Motorola 6847
The MC6847 is a video display generator (VDG) first introduced by Motorola and used in the TRS-80 Color Computer, Dragon 32/64, Laser 200, TRS-80 MC-10, NEC PC-6000 series, Acorn Atom, and the APF Imagination Machine, among others. It is a relatively simple display generator compared to other display chips of the time. It is capable of displaying text and graphics contained within a roughly square display matrix 256 pixels wide by 192 lines high. 

The ROM includes a 5 x 7 pixel font, compatible with 6-bit ASCII. Effects such as inverse video or colored text (green on dark green; orange on dark orange) are possible.

It is capable of displaying nine colors: black, green, yellow, blue, red, buff (almost-but-not-quite white), cyan, magenta, and orange. According to the MC6847 datasheet, the colors are formed by the combination of three signals:  with 6 possible levels,  (or  with 3 possible levels) and  (or  with 3 possible levels), based on the YPbPr colorspace, and then converted for output into a NTSC analog signal .

The low display resolution is a necessity of using television sets as display monitors. Making the display wider risked cutting off characters due to overscan. Compressing more dots into the display window would easily exceed the resolution of the television and be useless.

Video modes

Signal levels and color palette
The chip outputs a NTSC compatible progressive scan signal composed of one field of 262 lines 60 times per second.

According to the MC6847 datasheet, colors are formed by the combination of three signals:  luminance,  chroma and  chroma, according to the YPbPr colorspace. These signals can drive a TV directly, or be used with a NTSC modulator (Motorola MC1372) for RF output.

 may assume one of these voltages: "Black" = 0.72V; "White Low" = 0.65V; "White Medium" = 0.54V; "White High" = 0.42V. 

 (or ) and  (or ) may be: "Output Low" = 1.0V; "R" = 1.5V; "Input High" = 2.0V.

The following table shows the signal values used:

Notes:
  
1) The colors shown are adjusted for maximum brightness and only approximate (diferent color spaces are used on TV and web pages). 

2) At least on the Color Computer 1 and 2, the alternate palette of text modes (actually the text portion of semigraphic modes) was dark pink (or dark red) on light pink, of shades not listed here (and no dark orange), whereas the Color Computer 3, with a different chip, made it dark orange on orange. 

The first eight colours of this table were numbered 0 to 7 in the upper bits of the character set (when bit 7 was set, bits 4-6 represented the colour number), but ColorBASIC's numbering was 1 higher than that in text mode, as it used 0 for black.

See also
 Motorola 6845, video address generator
 Thomson EF9345
 Texas Instruments TMS9918
 MOS Technology VIC-II
 List of home computers by video hardware

References

Graphics chips
6847