Cables ... ?

Once you have got your high quality mega sharp picture you need to get that over to you screen with minimal loss

One area where there is more bull than almost any other is cables. When it comes down to video cables there really is no doubt about it. A video signal is a 75R sourced and terminated signal. In order to get the picture to it's destination with minimal degradation you need a 75R transmission line cable. To have a low level of interference it should be of the coaxial (screened) type. Your TV antenna down lead is (or at least should be) a 75R coax. In a transmission cable each unit length has a characteristic impedance of 75R and assuming that the source is 75R and the destination impedance is also 75R the only loss (in a perfect cable) would be due to the resistance of the copper in the cable which is very small indeed. There would also be a 1:1 VSWR and no reflections in the cable to cause ghosting or other picture nasties. The only questions are what is the loss of a particular cable over the sort of length I require, is the cable sufficiently screened and do I need it to be flexible. On the subject of screens there are 3 basic flavours. At the bottom of the pile there is the "lapped" screen where the strands are simply wound around the inner core. Next we have the braided screen where as the name suggests the screen is neatly woven around the center core. This is the one usually found on the better video grade cables. finally we have the solid foil screen. Here the screen is pieces of copper or aluminum foil wrapped around the inner core and usually held in place by a braided outer screen. Found in satellite down leads and very high quality RF cables it is generally regarded as the best. (Not to be confused with the metallised plastic foil which is on a par with lapped screens)

 A good video grade cable like URM70 has an impedance of 75R, a loss of less than 1.5dB per 10 meters at 100MHz (even lower at video frequencies) and an excellent braided screen, it is 6mm in diameter and is flexible.


TV grade coax is very suitable for video signals as it is a low loss 75R coax, but unless the runs are very long indeed the improvement over something like URM70 would not really be noticeable. TV coax (and the even better TV125 satellite cable) both have a solid center core which makes them stiff and prone to fracture if continually moved. TV125 has a loss of only 0.49 dB per 10 meter length up to 100MHz it is ideal for build in installations but for RGB a minimum of 3 runs would be required.


Audio low loss low capacitance cables are generally not best suited to video, particularly longer cable runs

The SVGA type of cables and the better scart cables have thin individually screened 75R coax built into the cable in the case of cables for RGB there will be either 3 or 5 of these. The new RGB cable from Keene Electronics (product code KBL28) is a case in point having 5 x 75R coax cables and four signal cables in one outer. The coax cable use a fully braided (woven as against lapped) screen and ad additional overall screen for excellent shielding