DIY video cables

[paulspencer]

I'm planning on getting a projector, and so I'd like to make some component video cables, but it would seem it's not quite as straightforward as audio interconnects.

How do I test that the impedance is correct? I understand they need to be 70 ohms at a certain frequency range, but will a quick test with a multimeter do the trick?

I've also read that most cables sold as video cables are not in fact 70 ohms, even many that claim to be. How important is this?

Has anyone compared video quality using audio vs video cables?

Does is make any sense adding a resistor to the cable to get the correct impedance?

[I_Forgot]

Cable impedance is a function of the geometry of the cable. Typical video cable is coaxial. It has an inner conductor covered by a dielectric, covered by a braided or foil shield. The impedance is determined by the diameter of the inner conductor and the thickness of the dielectric layer that covers the inner conductor.

You can't measure impedance with a DMM because it is a high frequency property. You also can't just add a resistor to get the right impedance.

75 Ohms is the standard impedance for video. You need to buy 75 Ohm cable. About the only thing you can do yourself is mount the connectors on the ends of whichever 75 Ohm coax you select.

The connectors also have an impedance which should be the same as the cable. Be sure to get 75 Ohm connectors. RCA plugs are not really a controlled impedance connector, so they really aren't good for high quality video. BNCs are available at 75 Ohms. Of course, if your equipment has RCA jacks, you need RCA plugs.

Each place where the impedance deviates from 75 Ohms will cause a reflection of the signal which will tend to reduce the quality of the image you see on your screen. Kinks in the cable, bad connectors, etc. all contribute to signal degradation.

You probably can't buy connectors and cable cheaper than you can get the already assembled cables, so there isn't much point in assembling them yourself unless you need some weird connectors or can't find a ready made cable that's the right length.

I_F

[paulspencer]

Thanks Mark.

It's unfortunate that RCA mess it up since I have no choice - the connections on DVD player and projector are both RCA. Still I'll get some coax 75 ohm cable, and there's nothing I can do about the ends.

I don't know about the US but it's much cheaper to make my own cables here. In fact the cost of a set of component video cables here can be more than I spent on cables for my entire system!

I found a good pair at Jaycar for $70 and it is only 1.8m. They also have cheaper 10m component cables for $50. I won't mention those cables you find from more exclusive hifi stores!

I can make my own for $9 for terminators and $12.50 for a 25m roll of coax cable. It's not a huge saving, but the cables will be consistent with the rest of the system, and the result will be a lot neater as I'll make the cables just the right length. I'll probably install wall plates, no cable to roll up, no wasted length.

I found a source of cable/plugs has very good prices, so if I do change at some stage, I won't be worried that much. FYI it is:
http://www.psraudio.com.au

[rabbitz]

Paul

Have a look at the Belden 1505A and is quite cheap.

http://www.diyaudiostore.com/digital/digi.htm

I think Jaycar have some bulk Belden video cables.

[ash_dac]

Hi,

I made some s-vhs cables a while back..

Shame about the RCA things. I just got hold of some of professional 75ohm cable from http://www.vdctrading.com/


RCA connector you should try dealing with that scart / euro AV connector!

I built my own as I tried three different manufacturers cables, and they all had really bad interfernce on them.


Kind regards,

Ashley.