In the late 1970s and early 1980s, many personal computers were marketed to
the home market and were designed to connect to color television sets. These
computers produced low-power radio frequency signals sent directly to the
antenna connection on the television. Some computers in this market also
produced composite video[1] signals for connecting to video monitors.The difference between a television and
a video monitor was that a television had a radio receiver. That receiver
demodulated the radio frequency signal, extracting the composite video and sound
signals. Therefore, a television had an RF receiver, whereas a monitor lacked
the receiver and directly accepted a composite video signal. Separate audio
connections or separate speakers were required for a monitor. What a television
and a monitor had in common was that they used the same video signal as
broadcast television of the day with its limited bandwidth and resolution.
However, bypassing the receiver with a direct composite video signal provided a
sharper image with better color.
Few televisions included a composite video input, only antenna or cable
inputs. If you had a computer that produced a composite video output but had a
television instead of a monitor, you needed an RF modulator to convert the
composite signal to an RF signal for the television. Like computers that had RF
outputs, these had a channel selector for channel 2 or 3 in North America.
An
RF modulator with a composite video input (yellow RCA connector) along
with right and left line-level audio inputs. The RF section has a
pass-through for a television antenna or cable input. The modulator can
be switched out of the circuit, connecting the antenna or cable to the
television.
S-100-based computers often used video terminals and sent signals over an
RS232 connection as if they were connected to teleprinters. Video terminals had
all the electronics to convert the RS232 signal into that needed to create the
display. In 1978, Digital Equipment Corporation (a big name in mainframe
computers at the time) introduced the VT100 video terminal. This terminal became
popular, and other companies made compatible monitors and emulators. The VT100
supported ANSI escape codes. These codes, often preceded by the binary code
00011001 (produced by pressing the Esc key), performed various functions like
moving the cursor to arbitrary locations on the screen, scrolling the screen up
and down, Erasing lines and characters, and controlling status lights outside
the screen. This new generation of ANSI terminals went beyond acting like an
electronic paper roll printer as early video terminals did.
The IBM PC required a plug-in graphics adapter. The output signal and monitor
type depended on the chosen adapter. Some adapters used a composite video signal
sent to a monochrome or color monitor. Others sent analog signals with separate
wires for synchronization signals. If such a system displayed color, it used
three wires to send separate signals for red, blue, and green.
Input for an early color monitor with BNC connectors for separate red,
blue, and green signals.
Starting with the MDA card described below, the computer sent TTL-level
signals, providing no way to have different brightness levels. Most
high-resolution monochrome monitors had green or amber (orange) displays.
An
IBM PC with a green monochrome display.
A
Nixdorf computer with an amber monochrome monitor.
With the CGA standard, the system used three wires to send separate
signals for red, blue, and green, with a fourth wire enabling two brightness
levels for each color.
