Atari 2600 Programming Tutorial
Session 2: Television Display Basics
By Andrew Davie (adapted by Duane Alan Hahn)
Hopefully you've been through the first part and have your editor, assembler, emulator and documentation ready to go. What we're going to look at now is a basic overview of how a television works, and why this is absolutely necessary prerequisite knowledge for the '2600 programmer. We're not going to cover a lot of '2600 specific stuff this time, but this is most definitely stuff you NEED TO KNOW!
Television has been around longer than you probably realize. Early mechanical television pictures were successfully broadcast in the '20s and '30s (yes, really!—see http://www.tvdawn.com/). The mechanical 'scanning' technology utilized in these early television systems are no doubt the predecessors to the 'scanning' employed in our modern televisions.
A television doesn't display a continuous moving image. In fact, television displays static (non-moving) images in rapid succession—changing between images so quickly that the human eye perceives any movement as continuous. And even those static images aren't what they seem—they are really composed of lots of separate lines, each drawn one after the other by your TV, in rapid succession. So quick, in fact, that hundreds of them are drawn every image, and many images are drawn every second. In fact, the actual numbers are very important, so we'll have a look at those right now.
NTSC, PAL, and SECAM
The Atari 2600 console was released in many different countries around the world. Not all of these countries use the same television 'system'—in fact there are three variations of TV systems (and there are three totally different variations of Atari 2600 hardware to support these systems). These systems are called NTSC, PAL, and SECAM. NTSC is used for the USA and Japan, PAL for many European countries, and Australia, and SECAM is used in France, some ex-French colonies (for example, Vietnam), and Russia. SECAM is very similar to PAL (625/50Hz), but I won't spend much time talking about it, as Atari SECAM units are incredibly rare, and little if any development is done for that format anyway. Interestingly, the differences in requirements for displaying a valid TV image for these systems leads to the incompatibility between cartridges made for NTSC, PAL and SECAM Atari units. We'll understand why, shortly!
Images Per Second (Frequency)
A television signal contains either 60 images per second (on NTSC systems) or 50 images per second (on PAL systems). This is closely tied to the frequency of mains AC power in the countries which use these systems—and this is probably for historical reasons. In any case, it's important to understand that there are differences. Furthermore, NTSC images are 525 scanlines deep, and PAL images are 625 scanlines deep. From this, it follows that PAL images have more detail—but are displayed less frequently—or alternatively, NTSC images have less detail but are displayed more often. In practice, TV looks pretty much the same in both systems.
But from the '2600 point of view, the difference in frequency (50Hz vs. 60Hz) and resolution (625 scanlines vs. 525 scanlines) is important—very important—because it is the PROGRAMMER who has to control the data going to the TV. It is not done by the '2600 (!!)—the '2600 only generates a signal for a single scanline.
This is completely at odds with how all other consoles work, and what makes programming the '2600 so much 'fun'. Not only does the programmer have to worry about game mechanics—but he or she also has to worry about what the TV is doing (for example, what scanline it is drawing, and when it needs to start a new image, etc.).
Let's have a look at how a single image is drawn by a TV. . .
A television is a pretty amazing piece of 1930's technology. It forms the images we see by shining an electron beam (or 3, for color TVs) onto a phosphor coating on the front of the picture tube. When the beam strikes the phosphor, the phosphor starts to glow—and that glow slowly decreases in brightness until the phosphor is next hit by the electron beam. The TV 'sweeps' the electron beam across the screen to form 'scanlines'—at the same time as it sweeps, adjusting the intensity of the beam, so the phosphor it strikes glow brightly or dimly. When the beam gets to the end of a scanline, it is turned off, and the deflection circuitry (which controls the beam) is adjusted so that the beam will next start a little bit down, and at the start (far left-hand-side) of the next scanline. And it will then turn on, and sweep left-to-right to draw the next scanline. When the last scanline is drawn, the electron beam is turned off, and the deflection circuitry is reset so that the beam's position will next be at the top left of the TV screen—ready to draw the first scanline of the next frame.
This 'turning-off' and repositioning process—at the end of a scanline, and at the end of an image—is not instantaneous—it takes a certain amount of time for the electronics to do this repositioning, and we'll understand this when we come to talk about the horizontal blank (when the beam is resetting to the left of the next scanline) and the vertical blank (when the beam is resetting to the top left scanline on the screen). I'll leave that for a later session, but when we do come to it, you'll understand what the TV is doing at these points.
A fairly complex—but nonetheless simple-to-understand analog signal controls the sweeping of the electron beam across the face of the TV. First it tells the TV to do the repositioning to the start of the top left line of the screen, then it includes color and intensity information for the electron beam as it sweeps across that line, then it tells the TV to reposition to the start of the next scanline, etc., right down to the last scanline on the screen. Then it starts again with another reposition to the start. . . That's pretty much all we need to know about how that works.
The Atari 2600 sends the TV the "color and intensity information for the electron beam as it sweeps across that line", and a signal for the start of each new line. The '2600 programmer needs to feed the TV the signal to start the image frame.
A little side-track, here. Although I stated that the vertical resolution of a TV image is 625 lines (PAL) and 525 lines (NTSC), television employs another 'trick' called interlacing. Interlacing involves building up an image out of two separate 'frames'—each frame being either the odd scanlines, or the even scanlines of that image. Each frame is displayed every 1/30th of a second (30Hz) for NTSC, or every 1/25th of a second (25Hz) for PAL. By offsetting the vertical position of the start of the first scanline by half a scanline, and due to the persistence of the phosphor coating on the TV, the eye/brain combines these frames displaying alternate lines into a single image of greater vertical resolution than each frame. It's tricky and messy, but a glorious 'hack' solution to the problem of lack of bandwidth in a TV signal.
The upshot of this is that a single FRAME of a TV image is actually only half of the vertical resolution of the image. Thus, a NTSC frame is 525/2 = 262.5 lines deep, and a PAL frame is 625/2 = 312.5 lines deep. The extra .5 of a line is used to indicate to the TV if a frame is the first (even lines) or second (odd lines) of an image. An aside: about a year ago, the #stella community discussed this very aspect of TV images, and if it would be possible for the Atari to exploit this to generate a fully interlaced TV frame—and, in fact, it is possible. So some 25 years after the machine was first released, some clever programmers discovered how to double the resolution of the graphics.
Back to basics, though. We just worked out that a single frame on a TV is 262.5 (NTSC) and 312.5 (PAL) lines deep. And that extra .5 scanline was used to tell the TV if the frame was odd or even. So the actual depth of a single frame is 262 (NTSC) and 312 (PAL) lines. Now, if TV's aren't told that a frame is odd, they don't offset the first scanline by half a scanline's depth—and so, scanlines on successive frames are exactly aligned. We have a non-interlaced image, displayed at 60Hz (NTSC) or 50Hz (PAL). And this is the 'standard' format of an Atari 2600 frame sent to a TV.
In summary, an Atari 2600 frame consists of 262 scanlines (NTSC) or 312 scanlines (PAL), sent at 60Hz (NTSC) or 50Hz (PAL) frequency. It is the job of the '2600 programmer to make sure that the correct number of scanlines are sent to the TV at the right time, with the right graphics data, and appropriate control signals to indicate the end of the frame are also included.
One other aspect of the difference between TV standards—and a consequence of the incremental development of television technology (first we had black and white, then color was added—but our black and white TVs could still display a color TV signal—in black and white)—is that color information is encoded in different places in the signal for NTSC and PAL (and SECAM) systems. So, even though the programmer is fully-responsible for controlling the number of scanlines per frame, and the frequency at which frames are generated, it is the Atari itself which encodes the color information into the TV signal.
This is the fundamental reason why there are NTSC, PAL, and SECAM Atari systems—the encoding of the color information for the TV signal! We get some interesting combinations of Atari and games, for example. . .
If we plug a NTSC cartridge into a PAL '2600, then we know that the NTSC game is generating frames which are 262 lines deep, at 60Hz. But a PAL TV expects frames 312 lines deep, at 50Hz. So the image is only 262/312 of the correct depth, and also images are arriving 60/50 times faster than expected. If we were viewing on a NTSC TV, then the PAL console would be placing the color information for the TV signal in a completely different place than the TV is expecting—so we would see our game in black and white.
There are several combinations you can play with—but the essence is that if you use a different '2600 variant than TV, you will only get black and white (for example, NTSC '2600 with PAL TV or PAL '2600 with NTSC TV) as the color information is not in at the correct frequency band of the signal. And if you plug in a different cartridge than TV (NTSC cart with PAL TV or vice-versa) then what you see depends on the television's capability to synchronize with the signal being generated—as it is not only the incorrect frequency, but also the incorrect number of scanlines.
All of this may sound complicated—but really all we need to do is create a 'kernel' (which is the name for your section of an Atari 2600 program which generates the TV frame) which does the drawing correctly—and once that's working, we don't really need to worry too much about the TV—we can abstract that out and just think about what we want to draw.
Well, I lie, but don't want to scare you off TOO early.
Next time, let's have a look how the processor interacts with hardware, I/O and memory.