From: jhwhit01@ulkyvx.louisville.edu
Newsgroups: sci.electronics
Subject: Re: 8-bit video chip needed
Date: 22 Nov 92 21:59:09 GMT
Distribution: usa
Organization: University of Louisville

In article <sad.722460663@camelot>, sad@camelot.bradley.edu (Shrijay Dalal) writes:
> Anyone know which are good 8-bit video chips to use to display to a
> composite screen (TV) and is relatively easy to use and come by?
  
Texas Instruments TMS9918A is probably one of the easiest 8-bit video display
processors to use if you can live with its limitations.  The best source is
probably pulling it from its socket from a used 99/4A home computer (which run
from $5-30, usually, depending on what extras come with it).  The 9918A does
need the heat sink attached to it in that computer.  JDR Microdevices sells
what they call a TMS9918A for $19.95.  But you might get the near equivalent
TMS9118 instead (a later version, missing an output clock signal but with a
better memory interface).  These VDP's (and their "color-difference" versions)
are limited to 256x192 pixels, and have a palette of 15 colors and transparent
(for multi-video applications).
 
The TMS9918A et al were the chips spec'd for the original MSX standard developed
in Japan.  [The microprocessor used was the Z80.]  Texas Instruments did do
more development in the line, but they never did release the AVDP (512x212
pixels etc.).  However, after TI decided to go a different route with the
TMS32010 and following graphics co-processors, Microsoft (the "MS" in MSX) and
Yamaha developed the MSX-II video chip, known alternately as the V9938 or the
YM2701.  It is compatible with all the TMS9918A modes, but supports composite
(NTSC or PAL) resolutions up to 512x212 (non-interlaced), 16 out of 512 colors,
and 512x424 interlaced with same color limitation.  In 256x212(424) mode each
pixel can be one of 256 colors.  This chip, or a variant thereof, is what I've
been told is used in the Sega GameMaster (?) machine.
 
A latter chip known as the V9958 or YM2703 (apparently used in Sega Genesis)
the graphics modes as the V9938, but the composite out signal was dropped to
provide alternate features.  It does have the same linear RGB outputs as the
V9938, though, which are readily converted to composite with external encoding
circuits.  One interesting new feature of the V9958 is the YJK mode, which is
sort of like the original Amiga HAM mode.  In the 256x212(424) pixels mode,
the 8-bit values for each group of four adjacent pixels are used to expand the
{number of colors on-screen at once.  This is done in a way that is not easily
described, though Yamaha says it allows up to 19,268 colors to be displayed.
 
One caveat about using the V9938 or V9958:  They are in 64-pin Shrink DIP.
This means they take up about the same real-estate as a 40-pin DIP, and it is
next to impossible to find a breadboard or protoboard with the tighter spacing.
DigiKey does sell a 64-pin Shrink DIP socket, so if you can drill your own
circuit board you can use the V9938 or V9958 if you wish.
 
The V9938 and V9958 chips are in the $30-40 range, with the V9958 about $3 more
than the V9938.  However, dealers only know them by the Yamaha part numbers
{YM2701 and YM2703.
 
Jeff White       jhwhit01@ulkyvx.louisville.edu
Newsgroups: sci.electronics