BY ROBERT LANG
One of the things that has always interested me is
the ability to superimpose text on a TV picture.
Since the early days of the microprocessor, a way
to interact with text has been a requirement. In the days
before computer monitors (yes, there was a time), one of
the first such devices was Don Lancaster’s TV typewriter.
In the late 1970s, if you built your own home computer
from a kit, the TV typewriter was one of the few input/output devices available at a reasonable cost, but it could not
superimpose text on a picture, only text on a black background. Well, my TV typewriter has been in the closet for
many years, and I pulled it out the other day to take a look
Figure 1. The original TV typewriter.
at it. The TV typewriter uses 68 integrated circuits to display
text on a TV screen, but I think that the chip count can be
improved upon with a modern microprocessor.
One of the things I have shied away from since getting involved with electronics is TV interfacing. No doubt,
I was scared off by the complexity of the original TV typewriter. To overlay textual information on a TV picture, timing is everything. The microprocessor must put out information at exactly the right point when the proper line on
the TV screen is being drawn. Fear not, this can now be
accomplished with three integrated circuits. We will build
a unit that can superimpose text data on a TV picture and
use an RS-232 interface to update the text. After we complete the unit, we will use it to explore the world of subliminal messaging.
NUTS & VOLTS
The National Television Standards Committee
(NTSC) sets the analog television standard for the US;
this format itself is also informally called NTSC and is
described on their website listed in Reference 1. While
standard for the US, it has also been adopted in other
countries, as well, such as Japan, while many countries in
Europe use phase alternation line (PAL). The NTSC format consists of 29. 97 (nominally 30) interlaced frames of
video a second, each consisting of 480 lines of vertical
resolution out of a total of 525 (the rest are used for sync
and vertical retrace). Thus, the horizontal scan frequency
is 525 x 29. 97 or 15,734 Hz. Each scan line takes approximately 63. 5 microseconds. NTSC interlaces its scan
lines, drawing odd numbered scan lines in odd numbered
fields and even numbered scan lines in even numbered