RCA
Direct Answer
The RCA connector, introduced by RCA Corporation around 1937, is a de facto industry-standard connector for consumer audio and composite video -- there is no formal mechanical specification for the connector itself, though the composite video signal it typically carries targets a nominal 75-ohm impedance.
Source: De facto industry standard (RCA Corporation, c. 1937)
Specs
| Outer Shell Diameter | ~8.3 - 8.5 mm |
| Center Pin Diameter | 3.175 (1/8 in) mm |
| Composite Video Impedance (nominal, signal-level) | 75 ohm |
| Passband | 0 - 100 MHz |
A Standard With No Standard
Despite being one of the most widely used connectors in consumer electronics history, the RCA connector has no dedicated formal mechanical specification from a body like the EIA or IEC -- it functions purely as a de facto standard, established by RCA Corporation's own products and copied industry-wide because it worked and was cheap to manufacture. The 75-ohm figure often quoted for "RCA video" actually describes the composite video signal standard the cable is expected to carry, not a guaranteed electrical property of the connector itself; the plug's physical dimensions don't provide a true 75-ohm impedance match by design.
RCA Corporation introduced the connector by 1937, first using it internally to link a turntable to an amplifier inside its own U-109 radio-phonograph console. It migrated to an external, rear-panel connection on consumer radios by 1938, and the same basic plug -- unchanged in essentials -- went on to become the standard connector for consumer stereo audio (in white/red pairs) and composite video (in yellow) for the rest of the 20th century.