PAL/NTSC Upgrade
For Atari 600XL/800XL/65XE/130XE and XE Game System Version 1.0 (7/95) by Nir Dary
(Documentation by Nir Dary and Ben Poehland)

This upgrade gives your NTSC 8-bit Atari computer the option to run PAL software, games, and demos without messy VBI problems that cause screen jitters or system crashes. You'll be able to enjoy any PAL software in full color on a regular NTSC monitor or TV and still run your NTSC software as usual. Some PAL software will run on NTSC machines, but usually it won't: a real frustration for North American 8-bitters who crave the fabulous European graphics.
The European market offers lots of software that never appeared in North America due to the incompatibility between the American and European video systems. Games with amazing graphics are still actively being developed in Europe, particularly in the former Eastern-bloc countries such as Poland. On the Files included with this page you'll find a variety of demos and games demonstrating just how good the European software can be. You might want to try running this demos before upgrading your computer, so you'll be able to see the before-and-after difference.

The main difference between PAL (Phase Alternated Line) and NTSC (National Television System Committee) is that PAL generates 25 full video frames per second while NTSC produces 30 frames a second. Due to screen interlacing, in which first all the odd lines and then all the even lines of the frame are presented, the vertical scan frequencies for PAL are 50Hz (50 scans per second) and 60Hz for NTSC. It is mainly the United States and Canada that use NTSC video, while most of the rest of the world uses PAL.

To the casual North American television viewer, the difference in vertical scan frequency will manifest itself as a slight flicker of the PAL video screen compared to an NTSC display. You might wonder why Europeans tolerate these flickering screens. There are two answers to that question: one technical, the other psychological. Technically, the advantage of PAL video over NTSC video is that the PAL system gives better resolution (finer detail) than NTSC does. Psychologically, Europeans don't have to "tolerate" flickering video screens, because they don't see the flicker! It is only people from North America, who have grown accustomed all their lives to the faster scan rate of NTSC video, who notice the flicker when they're presented with the slower-scanning PAL display. American visitors to Europe often praise the greater clarity of PAL video screens while complaining that they flicker; European visitors to America don't notice any flicker on American TV's (they're already used to the slower scan rate, so the faster scan rate of NTSC makes no impression on them), but often complain the picture on an American TV is fuzzy. For consumers, then, the bottom line is that the NTSC system sacrifices video clarity to avoid flicker, while the PAL system accepts more flicker to gain an improvement in clarity.

You should be aware that by installing the PAL/NTSC Upgrade in your computer, you'll be introducing this so-called "PAL flicker" into your video display. Due to the psychological nature of the flicker phenomenon, there isn't any electronic "fix" for it. I've received a variety of opinions about this issue. Some users find it very annoying while others seem not to notice. Results of an informal poll of users who observed a recent public demonstration of the PAL/NTSC Upgrade suggests responses to PAL screen flicker depend on the viewer's age. Children under 12 seem not to notice it. Teenagers and young adults might notice it to some degree, but don't seem bothered by it. Adults over 30 are most likely to notice the flicker or to find it objectionable.

The ambient light in the room does make some difference. A brightly lighted room seems to minimize the flicker, while a dimly lighted room accentuates it. Less clear is whether the nature of the room lighting makes a difference. Some people seem to feel the flicker is more noticeable under fluorescent lights than with incandescent lighting, while others find the flicker equally objectionable under both types of light. Perhaps the best solution here is use some form of "DC light", such as sunlight, to illuminate your computer room.

If you use your Atari computer primarily for text-based applications (word-processing, telecommunications, databases, programming, etc.) where you're staring at your video screen for long periods of time, I recommend you NOT install the PAL/NTSC Upgrade in such a system. Even if you don't initially find the flicker objectionable, a prolonged period of staring at a static 50Hz video display might induce fatigue.

Instead, I recommend that you install the Upgrade in a secondary machine that's used primarily for graphics and/or videogame playing. (Of course, if graphics and games are already your main applications, then go for it!) Since screen displays in videogames are constantly changing, any flicker present will likely go unnoticed because you're too busy keeping up with events on the screen. Users of videogame software will benefit most from this Upgrade.

The PAL/NTSC Upgrade Kit consists of a PAL ANTIC chip (CO21698), a replacement OS ROM chip, 40-pin and 28-pin sockets (if required), software disk(s), and documentation. Make sure you have these items before proceeding.

For XL owners with socketed chips, this is a plug-in upgrade consisting merely of replacing the original ANTIC and OS chips with the ones supplied in the kit. Count yourself lucky if your chips are already socketed!

For XE owners and XL owners with soldered chips, you'll have to desolder your ANTIC and OS chips and install sockets in their place. WARNING: if you are unskilled in soldering/desoldering procedures, get some help! Don't do it yourself! Desoldering a large computer chip without damaging either the chip or the circuit board is very tricky. I will not be held responsible for the failure of this upgrade or incidental or consequential damage to any equipment or person attempting to install this upgrade or any variation of it.

Installation of the upgrade is straightforward. For soldered chips, required tools are:

Medium-size (#1-point) Philips screwdriver;

Flat-bladed screwdriver, knife blade or metal letter opener;

Needle nose pliers;

15 to 25 watt soldering iron;

Squeeze-bulb desoldering iron with a fresh nipple-tip (about $9 at any Radio Shack store);

Thin gauge rosin-core solder, about .015" diameter.

Unplug your computer from any cables attached to it, remove the screws holding the case together, and lift off the top half of the case. Remove any screws fastening the motherboard to the bottom case half, and remove the motherboard from the case. Then remove the metal RF shield from the motherboard.
Locate the 40-pin ANTIC chip (screened U7 on the 800XL or 130XE) bearing Atari part number CO12296 or CO21697. On the 800XL it's the second 40-pin chip from the left on the lower row; on the XE it's the second 40-pin chip from the top in the middle row. If the chip is in a socket then you're lucky: simply unplug the old NTSC ANTIC chip and replace it with the PAL ANTIC chip from the Upgrade kit. Don't discard the old ANTIC chip; place it in a static-free package for safekeeping.

If your ANTIC is soldered to the board, you have to desolder it and replace it with a socket. Turn the motherboard over, foil side up, and locate the ANTIC chip. With a hot iron apply a small amount of solder to all 40 pins, taking care not to dwell on each pin for more than a few seconds. Now warm up the desoldering tool. Squeeze the bulb and hold it, and place the hot nipple firmly over the first pin. In a few seconds you'll see the solder liquefy. Maintaining firm contact with the pin and foil pad, release the bulb so it sucks up all the molten solder. The procedure should take less than 10 seconds. Repeat this procedure for the remaining 39 pins. Now check each pin for freedom by seizing it with pliers and shaking it gently. If any pin is stuck, it isn't completely desoldered. Add more solder to the pin and desolder it again. When all pins are free, turn the board over and gently pry the chip up with the flat-blade screwdriver. It should lift out easily with no resistance. If it doesn't come out easy, DON'T FORCE IT! (Forcing it will wreck the delicate foil runs on the board and ruin your computer!) Repeat the desoldering procedure on the stuck pins until the chip comes out easily. Once the chip is removed, use the desoldering tool to clear any holes plugged with solder debris. All the vacated holes must be clean.

Solder the 40-pin socket to the board where the ANTIC chip was, and insert the new PAL ANTIC into the socket. Make sure the PAL ANTIC is facing in the same direction as the rest of the chips on the board.

Special Note For Users With RAM Upgrades: if you have a RAM upgrade with wires attached to the ANTIC chip, carefully desolder those wires when you remove the old ANTIC. The new PAL ANTIC CO21698 is functionally equivalent to the newer NTSC ANTIC, CO21697. If your old ANTIC was CO12296, consult the documentation for your memory upgrade for the correct re-wiring. In general, the wiring is simpler for the newer ANTIC chips CO21697 or CO21698.

Now locate the 28-pin Operating System chip, part number CO61598, designated U5 on the 800XL and 130XE motherboards. On the XL motherboard it's the second chip from the top on the upper right; on the XE it's located in the middle close to the front. If the chip isn't socketed, desolder it from the board and install a 28-pin socket in its place using the procedure described above for the ANTIC chip. Now plug in the new OS chip, taking care that it faces in the same direction as the rest of the chips on the board. Store this chip along with the ANTIC chip removed earlier, in a static-free package. This completes the installation.

TROUBLESHOOTING: Test your computer before reinstalling it back in the case. Attach the power cable and video connector, and turn on your video monitor. Then power up the bare motherboard (it will boot up without disk drives or keyboard plugged in). If you don't see the familiar READY message from BASIC, turn off the power immediately! Check both of the chips you installed, making sure they're correctly oriented (notches facing in same direction as the other chips on the board) and that they are seated firmly in their sockets with no pins bent-under. If it still doesn't work, remove the chips and carefully inspect your work with a magnifying glass. Keep a sharp lookout for solder bridges between adjacent pins, or for broken foil traces. If problems persist, try plugging back the original chips one at a time. If the computer boots up OK with one or both original chips replaced, then probably the new chips were damaged in shipping. I will gladly replace any chips for free upon receipt of the chip you believe is damaged or non-functional.

Once you are satisfied the computer is working OK, reinstall it back into the case. You'll notice that your video display is a slightly different size than before, with video information disappearing off the top/bottom or edges of the screen. Readjust the vertical and horizontal controls on your monitor until you once again have a satisfactory display that doesn't run off the screen and is stable.

Now... run the files that included with this page and enjoy!

PAL uses a 50Hz vertical scanning frequency while NTSC works on 60Hz. PAL video produces 25 full picture frames per second with 625 scan lines per frame, while NTSC video produces 30 pictures per second with 525 scan lines per frame. The increased number of scan lines in the PAL screen frame yields a 19% improvement in video resolution compared to NTSC, at the cost of a slower scan rate which for some people can cause the screen to appear to flicker.

The true clock frequency for an Atari 8-bit PAL computer is 1.7734470 MHz, while for the NTSC 8-bits the clock frequency is 1.7897725 MHz. Atari PAL computers have two clock crystals, while Atari NTSC computers have only a single crystal. The NTSC computers are simpler because the color clock can be divided out of the system clock. This isn't possible in the PAL machines, as it would require division by 2.8 (a number that doesn't set well with digital systems!). The second crystal is therefore provided in PAL machines to serve as the source of the color clock signal.

A VBI (Vertical blank Interrupt) executes every 1/50 of a second in PAL computers while in NTSC computers it's generated every 1/60 of a second. Thus a PAL computer has 35468 clock-cycles/VBI while an NTSC computer has 29829 clock- cycles/VBI. do the math you'll see that duration of a VBI in the NTSC computer is only 84% of the duration of a VBI in a PAL machine.

The old 400/800 models had different operating systems for PAL and NTSC. The newer Atari 8-bits (XL/XE series) use the same OS for both systems. In these machines the OS itself figures out which video system hardware the computer is using and adjusts itself accordingly.

So, how does the OS detect if the computer is PAL or NTSC? Quite simple, address $D014 in the GTIA chip holds a flag to determine NTSC or PAL:

Bit 1-3 clear (xxxx000x) = PAL

Bit 1-3 set (xxxx111x) = NTSC

OS address $62 (decimal: 98) is called PALNTS, and it's used as a flag to determine whether the computer's video is PAL or NTSC. After installing a PAL ANTIC chip into an NTSC computer, the value at GTIA address $D014 will still be the value for an NTSC computer, so the wrong data will be stored at address $62. (The computer will still "think" it's an NTSC machine even though it now is processing video data using the PAL system.)

In order to "trick" software programs written for PAL machines to make the software think it's running on a real PAL machine, a minor modification of the OS is required. The required modification was very simple: I just changed address $C386 to be "1" instead of "0". Following is the relevant section of Atari OS source code to illustrate what I did:

C374 AD 14 D0
LDA $D014
C377 29 0E
AND #$0E
C379 D0 08
BNE $C383 ;Branch on not zero
C37B A9 05
LDA #$05 ;PAL detected
C37D A2 01
LDX #$01
C37F A0 28
LDY #$28
C381 D0 06
BNE $C389 ;skip NTSC part
C383 A9 06
LDA #$06 ;NTSC detected
C385 A2 01
LDX #$01 ;(used to be LDX #$00)
C387 A0 30
LDY #$30
C389 8D DA 02
STA $02DA ;KEYREP - Rate or key repeat
C38C 86 62
C38E 8C D9 02
STY $02D9 ;KRPDEL - Auto delay rate

If you're using a modified OS (OMNIMON, OMNIVIEW, ARGS, etc.) or any type of OS that comes on 27128 EPROM, and would like to have your OS modified, simply contact some one who can and knows how to read and burn EPROM's. simply tell him to change address $C386(hex) to '01', that's all!. For those of you using the UltraSpeed+ OS from Computer Software Services, and would like to keep this OS installed in their machine with the PAL/NTSC Upgrade, I will provide a routine that copies the ROM OS into RAM, performs the update and then initiates a coldstart. This should work on most programs, but not all of them. This is the best I can do for now; the US+OS comes in a protected epoxy case, so there's no way to read it on an EPROM burner, modify it and burn it back. Keep in mind that not all PAL software checks the PALNTS flag; I ran across only two European programs that had trouble running without the updated OS. The routine will be included in this page as soon as it's done!!!, sorry!!! I haven't had any time to work on it yet!.