Saturday, April 8, 2017

The Amazing Technology in the Nintendo Game Boy

Truly nothing like the Game Boy had ever been seen before by the general public when it was released in 1989.  Handheld gaming prior to that was confined to simple, single hand-held games like the Coleco mini-Arcades, the Nintendo Game and Watch series and the ubiquitous Tiger Electronics Hand-helds.  These were simple games that were driven by pre-programmed microcontroller chips and drove an LCD display that was only capable of displaying a series of fixed patterns.  Although the patterns could have a high level of detail, the limitations of the display severely limited the complexity and longevity of these games.

The Game Boy's best-known predecessor, the Milton Bradley Microsivion, used a 16x16 display.  The Microvision was not very successful and its games were put on pre-programmed microcontrollers that plugged into the main unit.  These microcontrollers operated at a very low speed of 100KHz, and provided only 64 bytes of RAM and 1-2KB of ROM for a game.  The low resolution of the display also placed severe limitations on the games that could be made for this system.  The Epoch Game Pocket Computer was released in Japan in 1984 and used a 75x64 resolution display, but it was not very successful and only had five games released for it.

The Game Boy of 1989 was a fully programmable console, giving 8KB of RAM, 8KB of Video RAM, four channels of stereo sound, a serial game link port for peripherals and multiplayer and an 8080/Z80 hybrid CPU running at 4.193404MHz.  The NES came in a large box and required eight ICs to get all the essential functions working.  The Game Boy whittled this down to four.  The CPU, APU and PPU were combined into one 80-pin chip. The two RAM chips and an audio amplifier comprise the other chips on the main board, and the LCD controller chip can be found near the screen.  A D-pad and four buttons were available, just like the NES controller.  Cartridges immediately had standardized memory bankwitching controllers available to make ROM available up to 512KB and 8KB of battery backed RAM.  Eventually there would be games on the original Game Boy that required 4MB ROMs, far larger than any NES game.  The screen had a resolution of 160x144 and could produce four shades of one color.    

With this hardware, Nintendo and its 3rd Party Developers could give players games that could match the sophistication of the NES, if not the color.  The Game Boy could do smooth scrolling both horizontally and vertically (because that 160x144 is a window into a 256x256 playfield), easily display a non-moving status bar, perform color cycling with multiple palettes, display an appropriate number of hardware sprites for the smaller screen and output near NES-quality audio in stereo.  Graphics could appear very detailed, even in four shades of gray, on the dot matrix display.  Every pixel was clearly distinct, something of which could not be said of the NES.

Perhaps the most amazing element of the Game Boy, and certainly its most challenging, is its LCD screen.  The Game Boy uses a SuperTwisted Nematic Passive-Matrix monochrome LCD display.  This screen is composed of six layers.  From front to back :

The first layer is a vertically-polarizing filter coated onto the top glass.

The second layer consists the top glass with translucent electrodes vertically etched into it

The third layer holds the liquid crystals.

The fourth layer consists of translucent electrodes horizontally etched into the bottom glass

The fifth layer consists of the horizontally-polarizing layer.

The sixth layer is the reflective layer.

This diagram from wikipedia gives a good representation of the construction of the Game Boy's screen if you replace the number segments in layer 2 with pixels :

The Game Boy has the famous pea-soup green/yellow color, and that is due to the reflective and polarizing layers.  The glass layers themselves are not really tinted.  Given that the human eye is most sensitive to wavelengths in the green portion of the visible spectrum, it is probably no accident that many, many LCDs of this period and since have the pea-soup green backing.  The Game Boy knockoffs like the Watara Supervision, the Bit Corp. Gamate, the Welback Mega Duck and the Timetop GameKing also use green/yellow screens.  The contrast of these displays leaves something to be desired given the technology of the time, hence the greenish hues.  The Game Boy Pocket and its monochrome contemporaries like the Neo Geo Pocket, the Wonderswan and the used gray background screens.

The two polarized layers are oriented 90 degrees to each other.  The top polarizer only allows light to pass traveling in a vertical orientation.  The second polarizer blocks light traveling in a horizontal orientation.  When the screen is off, the liquid crystals are not active and do not change the polarization of the light, so you are seeing the reflective layer.  When the screen is on and a pixel is turned on, that tells the liquid crystals in the area covered by a pair of electrodes to twist their orientation.  This in turn can rotate the direction of the light from vertical to horizontal.  The more horizontal the turn, the less you will see of the reflective layer and the more you will see of the polarized layers. Depending on where the contrast dial is, you can see a totally dark or totally clear screen, depending on the amount of current given to the screen.  The best visibility is when there is something of a greenish hue to the off pixels.  Also, note that the Game Boy can provide two intermediate settings, which gives you the four shades.

The Game Boy uses a Super Twisted Nematic (STN) panel as opposed to a regular Twisted Nematic (TN) panel.  Non-STN panels were not highly regarded because the contrast was so poor.  The original IBM PC Convertible first used TN panels and they were not appreciated.  The second screens available for the IBM PC Convertible used STN panels.  Upgrading to STN panels improves the contrast of the display significantly, allowing for distinct shades of pixels.  While there is ghosting on Game Boy screens, it was often confined to early games.  Later games tend to reduce the speed of animation to limit ghosting.   The Game Boy is a passive matrix display where a not-insubstantial amount of time is needed to refresh the state of the pixels.

The Game Boy addresses its screen in a matrix arrangement, columns and rows, much like a computer keyboard.  The electrodes cover a whole column or row, while active matrix displays assign a transistor to each pixel or subpixel.  Because of this, there is an issue of dead screen lines, not dead pixels.  This typically happens when a connection on the vertical ribbon cable breaks off.  Fortunately it can often be reflowed with a soldering iron.  The horizontal lines are much more difficult to fix due to the construction of the screen and cables connecting it to the board, but the incidence of dead horizontal lines is much more uncommon.

The Game Boy's rivals at the time, the Atari Lynx, the Sega Game Gear and Sega Nomad, used color STN panels with CCFL backlights.  Backlights do not rely on ambient light but project light through the glass and polarizing layers.  CSTN panels display color via filters.  Each pixel is divided into a red, blue and green subpixel strip, and the liquid crystals' blocking of light for each subpixel determines the color of the actual pixel.  Because the addressing is by rows and columns instead of individual pixels, once a connection is broken a whole line of subpixels will go.  The low resolution of the screens tends to give them a stripey look.  The Turbo Express was an early TFT active matrix display with a CCFL backlight where each pixel was controlled by a transistor/capacitor gate.  Active matrix displays are more complex to build but offer faster screen response times.

The CCFL backlights were essentially the downfall of these early systems.  CCFL backlights are comparatively thick and very power hungry.  You could easily get 30 hours with fresh batteries in a Game Boy but maybe 5 hours with a Game Gear.  However, until the early 21st century, they were the only cost-effective portable illumination solution.  However, with an LED backlight mod, you can add 10 hours to your Game Gear's battery life.  The Game Boy Light used an electroluminescent backlit screen not unlike the screen of my Tandy 1400 LT,,  or the last screens of the IBM PC Convertible.  When turned on they add noise, only work with monochrome displays and are not the most efficient backlighting source now available.

The backlighting mod for the Game Boy and Game Boy Pocket uses bright LEDs and a reflective diffuser layer to provide and project light through the glass layers of the LCD screen.  In order to install these mods, you must pull away the adhesive-backed reflective and polarizing layer off the LCD.  The backlight mods come with a polarized film.  Without the polarized film, you will see nothing because all light is reflected off the backlight/reflective layer.  No light is blocked.

The polarizer film can be oriented horizontally or vertically by turning the film 90 degrees.  If oriented vertically, the shades will appear inverted compared to how they should look.  In this instance, the off state of the liquid crystals keeps the light going through vertically, which gets blocked by the second polarizer layer.  The on state allows light from the reflective layer.  Turn the film again and you can see normal shades if that is what you want.

Some clever individuals found that if you run the two video signals through an inverter, you can significantly improve the contrast of the pixels.  This is called the bivert mod, and the chip used is a 74HC04 hex inverter.  An inverter turns 0s into 1s and vice versa, so light pixels become dark and dark pixels become light.  It also acts as an amplifier.  While you can do the bivert mod on a non-backlit Game Boy, the resulting graphics will be inverted.  A backlight mod and a properly oriented polarizer film will restore the normal look to the graphics.  I might suggest that you could run the inverted signal through another gate on the chip (there are six inverters on an '04) to get proper graphics, but that may amplify the screen too much!

The Game Boy Color and Game Boy Advance screens graduate to Color Active Matrix TFT screens, but are not backlit.  They cannot be backlit because their panels have a third pane of glass on the back of the reflective layer, and that glass pane cannot be removed without destroying the screen.  Frontlight kits like the Afterburner appeared soon after the GBA was released, and now there are other kits available for both systems.  The frontlight kits are a translucent pane of glass with bright LEDs on the bottom, the glass pane diffuses the light over the screen.  The addition of an optically translucent adhesive called LOCA between the backlight and LCD glass offers a substantial improvement of the color diffusion and contrast.

1 comment:

Anonymous said...

"I might suggest that you could run the inverted signal through another gate on the chip (there are six inverters on an '04) to get proper graphics, but that may amplify the screen too much!"

Putting two or more inverters in series won't cause any additional amplification. The output of any given inverter will essentially be rail-to-rail as it is, so there should be no problem with this.