hr@uicsl.UUCP (11/11/85)
Some time ago, I asked about how to easily convert RGB to to composite
video. Here is a summary of the responses.
1)
Use the MC 1377 matrix chip! You feed it sync, R, G, B (of course,
all properly DC restored so that the black level is 0.0 volts and peak
white is 1.0 volts on the RGB inputs). All it requires is
a crystal ( 63/88 * 5.000 mHz for NTSC, ~~4.43 mHz for PAL-B)
five capacitors and two resistors
an 8.2 v CLEAN precision reference for pin 16
12 volts DC
The inputs are R, G, B, and sync. No provision is made, however, for
proper black level setup intervals, and no gamma precompensation is provided.
This is NOT NTSC by a long shot (i.e. you can't hook it to a television
transmitter) but it will probably work on most monitors quite well.
If you want it to work a LOT better, lock a 14.3182818 mHz source to
the horizontal scanning frequency. (The horizontal scanning frequency can
be obtained by differentiating the composite sync, triggering a one shot,
and some combinational logic). With this pseudo SC-H (video-ese for
subcarrier-horizontal) genlock, you won't get a lot of artifacts and junk
in the image due to imperfect subcarrier phasing. (Owners of heterodyne
type VTRS will recognize this as lines which are diagonal and parallel to
each other, and the angle between those lines and the scanning lines
changes constantly with time base errors . . .)
Back to the subject: One more thing: this chip does not provide the
correct matrix precompensation and delay for 'true' NTSC, so you will
be limited to a 0.5 mHz video bandwidth for saturated colours. If you are
trying to (broadcast)(record)(display) something like a Vectrix RGB output,
you can forget it, since these units will have R, G, B video components
well up to 4.2 mHz. YOU ALSO MAY GET STRANGE BEATS AND FLAKIES because
of the lack of precompensation and delay equalisation. . . (similar to
the effect when a finely structured cloth is telecast such as herringbone
tweed, etc.)
You may also have some gross interline flickering, too; in which
case you can always mutilate the equalising pulses in the vertical sync,
etc (ala Atari video games, etc.)
Hope this helps,
David Anthony
DataSpan, Inc
2)
This is taken from my copy of the Atari 800 logic.
comp video is sync + chroma + luminance
luminance ---> op amp --------+----+-----> comp video --> rf modulator
| |
sync ---> op amp --> diode -->+ |
|
color---> op amp --> 100pf -> 1k --+
--
--
{ihnp4|vax135|allegra}!lznv!nrh
Nigel The Mad Englishman or
The Madly Maundering Mumbler in the Wildernesses
3)
Someone else reminded me that, generally, one did not want to
go from RGB to composite since information was lost.
4)
In VOL. 1 NO. 1 of the "Computer Smythe", there is an
article entitled "Easy Color Conversion, convert RGB to
composite signals". The circuit uses an LM1886.
harold ravlin
{ihnp4,pur-ee}!uiucdcs!uicsl!hr