75-Ohm Video Cables

75-Ohm Video Cables

Any video engineer will tell you that the most important factor in determining accurate video signal transmission is tight control over 75-ohm impedance, especially when it comes to HDTV. Impedance is the amount of resistance to a current as it travels through a cable and is determined by the relationship between a cable's conductor and it's shield-ground. The most important thing to understand is that variations in the spatial relationship between a cable's conductor and shield-ground negatively effects impedance and, in turn, video signal quality. A Wikipedia definition of this phenomenon can be found here: http://en.wikipedia.org/wiki/Impedance_matching .
Any variations in a video cable's structure or in the way the cable is joined to its connectors cause something called reflection loss, a situation in which video signals "bounce" back to your transmitting device. A Wikipedia definition of this phenomenon can be found here: http://en.wikipedia.org/wiki/Reflection_loss. Reflection loss degrades signal integrity and ultimately effects video quality.

For example, if your transmitting equipment and your receiving equipment (video display) use standard 75-ohm video transmission (as nearly all video displays do), your display expects to receive an accurate 75-ohm video signal. However, as your video signal departs your transmitting device at 75-ohms it will be altered by the components of your video cable in the following in order:

connector A --> connector joint --> cable itself --> connector joint --> connector B.

It isn't possible to perfectly preserve 75-ohm impedance, but the goal is to minimize changes to the impedance and your video signal.

Following our example, solder-style RCA connectors (originally designed for audio use) cause an excessive amount of back reflection because they average around 25 ohms impedance. Each cable has two connectors, so two bottlenecks exist in any cables featuring solder-style or mass-production type RCA connectors. Also, the only way to solder an RCA to coaxial cable is to destroy the spatial relationship between the cable's conductor and shield-ground. This is done by twisting the cable's "surrounding" braid shield into a thick copper wire which is soldered to a single point on the side of the RCA connector. Structural changes such as this lead to even greater reflection loss and signal loss. Again, there are two solder joints per cable, so this negative effect is doubled. Also, low quality cable (even if expensive) frequently has great variations throughout the cable itself, and these variations lead to further reflection loss and signal loss.

As you can see below 75-ohm style RCA connectors retain the "equidistant" relationship between conductor and ground when crimped to standard coaxial video cable. Again, retaining this equidistance is the key to preserving 75-ohm impedance by reducing variations between the conductor and ground as the signal moves uniformly through the cable and connectors, without bottlenecks.