Losing colour from camera over a long run of cable

[DanSteely]

Hi All,

 

I am using a small ebay security type video camera for stage relay purposes and have lost colour when connected via 60m of BNC 75 ohm cable.

 

When it connected on the bench directly to the Sony pro CRT monitor there's no problem. I've measured the resistance of the cable and it comes out at 3 ohms for the core and 2.8 for the screen for the 60m. I also measured the capacitance which equates to 4320pF (4.32uF)

 

The camera has no controls and has standard 12V in via 2.1mm and V out on BNC.

 

It's not the end of the world if the relay is in B&W but I'd like to know the reason for the problem and if it's fixable.

 

Thanks and best regards.

 

Dan

[DrV]

I also measured the capacitance which equates to 4320pF (4.32uF)

First off, that's 4.32nF not 4.32uF.

 

Have you tried putting a 75ohm terminator at the monitor end? It may be that the colour burst is getting corrupted by reflections in the cable sufficiently that the monitor no longer recognises it.

Dave

[Dave m]

Some monitors have a 75 ohm switch to add a terminatorYou might also try a video line amp? Although easier to do when you have one to hand

[david.elsbury]

Yes this has happened to me- but it was a handycam with almost 100m of random cable including musiflex XLR cable and thin nasty screened audio cable.

 

( I was at school and didn’t know any better........)

[sandall]

Unlikely to be the cable, unless it's been damaged - as a test I've run similar lengths of mini RG59 versus mic cable. The picture was slightly softer through the mic cable, but with no great loss of colour.

[Owain]

Have you checked the power voltage in both locations, I've found ebay cameras to lose colour on low voltage (and also sometimes to have some strange voltage requirements)

 

Also if it's in low light location it may simply be switching to night / IR mode.

 

 

[gyro_gearloose]

You may need to power the camera locally as the voltage drop along 60m of cable might be too much. Some cameras I’ve installed at work mention needing to do this in their instructions.

[DanSteely]

Hi All,

 

I omitted to say that the camera is powered locally so is not suffering from 60m of voltage supply loss.

 

The terminator worked when I connected it via a BNC T connector.

 

Many thanks for all.

[richardash1981]

For the benefit of anyone reading this:

• The camera and monitor are composite video!
• Colour is transmitted separately from the luminance (black and white) part of the picture, but down the same cable.
• The transmission is done on a carrier wave ("colour carrier") which is at 3.8MHz
• 3.8MHz is higher in frequency than any part of the black and white picture
• So it you have enough high frequency loss, it looses the colour carrier first, whilst the B&W picture looks OK (ish)
• Because of the high frequency, colour carrier is also more prone to problems with termination, reflections and so on because the wavelength of the signals is shorter - about 20cm I think.
• Most professional type video monitors have high impedance inputs so that signals can be looped through them to another destination
• As a result, you need to fit a terminator (or use the switch if provided) at the end of the line to get the correct signal levels.

[DrV]

.[*]The transmission is done on a carrier wave ("colour carrier") which is at 3.8MHz[*]3.8MHz is higher in frequency than any part of the black and white picture

That's the NTSC frequency. PAL-I uses 4.43 MHz (4.43361875)

[KevinE]

Without splitting hairs even more, the colour carrier (tv talk: subcarrier) isn't sent seperately from the luminance in a composite signal as used by CCTV. It's superimposed onto it.

And this is different to the colour burst, which is another thing altogether.

 

What the monitor was probably doing was failing to lock to the colour burst and so bringing in the colour killer.

 

PAL is designed to maintain correct colour even in the face of poor signals, but if the reference oscillator in the monitor (or TV for that matter) can't phase lock to the colour burst, it will (should) revert to a black and white picture to prevent the wrong colours from appearing on the picture as a result of trying to make sense of a noisy subcarrier.

[Jivemaster]

Ultimately with composite video some kit has 75ohm termination and some has high impedance. High is useful if you need to put lots of loads on one line or even just a monitor and a recorder, but the composite feed works best when presented with a 75ohm load so several high impedance monitors don't load it enough til you add a 75ohm terminator which can be an appliance. My composite monitor has switchable termination resistor. For short runs this may not matter, but for tens of metres it certainly does.