DC pulse carried over an analog signal

[pcleats]

Hi all,

Here is my challange. I have a camera that is both color and black and white. It has IR leds that come on automatically when it gets dark. The problem I'm having is that as it begins to get dark the leds will flicker on and off causing white spots in the viewed image. There is a small pot that can be adjusted, but its not practical and hard to access.

What I would like to do be able to control when the leds actually come on. Sounds simple right, but heres the catch I can't use any additional wires. I need to be able to carry a digital pulse over the video line to a small control circuit inside the camera.

Any ideas on the best way to do this?


Thanks for all the help.

Patrick

[stevech]

is it sensing light/dark through the lens or via a separate photo-sensor? If the latter, you could "shine" light on this sensor to affect when the LEDs come on.

[pcleats]

is it sensing light/dark through the lens or via a separate photo-sensor? If the latter, you could "shine" light on this sensor to affect when the LEDs come on.

It does have a photocell which I will disconnect. I have to be able to control when the leds come on. The cameras could be located in remote/hard to reach places.

Thanks

Patrick

[GTBecker]

I can't use any additional wires. I need to be able to carry a digital pulse over the video line...

My first suggestion is to find a camera that does what you need.

If you must use the video line to control the lighting, you can do it in the vertical blanking period of the video signal, when no information is normally conveyed. That might not be trivial, but it can be done. We used that technique years ago to carry keyboard data from a video terminal.


Tom

[pcleats]

My first suggestion is to find a camera that does what you need

I have done much research, but by no means exhaustive. I have not located a camera that will do exactly what I need. The application is unique and the cameras MUST be both color in the day time and black and white at night. The IR leds in the cameras are always replaced with invisible IR leds in the 910-950 nM range. The ability to turn the cameras on remotely will make them more useful.

If you must use the video line to control the lighting, you can do it in the vertical blanking period of the video signal, when no information is normally conveyed. That might not be trivial, but it can be done. We used that technique years ago to carry keyboard data from a video terminal.

If you have done it in the past how about shedding a bit of light on the technique, so I can implement it in my project!


Thanks

Patrick

[GTBecker]

> ... a bit of light on the technique...

Well, it was 30 years ago, but...

You must make the video line bidirectional, and it must be able to turn
around quickly. The line is usually a terminated 75-ohm unbalanced coax
that carries megahertz-bandwidth analog signals, so you'll need a
compatible driver/receiver switch at each end. The switches must be
independently well-timed by extracting the vertical sync pulse from the
camera's video signal (video separation chips should make this easy
today). At the appropriate time in each frame, the switches will allow
a brief glimpse of the line state during the Vertical Blanking Interval
from the camera end, and allow a pulse or pulse group, injected onto the
line at the control end, to be detected.

There are other uses for the VBI, too. Perhaps most common is a VITC
time code, but bidirectional audio and camera control commands can be
placed there, as well. Devices exist that insert VITC onto a video
cable. I did some Googling ("bidirectional video vertical interval
period") and found a Panasonic pan-tilt-zoom camera head that uses this
mechanism to get position commands to it, and many patent mentions.
Perhaps you can find some ideas from those applications.


Tom

[twesthoff]

Is the camera powered over the coax, or by separate wire?


-----Original Message-----
From: ZX-24 [mailto:zx24.forum@zbasic.net]
Sent: Tue 10/10/2006 12:48 PM
To: zx24.forum@zbasic.net
Cc:
Subject: RE: DC pulse carried over an analog signal

... a bit of light on the technique...

Well, it was 30 years ago, but...

You must make the video line bidirectional, and it must be able to turn
around quickly. The line is usually a terminated 75-ohm unbalanced coax
that carries megahertz-bandwidth analog signals, so you'll need a
compatible driver/receiver switch at each end. The switches must be
independently well-timed by extracting the vertical sync pulse from the
camera's video signal (video separation chips should make this easy
today). At the appropriate time in each frame, the switches will allow
a brief glimpse of the line state during the Vertical Blanking Interval
from the camera end, and allow a pulse or pulse group, injected onto the
line at the control end, to be detected.

There are other uses for the VBI, too. Perhaps most common is a VITC
time code, but bidirectional audio and camera control commands can be
placed there, as well. Devices exist that insert VITC onto a video
cable. I did some Googling ("bidirectional video vertical interval
period") and found a Panasonic pan-tilt-zoom camera head that uses this
mechanism to get position commands to it, and many patent mentions.
Perhaps you can find some ideas from those applications.


Tom

[serdahl]

Hi all,

Here is my challange. I have a camera that is both color and black and white. It has IR leds that come on automatically when it gets dark. The problem I'm having is that as it begins to get dark the leds will flicker on and off causing white spots in the viewed image. There is a small pot that can be adjusted, but its not practical and hard to access.

What I would like to do be able to control when the leds actually come on. Sounds simple right, but heres the catch I can't use any additional wires. I need to be able to carry a digital pulse over the video line to a small control circuit inside the camera.

Any ideas on the best way to do this?


Thanks for all the help.

Patrick

It sounds like the original problem is the "flickering" IR Leds. The ambient light sensor (photocell) circuit and its threshold setting (small pot) and its hysteresis are not functioning properly. Fixing the performance of the light sensor circuitry controlling the IR Leds would be the most direct. This may take some additional circuitry, but it solves the problem at its source.

My experience with Panasonic cameras has been that the cameras do a good job of switching from color to BW (and back again) based on the "color content" and the "light level". In my application, the IR Led light source (and its 12 VDC power souce) was separate from the camera and it had its own "ambient light level detector". By putting a small piece of "milky" plastic (as diffuser) over the photocell and adjusting the threshold, I was able to have the IR Leds come on about 15 minutes before the camera switched to BW and go off about 30 minutes after the camera went back to color.

I would need the camera model number and your power wiring diagram to be of further help.

Best Regrds, Eric

[pcleats]

Thanks serdahl,

I will try your idea.

All of the circuits I have looked at have been a bit on the complicated side. I would still like to check into how I could control them remotely so I could do it if I needed to.

Patrick