Singing the NTSC Red-green Blues
Tom drives me crazy. He thinks he knows everything. The worst part is, he does. So even though he has all the personality of a sea urchin, sometimes I find myself asking him questions. The first time I picked his brains was a few years ago when I got my original IBM PC. I asked him how I could put my computer animations onto video. He laughed.
"You can't do it, Scott. But you're going to try anyway, so why are you soaking up my precious time?" he said.
Like I said, a sea urchin, but I resolved to act civilized. "I was able to tape off of my old Apple II with no problem! Why can't I get video out of my brand-new PC?" I asked.
"You're using EGA mode, so the pixels aren't square and you'll lose the aspect ratio. The rasters aren't even related, let alone synced, so you have to convert your scans. Going to interlaced video, it'll flicker enough to give you seizures and to top it all off, the colors will be illegal," he replied.
I had no idea what he was talking about, so I went for the bluff: "Other than that it'll work?"
Tom snorted. "That and the fact that the hardware you need will set you back a cool fifty kilobucks!" He glared at me. "And don't get any stupid ideas about taping the screen with your camcorder. The rep rates don't match so you'll get roll bars."
He had me totally lost on that one. The only thing I could recall from his windy techno-tirade was "taping the screen with your camcorder." I slapped my head. Of course! I immediately set out to try it.
My first results I called "A Study in Screen Glare." I decided to continue the experiment at night, with the lights out. My next results I called "Wash Me," in honor of the fingerprints, grime and unidentified baby fluids that my twins had deposited on the monitor.
On my third experiment, I could actually see the screen. That's when another piece of Tom's sermon came back to haunt me: "you'll get roll bars." At the time, I thought he was talking about those extra couple of pounds I've been trying to lose. Instead, I saw an awful horizontal bar that scrolled down the screen, over and over again. This rotten little artifact pretty much overwhelmed my test animation.
Ultimately, I was able to reduce the roll-bar effect by using dark backgrounds. I finished my animated video with this technique, but I don't recommend it. The results are marginal, and you have to clean your monitor.
Time Marches On
A couple of years later, when the cost for outputting video came down from $50,000 to $500, I got a chance to try again. I recorded my same old EGA-mode animation, eagerly anticipating the vast improvements this sophisticated hardware was about to bring into my life. Instead, it made me remember another thing Tom warned me about: non-square pixels. I thought he was over the edge on that one, but the evidence was staring me in the face: my whole animation looked squashed! All my circles had become crushed ovals, and the text was ridiculously squatty. I was not impressed.
And just as Tom said, there was a hideous flickering of all the horizontal lines. Oh yeah--I almost forgot--the colors were wrong and there was some bleeding too. That must be what Tom meant by illegal colors. Apparently, Tom was right about everything. I hate that.
The worst part was that my brand-new, hi-tech video hardware compared poorly with just aiming a camera at the screen. This was depressing. My inexpensive Amiga did a much better job, so that's what I used for the next few months.
Finally, IBM got real and introduced the VGA card with square pixels. In the 640x480 graphics mode, the computer image has the same proportions (or as Tom would say, aspect ratio) as a TV screen, making it video-friendly.
Meanwhile, on the Apple front, things were starting to move, too. The Macintosh had square pixels right from the start, but it took Apple a while to realize that customers wanted color. Finally, the Mac was delivering color graphics and the world was a better place.
Here and Now
These days, computer video is looking pretty good. You can buy video output cards that run the gamut from prudent to pricey. Mac boards like the Spigot II Tape from Radius are great starter cards. You can use one of these to digitize, edit and output your home videos. The resolution is not exorbitant, but neither is the price. For output and play-through capabilities on the PC, try the VideoVGA card from Truevision.
At the mid-range, you'll find cards like the Truevision Targa 2000, using state-of-the-art technology for both the PC and the Mac. This card is at the core of high-end stand-alone systems like the Avid.
For the financially well-endowed, there are cards from companies like Folsom, RGB Spectrum and Lyon Lamb. They have some really top-notch cards with component output and built-in panning and zooming, but be prepared for price tags that go from eight to twenty-five thousand dollars!
Flicker is still a problem, but less so. Video output cards now routinely include flicker filters. There is more than one way to filter a flicker, though, and some companies have done it better than others. I was still having some problems with twinkling text, so I ended up calling Tom again.
"What's on your mind, if I may be allowed to stretch the term?" he asked.
I'm going to act civilized, I reminded myself. I told him about my problem.
"Well, I'm not surprised you're getting flicker. Didn't I tell you that would happen? Your computer display isn't interlaced, and NTSC is. That means NTSC displays all the odd lines first and then all the even lines. As any school-child knows, these half-resolution images are called fields and they are usually similar to each other. But on a computer, a thin horizontal line may exist in one field, yet have no trace in the other. That's gonna flicker. The solution is to reduce the contrast, or better yet, don't use thin horizontal lines. Double 'em up and your problems will go away," he told me.
Double the lines, huh? Easy for Tom to say. But by using fat or bold-faced fonts, I found that things did look a little better. Anti-aliasing, which smoothly blends the edges of letters into the background, also helped. I noticed that frilly fonts with a lot of serifs and fine lines flickered like mad. I switched to sans-serif fonts like Helvetica and Optima and things really mellowed out.
In my graphics programs, I made sure that there were no thin, contrasty lines by painting with a bigger brush. I made all my lines two pixels wide at minimum.
In pictures I had already made, I lowered the contrast a bit or blurred them very slightly to fatten up the lines. With this bag of tricks, I finally conquered flicker. Well, mostly.
Now I was on to my next problem: illegal colors. I've always been a law-abiding citizen. I sure didn't want my picture hanging in the Post Office for pushing illegal pixels. There had to be a way to rehabilitate my palette.
I dreaded another lecture from Tom, so I went straight to the very last resort.
I read the manuals.
I found out that there's a feature in many software packages to make any color palette more palatable for NTSC. In Photoshop, you can find it in the Filter menu. It's in the Video submenu and it's called the NTSC Colors Filter. This filter is available to all programs that accept Photoshop plug-ins, like Premiere and DeBabelizer. Plug-ins are a wonderful sign that software designers are finally tuning in to our needs--and it's about time!
Check out Adobe's Premiere. Its color picker warns you whenever you select a color that's out of line. You should play with its color wheel to see just what an illegal color looks like. Notice that the illegals are all the brightest, most saturated colors. Now you know why TV is so dull.
CoSA's video editing program, After Effects, offers two different ways to tame your palette. The default method is to reduce luminance, keeping the NTSC signal within the maximum allowed brightness. Just darkening the image seems like the coward's way out, but the alternative isn't much better. It lets you legalize your unruly hues by reducing their saturation. That sounds like what you want, but in fact it changes the relative coloration and doesn't stay true to the original. I've had better luck with the luminance method, but you should try them both.
After Effects also allows you to specify the maximum signal amplitude. NTSC measures signal brightness in IRE units (Institute of Radio Engineers). Your video signal should be between 7.5 and 100 IRE to be legitimate. If you want a little more juice in your image, you can push it to 110 IRE units. If you dare to walk on the wild side, you can push it to 120, but beware: some equipment will "clamp" any signal that strays above 100 IRE. When this happens, you'll lose all detail in the brightest areas of your image. Use discretion.
In addition, After Effects has a very handy feature called Key Out Unsafe. This lets you preview just which delinquent pixels are violating the current IRE curfew.
Putting it All Together
With these tools at hand, I finished another animation just the other day. Tom dropped by to see it, which I thought was unusual until I realized he was only there to tell me how bad it looked.
"There's a little flicker around the opening titles," he started, "and it looks like you pushed the saturation to the wall. Was that a dropped frame? Ah, there's a little bit of banding over here, see? Say, is that a crack in your 3D model?"
He droned on, a litany of criticism. Finally, when the video stopped, he stopped too. Then he punched me in the arm, another one of his endearing traits. "Not bad, Scott. For a complete bozo, you do pretty good work."
I decided to take it as a compliment.
Scott Anderson is the president of Wild Duck Software, a computer graphics development company.
Never Twice the Same Color
Although they won't always admit it, most video engineers know that our color TV standard is a compromise at best. One engineer I know swears that NTSC stands for "Never Twice the Same Color." It may not be that bad, but NTSC does pose some problems for us computer folk.
Typically, a video frame contains about 486 lines with 720 dots apiece. This is luminance resolution. The color resolution, however, is closer to 360 pixels wide by about 240 pixels high. This is surprisingly poor resolution. It's almost like NTSC draws a monochrome image with a fine ink pen, then comes in and paints the color information with a big, sloppy brush. The key is to avoid graphics or text that rely on fine color details.
TV refreshes the image a lot (60 times per second). But this has its own price: the phosphors on the screen must be short-lived. After the electron gun hits a phosphor, it must glow and then die out quickly for the next frame. If the phosphor holds its glow too long, everything that moves will leave a ghostly trail.
Unfortunately, with fast phosphors you can get flicker as the image dies out before it gets refreshed. To overcome this nasty artifact, the engineers decided to interleave the lines, doing the odd lines on one sweep and the even lines next. That way, as the odd lines are fading, the evens are being updated, and the eye sees flicker-free, continuous motion. That's interlaced video.
But while interlacing is just fine for action video, it's hard on the eyes when displaying static text. For instance, on an interlaced screen, the horizontal line at the top of the letter 'T' will show up on every other field. It will be on the screen for 1/30th of a second, then off for 1/30th. The cute trick originally invented to cure flicker thus ends up causing flicker!
Another problem is the screen refresh rate. The NTSC standard is thirty frames per second, or 60 fields per second. But a computer screen refreshes anywhere from 60 to 75 times per second. Converting from one rate to another is the job of a scan converter.
Color is the last problem. On your computer screen, each pixel consists of a distinct red, green and blue value. For NTSC, these colors need to be converted into hue and saturation. This is the job of an encoder.
Turning a computer image into an NTSC signal is no easy task. Ideally, you should look for a single card that performs as many of these conversion tasks as possible. Not only will installation be easier, but everything will be more efficient, resulting in faster throughput.