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ryanm
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Joined: 25/09/2015
Location: Australia
Posts: 198

Posted: 08:14am 25 Jul 2024

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Hi All,

I'm hoping with all the people doing low level TV signal output on micros some one might be a bit of a guru with analog video signals.

I'm repairing a borehole inspection camera on a (I believe 250m) cable. Unit was originally working on an old cathode ray TV with a built in VHS recorder. My first job was to move to a digital recording system and I wasn't getting any colour with the PAL recorder. After a bit of googling I thought it might be a SECAM signal and inserting a PAL/NTSC/SECAM converter off ebay inline fixes the colour issue, with some loss of quality due to a cheap unit.

Two months later and the camera fails. I've got a new PAL camera designed for a drone. I plug it straight into my recorder and I'm getting a good colour image. When I run it through the longer cable it is colour for about a second when the camera turns on then goes to black and white again. Swapping the camera over to NTSC (which the recorder supports) still gives a black and white picture.

Does any one have an idea of what is going on here?

phil99

Guru

Joined: 11/02/2018
Location: Australia
Posts: 1979

Posted: 08:45am 25 Jul 2024

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250m is a long cable for a camera to drive directly.
High frequency attenuation may be affecting the two chroma signals which for PAL are quadrature modulated on a 4.433MHz subcarrier. Or weaken the 4.433MHz colour burst signal that provides a frequency and phase reference for colour decoding.

A video amp at the camera end, with provision for boosting the higher frequencies a little may help.

Edit
If the cable is accessible check for any kinks in it.
A kink would cause a localised change in its signal impedance. If bad enough signal reflections from that might mess up the colour burst, preventing decoding.
Edited 2024-07-25 22:11 by phil99

ryanm
Senior Member

Joined: 25/09/2015
Location: Australia
Posts: 198

Posted: 09:50pm 25 Jul 2024

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It's annoying that the colour signal comes in for a second then dissapears, so I know the information is getting through the cable.

My guess is that the colour signal is falling just out of tolerence of the digital recorder's firmware so it's dropping it, while the old CRT TV probably has an anlalog circuit on the input which is passing through the signal at degraded quality.

Cable is in really good nick. It's got a fairly significant stainless braid under the insulation.

I'm ording a THS7316 to see how that goes.

phil99

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Joined: 11/02/2018
Location: Australia
Posts: 1979

Posted: 10:38pm 25 Jul 2024

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Looking at the datasheet for the THS7316 it should be ok.
In Fig. 1, to compensate for cable loss experiment with lower series output resistors. Reduce the 75Ω to 68Ω or 56Ω.

Or perhaps try a series R-C across the 75Ω to boost just the higher frequencies.
150Ω in series with 100pF to start with then try values up to 470pF.

ryanm
Senior Member

Joined: 25/09/2015
Location: Australia
Posts: 198

Posted: 11:57pm 25 Jul 2024

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Thanks Phil,

Will give it a go. At least I can understand your second post properly. Honestly this is me reading the first one.

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