Quick Answer / Summary: Composite wiring consists of three cores in one jacket: an inner 75-ohm coaxial line for the yellow CVBS video signal, and a twisted pair for the red and white audio channels. Each core terminates to an RCA center pin (signal) and shield (ground). The video core’s 75-ohm discipline prevents ghosting; the audio pair’s twist cancels hum. Color code is fixed: yellow video, white left, red right.
Composite wiring is more structured than the three-plug cable suggests. Inside one jacket there are two different electrical constructions: a 75-ohm coaxial line carrying the video, and a twisted pair carrying the audio, both terminated to RCA connectors at the center pin and shield. The audio pair and the video core have different jobs and different rules, and understanding which is which is the key to diagnosing composite problems at the wiring level rather than guessing at cables.
One Jacket, Three Cores
A composite cable contains three separate electrical paths that happen to share an outer shield, jacket, and connectors. The video path is coaxial – center conductor, dielectric, individual shield – tuned to 75 ohm. The audio paths are two conductors each with no individual shields, twisted together so that interference couples equally into both and cancels at the receiving end. All of it sits inside an overall foil and braid shield, then jacket. The construction is compact because the outer shield serves all three cores simultaneously, which is efficient but also why a defective outer shield affects video and audio at once.
Why the Video Core Is Coax and the Audio Cores Are Not
The formats have different requirements. CVBS is a modulated carrier with meaningful content up to several megahertz, and it behaves like a transmission line: it reflects at impedance discontinuities, and those reflections appear as ghost images. That is why the video core is coaxial with a controlled 75-ohm impedance – a transmission line, not just a wire. Audio at line level is a low-frequency signal with far less bandwidth demand, and its main enemy is hum rather than reflection, which is why a twisted pair is the correct construction and coaxial geometry would be unnecessary cost. Using a twisted pair for video is the shortcut that produces ghosting; using coax for audio is unnecessary but harmless.
Termination: Center Pin and Shield
Both core types terminate at the RCA connector the same way: signal conductor to the center pin, return conductor to the outer shield ring. The quality of that termination is where the two most common field failures originate. A center pin that is not fully seated or has been deformed makes intermittent contact, producing the wiggle-to-work symptom. A shield that is not fully clamped or has been stressed mechanically loses its reference, producing hum on audio and a soft, unstable picture on video. When re-terminating a composite cable, solder or crimp both conductors with the specified strain relief, and verify continuity and pinout on both ends before returning the cable to service.
Inside a Composite Cable
| Core | Construction | Impedance | Role |
|---|---|---|---|
| Video (yellow) | Coaxial: center, dielectric, shield | 75 ohm | CVBS carrier transfer without reflection |
| Audio left (white) | Twisted pair, one conductor | Unbalanced line | Left channel at -10 dBV |
| Audio right (red) | Twisted pair, one conductor | Unbalanced line | Right channel at -10 dBV |
| Overall | Foil plus braid shield over all cores | n/a | Hum and RF immunity for the assembly |
| Termination | Center pin plus shield ring | n/a | Signal and ground at each RCA |
Real-World Application Scenarios
Repair and re-termination of field-damaged composite cables, where knowing which core is coaxial matters for the fix. Diagnosing ghosting versus hum, where the symptom points to either the video core’s impedance or the overall shield’s integrity. Manufacturing and QC, where continuity, pinout, and impedance on the video core are the three test items. Cable selection, where the wiring structure – coax versus pair on the video path – is the specification that separates grades.
Recommended Internal Resources
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Frequently Asked Questions (FAQ)
Q: Why is the video core in a composite cable coaxial but the audio cores are not?
A: CVBS video behaves like a transmission line and reflects at impedance mismatches, so it needs a controlled 75-ohm coaxial geometry to avoid ghosting. Audio at line level is low-frequency and its main risk is hum, which a twisted pair cancels adequately – coaxial geometry would add cost without benefit.
Q: What causes hum on a composite cable but not on the video?
A: Hum usually means the overall shield or the return path is compromised – a loose shield clamp, a broken drain connection, or a ground loop – and low-frequency fields are coupling into the audio conductors. Video may still look acceptable because the effect is different on each signal type.
Q: How do I check a composite cable’s wiring?
A: Three tests: continuity on each core end to end, pinout verification (yellow to yellow, white to white, red to red at both ends), and impedance on the video core, which should read close to 75 ohm. The first two catch most failures; the third catches the ghosting class of problems.