Waveform 12 Analog Video, part 1

[HD Library]

Originally published 11/04/2004, HDTV Magazine, editor Dale Cripps

Updated 09/21/2005

By Richard Fisher

If I were to simply state my conclusions in this matter most will be left with unanswered questions due to all the misinformation in the market and my recommendations are seen as simply another mans opinion rather than a statement of science. When ever you go beyond a 1 foot cable connection you are dealing with a cable circuit that requires proper impedance matching of the cable, connectors, termination of those connectors and source and destination termination on the products. The longer the cable the more critical these elements become.

Analog is such a game because you can get away with so much shoddiness and poorly performing stuff and by god it all still works regardless of the artifacts induced that do not seem to bother the end user or are not clearly visible.

RCA connectors are limited and all insiders know it. 75 ohm BNC is the correct connector for video applications but they take up excessive real estate, are not assembly line friendly and more expensive.

Most RCA connectors, male and female, are for audio applications and measure 50 ohms. None allow proper termination of the ground plane of the coax and without that you still will not get the correct impedance. The tip itself relies on a single point of contact along both large open surfaces making it susceptible to corrosion and oxidation. If you were to see the assemblies you would also clearly see that there is no mechanical integrity in nearly all cases of consumer product. Due to the audiophile world the quality of RCA connectors and cabling has improved dramatically but rarely do you see this variety on your video equipment. Even with what is considered one of the best, Canare, you still cannot properly terminate the ground plane of the coax for 75 ohm. BNC does terminate properly and also provides a full surface contact area on the center pin and crimping termination rather than solder for integrity, prevention of corrosion and oxidation plus higher bandwidth response.

S-video uses a DIN connector and in nearly all cases low quality, serve-shield miniature coax cable. With runs less than 6 feet it is quite effective in consumer applications but beyond that the losses can be quite heavy even to the point of causing a timing error between the luma and chroma signal called Y/C delay. For long runs use a 6 inch S-video breakout cable that will have two RCA or BNC connectors on the end so you can use thicker low loss video cables for the length of your run. Even at that S-video is not physically capable of being terminated properly due to the wire and connector being used.

VGA connectors suffer from the same problem with impedance and are also limited by the size of the cable for this type of connector. 20 feet is considered pushing it. In most distance applications with this connector you would use a 1 foot breakout cable with BNC to BNC using runs of low loss video cabling.

Then comes adapters which change one type of connection to another. These are best avoided and we suggest you simply have your cables terminated with the correct connectors for your application. Adapters can throw off the impedance of your cabling circuit. We just ran into such a dilemma when we pushed a DVI cable to it