Saturday, October 10, 2015

Advantages of the 160x200 16-color Tandy/PCjr. Resolution

160x200 in 16 colors is a resolution officially only supported on the Tandy 1000s and the IBM PCjr.  Despite having the memory (16K) available, CGA can only do 160x200 in 4 pre-defined colors (by doubling pixels in the 320x200 mode) or 160x100 in 16 colors (by tweaking the 80-column text mode)  EGA can also support 160x200 unofficially either by doubling pixels or adjusting the parameters in its CRT controller, but this was seldom supported on EGA-supporting titles.

One of the most substantial benefits to the 160x200 16-color mode is that it uses half the video memory of the 320x200 16-color mode (16KB vs. 32KB).  The CPU needs only send half as much data to the video memory, improving performance on any machine.

One drawback with the 160x200 16-color mode is that you can only uses 20-columns of text at the standard 8x8 pixel IBM character set.  In this case, developers typically chose two options.  First, they could use their own character set which would fit into a smaller pixel matrix.  Maniac Mansion is an example of this option, it uses 4x8 characters for the Tandy mode and the standard 8x8 characters for CGA, MCGA, EGA or VGA modes.  Second, they could use the 320x200 16-color mode, which is what King's Quest and its sequels use.  In King's Quest, the pixels for the graphics display are doubled, improving performance to a good extent but not as much as if the true 160x200 mode was used.

Another benefit, which is under-appreciated these days, is that games using this mode look very good on a composite monitor.  The IBM PCjr. and the early Tandy 1000s (1000/A/HD/EX/HX/SX/TX) have a composite video output jack.  In a 320x200 mode, these computers show artifact colors that are markedly different from an IBM CGA card.

But in a 160x200 16-color mode, their composite colors correspond to their RGB colors quite well.  This is because in this mode, the pixel clock is the same as the NTSC color burst frequency, 3.58MHz.  Because the pixel clock is aligned with the color clock, the TV's color decoding circuitry can keep pace with the color changes being sent to it.  Even in a psuedo-160x200 game like King's Quest, the doubling of the pixels makes the graphics look as they should, color-wise.

By contrast, the pixel clock in a 320 pixel mode is 7.16MHz and 14.318MHz in a 640 pixel mode.  In those resolutions, the color decoding circuitry cannot keep up, leading to artifact colors.  The timing of the PCjr. and Tandy video controller chips is "off" compared to the IBM PC CGA, leading to different artifact colors. The 320x200 16 color mode does not look good on a composite monitor, the artifacting becomes ugly at this point, especially when it comes to dithering.

The final benefit to the 160x200 mode is that the end result looks acceptable on a video capture device. Capturing 320x200 or 640x200 graphics from a CGA card, PCjr., Tandy or EGA card that gives a good representation of RGB color is much, much more difficult.  First, you need a board that will convert the digital RGB signal to an analog RGB signal and preferably keeps color 6 brown, not dark yellow.  (From the composite video output, color 6 will look closer to dark yellow than to brown.)  Second, you will need a hardware device that can capture 15kHz RGB.  They do exist, but most people would probably use a scan-line doubler and send it to a VGA capture device.

You may think that only a handful of games used a 160x200 mode, but you would be surprised.  Here is a list of forty nine games that primarily use a 160x200 16-color mode for their graphics when run on a PCjr. or a Tandy 1000 :

Black Cauldron, The
Boulder Dash
Boulder Dash II: Rockford's Revenge
Bruce Lee
California Games
Demon Attack
Donald Duck's Playground
F-15 Strike Eagle
Gold Rush!
Indianapolis 500
King's Quest I: Quest for the Crown
King's Quest II: Romancing the Throne
King's Quest III: To Heir is Human
King's Quest IV: The Perils of Rosella
Leisure Suit Larry in the Land of the Lounge Lizards
Lost Tomb
Manhunter 2: San Francisco
Manhunter: New York
Maniac Mansion
Mickey's Space Adventure
Microsoft Flight Simulator (v2.0)
Mixed-Up Mother Goose
Murder on the Zinderneuf
Dr. J and Larry Bird go One-on-One
Pitfall II: Lost Caverns
Pitstop II
Police Quest: In Pursuit of the Death Angel
River Raid
Sea Speller
Silent Service
Slugger, The
Space Quest I: The Sarien Encounter
Space Quest II: Vohaul's Revenge
Super Bowl Sunday
The World's Greatest Baseball Game
Touchdown Football
Troll's Tale
Winnie the Pooh in the Hundred Acre Wood
Wizard and the Princess, The
Zak McKracken and the Alien Mindbenders


Anonymous said...

are there any games that use 640 200 16?

how does this compare with apple iigs and commodore 64 coco3?

also can u blog about tandy color computer 3?

google locked me out but im a regular here i owned a tandy 1000 and my neighbors had color computer and c64 and iigs

Great Hierophant said...

Sierra's ports of Japanese home computer games from Game Arts and Falcom use 640x200x16, as does Koei's ports of their turn based strategy games to DOS like Nobunaga's Ambition and Romance of the Three Kingdoms.

Most C64 games use a 160x200 mode, but the colors are a bit different and you cannot put colors wherever you like, which prevented ports of the AGI games to that system. The Sierra AGI games use an effective 140x192 graphics mode with an Apple IIe/IIc and a 160x200 mode with the IIgs. The IIgs, Amiga and ST look pretty much like the PCjr/Tandy/EGA/MCGA/VGA adapters.

If someone sends me a Tandy CoCo 3, I would be happy to write about it. I do not write about computers or consoles I can only fully experience by way of emulation.

macdeath69 said...

the 160x200 resolution was usefull to directly port graphics between different 8bit systems.

AppleII, Tandy/PCjr, Amstrad CPC, C64, I guess even Atari8bit computers had their most colourfull video modes using this resolution.

As computer market could need many different version on different uncompatible systems, any portability could mean less paid hourswork from specialised coders or graphic artists.

Many Amiga games were basically Atari ST games in 16 colours only, many Amstrad CPC games were ZX spectrum ports with jsut less colours, some games released on many platforms would get huge bits of graphics straightly ported with sometimes only insulting calculated palette conversions.