be analyzed by an engineer through a scope to 
determine their true quality. Naturally, an encoder built into 
the Targa board can't hope to match the stringent specifications 
as to what constitutes broadcast quality like an external $ 5,000 
professional encoder can do. The Targa, however, is well suited 
to all forms of industrial video applications.

Generic Truevision configuration (s), excluding board and 
software:

Computer:        Minimum                    Recommended

                 386SX 20MHz                386/33 or better
                 4Mb RAM                    8Mb RAM
                 80Mb fast hard drive       200Mb drive
                 monochrome card            VGA card
                 monochrome monitor         VGA monitor (**)
                 mouse                      digitizing tablet
                 13" Long Persistence	    19" Long Persistence
                 Phosphor monitor (*)       Phosphor monitor (**)

Due to the potentially large image files, especially if you have 
access to a scanner, some form of removable mass storage is 
highly recommended.

* The Targa Plus sends out both RGB and, due to the on board 
encoder, composite video signals. Therefore it would be possible 
to hook the Targa directly to a VCR or NTSC monitor that accepts 
composite. While this would provide you with a good idea of how 
your images will look if they are going out to video as your 
primary output medium, it will not give you a high quality 
interface to work from. Also, if you don't use a multiscan 
monitor, you will be limited to working at a fixed NTSC 
resolution. A quality display will greatly reduce eye fatigue and 
make you more happy and productive in the long run.

** The Targa Plus series can run what is called VGA pass through, 
meaning that you could have a one monitor solution if you already 
have a VGA card. Some software, however, requires a two monitor 
configuration and the only cost savings is that of the additional 
VGA display. To run the Targa Plus in an interlaced (video) mode, 
the Long Persistence monitor is required to reduce the amount of 
flicker. Also, be warned if you want to try running a single 
monitor with the VGA pass through: Truevision obviously can't 
test compatibility between the Targa and every VGA card to hit 
the market. There are some VGA adapters that will not function 
properly in this configuration. We have noted this problem with 
the higher end cards, especially those using the Tseng chip set.

Some application notes:

Video pass through: This technique of combining video with 
computer graphics can be accomplished in two ways: Overlay and 
Chroma Key. Overlay takes advantage of the alpha channel and 
allows live video to literally pass through the frame buffer. 
Chromakeying is the process that puts the television weatherman 
in front of the map. It involves a screen behind the subject that 
is a consistent color, usually blue or green so it won't 
interfere with flesh tones. When the image passes through the 
frame buffer, the key color is stripped out electronically and is 
replaced by the computer generated graphic. Sometimes you may 
have noticed, if he is wearing a blue tie, you can see the map 
through the weatherman.

A word about 3-D animation. Some people are convinced that 3-D is 
the ultimate combination of computer imaging and video. While 
this may be true, they may not have a full understanding of how 
complex it is to lay down rendered images to tape so that they 
appear in continuous motion. As we have calculated, a broadcast 
quality Vista frame at 756 x 486 x 32-bits can yield a file 
1.469664Mb in size. Obviously, unless you have an enormous RAM 
drive, your hard drive won't be capable of displaying files that 
large at 30 images per second for video output. Additionally, 
depending on your choice of 3-D animation software and the 
complexity of the scene, each image may take many minutes to 
fully render, especially if the PC calculates shadows, texture 
maps, reflections and the like. Therefore you will need an 
animation controller that will interface between the PC and your 
frame by frame editing VCR. Essentially, the controller holds the 
video tape at a specific frame until the PC is done rendering an 
image, it then tells the deck to record that image and then wait 
for the next one. This can be a time consuming process, as well 
as an expensive one: an animation controller costs a few thousand 
and a broadcast quality deck many times more. Additionally, you 
may require one or more of the following pieces of video 
equipment: A Time Base Corrector, a Sync Generator and encoder. 
Some of the more expensive pieces may be rented until they can be 
cost justified.
 
Kevin Freeman
