a couple of composite video questions

[tehtehteh]

hello

I have a setup I am trying to implement in my car, I have some queries about composite video

this is the setup as it is at the moment



I know a composite video cable should have a ground, but does this not make a ground loop in this setup?

for ease of installation I would like to repurpose an unused wire that is already in the car running between the screen and headunit, this would be the result, is this making sense or am I crazy?



my second question: the wire that I would like to repurpose is standard car 12v wiring, unshielded, would this make it unsuitable for the composite video signal?

 

[dorke]

1. No, it doesn't create a ground loop.
2.You can not use a single wire like the yellow one in your 2nd drawing.
The composite video needs a 75 ohm coaxial connection.
Using it in any other way will degrade the signal.
Absolutely not recommended!

 

[tehtehteh]

it's not clear in the diagram, but the majority of the cable is before the splitter, and it's not coax, so I figured with this and the fact that the ground is going into the same places anyway it makes both options essentially the same?

would it be possible for you to explain further why I am wrong? I still can't see the real difference between the 2 setups

 

[dorke]

The minimum needed is a signal+Gnd return wire.
It is essentially a transmission line created of both wires-will not work properly with just one.
Isn't there a coax terminal at the "boxes" ?

 

[tehtehteh]

the connections are RCA, a check with the multimeter tells me the outsides are indeed grounded, but if you see that the ground is split off like it is does that not mean I can remove that wire and essentially keep the same ground?

I undertsand the need for the ground in normal use, but if I know this will be a permanent connection with both input and output sharing the same ground it makes sense to me

I'm not saying your advice is wrong, I'm just saying I don't understand why I am wrong and would like to understand so I can learn a little bit

 

[AnalogKid]

Your second sketch has everything connected to the same reference potential, and at DC it might work for many applications (like door locks and dome lights). But video is not DC, and while the signal will get to the screen, it will pick up noise from the car's electrical system and will have serious frequency response and time-domain reflection problems. Look at the length and complexity of the video path to the screen, and compare that to the path for the return current back to the source. Conceptually (note that this is a grossly oversimplified description), think that every electron that leaves the source has to get back to it, and any differences between the two signal paths shows up on screen. Sharing the video return current with other currents in the same wire means a portion of those other currents modulate the video return current, which absolutely shows up on screen.

Separate from that is the quality of the signal path from the source to the screen. At megahertz frequencies, coax cable does much more than just shield the signal. This gets into the true nature of how a signal propagates down a wire, not a simple discussion.

ak

 

[tehtehteh]

thank you that makes sense to me

 

[dorke]

Beyond the issue of added noise that AK mentioned,

there is the issue of the video signal propagation (which has a bandwidth of about 5.5Mhz for PAL) through the transmission line(i.e coax).
The coax should have a 75 ohm impedance,otherwise there will be reflations which will totally distort the signal at the receiver side.
You can start reading about it here.

 

[tehtehteh]

it's a bit of a mystery to me how it's working at the moment then, because I'd say 3/4 of the length of the cable is just standard wiring, similar to a usb cable

 

[AnalogKid]

Because 80 - 90% of the video energy is not at the higher frequencies where transmission line effects dominate.

ak

 

[tehtehteh]

ok then I can probably get away with it then, I have 4 wires to spare, I can use 2 for ground return and 2 for signal

 

[tehtehteh]

hello

just a quick question to the composite video experts, they were talking about 75ohm coax is needed for the composite video signal, does this also apply to the impedance of any pcb trace I have carrying that signal?

 

[AnalogKid]

It should. The layout techniques that create a constant impedance signal path are stripline and micro-stripline.

ak

 

[tehtehteh]

thank you, I have looked these up and it seems like it will be micro for me as it's only a 2 layer board

this will be for several signals feeding into a bus, as it's a 2 layer board I will not be able to have a ground plane below the bus at the areas where the signals join, do you think in practice this will matter? there will be stereo audio also

 

[tehtehteh]

hello I am back again with another question, I am just wondering if I am passing these signals through a relay is there anything I need to take into account?