TTL RGB

TTL RGB (also called Digital RGB) is a video signal format that uses TTL (Transistor-Transistor Logic) voltage levels to represent color information. Unlike analog RGB which uses continuous voltage levels, TTL RGB uses discrete on/off states for each color channel.

Overview

In TTL RGB, each color channel (Red, Green, Blue) is either fully on (+5V) or fully off (0V). This limits the number of displayable colors but simplifies the circuitry required.

Color Depth

8 Colors (3-bit RGB)

Basic TTL RGB with one bit per channel produces 2³ = 8 possible colors:

R	G	B	Color
0	0	0	Black
0	0	1	Blue
0	1	0	Green
0	1	1	Cyan
1	0	0	Red
1	0	1	Magenta
1	1	0	Yellow
1	1	1	White

16 Colors (RGBI)

Adding an Intensity bit doubles the palette to 16 colors. When the Intensity signal is active, all colors are displayed at higher brightness. This was used in CGA monitors and some Japanese computer displays.

Signal Specifications

Parameter	Value
Logic high	+2.4V to +5V (TTL)
Logic low	0V to +0.8V (TTL)
H-sync	Separate, TTL level, typically active low
V-sync	Separate, TTL level, typically active low
Impedance	TTL (not 75Ω terminated)

Comparison with Analog RGB

Feature	TTL RGB	Analog RGB (RGBS)
Signal levels	0V / 5V (digital)	0-0.7Vpp (continuous)
Colors	8 or 16	Thousands to millions
Impedance	TTL	75Ω
Common connectors	EIAJ-8, DE-9	JP21, SCART, BNC
Typical use	1980s computers	Professional monitors, consoles

Common Connectors

EIAJ-8 - Rectangular 8-pin connector (Japanese monitors)
DE-9 - 9-pin D-sub (CGA monitors, some computers)
8-pin DIN 45326 - Round DIN (some computer outputs, though usually analog)

Compatible Equipment

Monitors

Sony Profeel Pro series (KX-21HV1, KX-27HV1)
Sony KV-1311CR, KV-14CP1, KX-14HD1
NEC PC-TV series
Sharp CU series
Various CGA-compatible monitors

Computers

IBM PC with CGA card
Fujitsu FM-X, FM-7
Pioneer PX-7 (MSX with laserdisc)
General PCT-55
NEC PC-8800 series (some models)

References

Digital RGB connector - MSX Wiki