04 - Cables, Bandwidth
Now is a good time to bring up the controversial topic of cables. There are a lot of people making a lot of money selling cables. They are the lowest tech part of the chain, and can be the subject of much over-marketing. For custom integration, the honest to goodness best thing you can do is make your own cables for room to room connections, and either use pre-made cables for component to component connections, or optionally custom make those as well. Custom made cables has some major advantages with your workflow, and usually results in a higher quality cable than what can be bought in the stores. RG59 can be used for most analog video applications since it has plenty of bandwidth, and is generally flexible enough to make interconnects. Crimp on RCA ends, colored boots, as well as labels describing to and from make for a professional looking installation.
The total distance you can go with analog video will depend on what bandwidth you need, and what type of cable you are using. Your goal should be to get the video signal there with no more than 3 dB of loss. Too much loss and you will start to see a softening of the picture.
For example, Belden 1505F is a high grade RG59 type cable that is flexible enough to make good interconnects. At 10MHz, it has 0.9dB of loss per 100 feet. So 300 feet should have about 2.7dB of loss, which will result in a fine picture for composite or 480i YPbPr.
In an example of HD video, the same cable at 75MHz has about 2.1dB loss per 100 feet. So at about 150 feet, there is slightly more than 3dB of loss. If the run is any longer, it might be wise to choose a better cable stock. Belden 1694A has a 75MHz loss of 1.6dB per 100 feet, so should be good for 200 foot runs.
Another reality factor is that most HD signals found in the home will not have the theoretical max bandwidth, but in fact much lower. The cable and satellite companies will probably be compressing the source signal enough to where an extra dB of attenuation at 75MHz may never be noticed.
Sheilding might be an important issue, depending on the installation. Poor shielding in a overly noisy environment could result in interference lines on the picture. It’s impossible to say how much shielding is needed unless you do a full site survey including viewing the RF spectrum on a spectrum analyzer. However RG59 and RG6 are designed to carry much higher frequency signals than HD video, so they are usually fine, even the types with single braid shields. The cheaper quality AV interconnects might skimp on the shielding to make them more flexible or just cheaper in cost, so longer runs of premade cables might be suspect.
Failures of YPbPr can look a couple different ways depending on the failure. If there is total failure of the Y cable, there will be no video. It contains all the sync pulses, and has to be there. Failure of the Pb or Pr lines will look like coloration problems, like someone cranked up the purple or green. Cables that have had a screw driven through the Y line or have been severely kinked may also look like the picture is too dim, lose sync often, or have the video softened and not look “HD”.