Length of individual RG59 runs for component connections (1 Viewer)

[Richard E Jones]

Some time ago I wired my home with RG59 for component video using RG59 as recommended by my cablers. I have selected a projector (the Sanyo Z3) and obviously would like to exploit the HD capabilities of it. The cable runs are

 

[Chu Gai]

1 &2) Yes, but as to whether there will be potential video degradation depends upon the disparity in lengths. Broadcast video standards (if I recall correctly) state the timing differences should not exceed 40 nanoseconds/100 feet. What they're getting at here is that in the manufacturing of coax, there's always going to be some variations in the properties of the cable. So if we were to cut 100 foot segments off a large spool and then measure the arrival time of an electrical signal through each, the maximum differences in arrival time should be 40 nanoseconds or less. In your case, let's assume that each coax is identical with respect to electrical properties with the only difference being length. If we calculate what length difference corresponds to 40 nanoseconds and assume a velocity of propogation of 0.6, we come up with 2.4 feet. So, Richard, if that's your length difference, I'd say knock yourself out. If not, just buy a 100 foot spool from someone (copper conductors, not copper over steel) and use that. Shouldn't cost you too much.

3) Not sure what you mean by composite. Some of the cable vendors I've spoken with such as AVCable have said they've observed no problems using RG59 component cables for lengths of 100 feet. Now you'll get a bit less signal loss with RG6 but you'll also find that it's a lot more inflexible especially if you go with a single center conductor as opposed to a stranded one.

4) Depends on you. No reason you can't used a wall plate. The insertion loss is trivial. Just make good, solid connections.

5) The engineering rule of thumb here is that the device needs 3x the bandwidth to avoid issues. 1080 is about 35 MHz so your receiver should have a bandwidth of about 100 MHz. I leave it to you to dig up the specs.

 

[Richard E Jones]

Thanks Chu Gai for your helpful response

Do you mean that if the length difference is > 2.4 feet I should expect some PQ reduction or it just won't work at all? I guess what I am really curious about is whether I will actually be able to display a HD signal (720p/1080i)over these 3 runs of RG59. If I can, but PQ suffers, what can I expect to see - colour loss, ghosting, contrast loss?

My question on composite (CVBS I think it is sometimes known as - the single RCA connector) is related to the above. Will the length issue potentially mean my PQ via component is worse than a composite signal over RG6?

 

[Chu Gai]

I'm making a fair amount of assumptions here Richard, mostly that the 3 cables you have are electrically identical with only length being the difference. You want the signals to arrive at virtually the same time so that they can be processed at the same time. When they're not, there will be some loss in resolution but honestly I don't know how signficant that word some is and whether it can only be seen on test patterns and not during actual playback. I'm of the general opinion that if I were buying 10 meter component cables, I'd be pissed off if one was a foot or two longer than the other. Hence, I'd try and keep them within a couple of inches. Since you've already got them in place, it's not going to hurt to try it, right?
If I've got a choice between composite and component, I'd choose the latter.

 

[John S]

I think you will be fine. Component video, will be much better than composite video.

 

[Leo Kerr]

The broadcasting house requirements may be considerably tighter than required by the end user, as they will be running it longer and/or through more equipment than you will, and thus may develop a greater timing difference.

Another, if not entirely accurate, way to determine how much distance difference is acceptable is thus:

1920*1080*30fps has a dot clock of about 62million.
To shift a pixel off by one requires a timing offset of 1/62,000,000 second. Signal travels through the wire at (close enough for our purposes,) the speed of light, which is what, 300,000,000m/s? Close enough (note there is a lot of rounding error going on here.)

Thus, to introduce a 1-pixel offset in a 1920x1080 image, you need about a 5 meter offset in cable length.

Or, don't panic.

Leo

 

[Bob McElfresh]

A un-broken path is always preferable. If you do go with a wall-plate - use BNC or CATV "F" style jacks. The RCA plug is rather poor at maintaining the 75 ohm path that video signals want.

 

[Richard E Jones]

Guys, thank you all so much for your input. I am now reassured!