[MOVED] Composite video switching

[kristaps_r]

Hi.

I am building circuit that will switch video input between 2 cameras (V1, V2). I have made prototype on my breadboard and I use FSA3157P6X analog switch.

I am having problems with interference between both video inputs. On TV that is connected to Vout I see video from selected camera overlayed with dark frame from other video source.

Can anyone help me solving this problem. Maybe I need better switching IC. I am looking for cheap solution with 2 inputs and small footprint.

Circuit in attachment.

Ragards,
Kristaps

 

[keith1200rs]

Re: Composite video switching

Try a dual switch so you can terminate the unused camera i.e. so it isn't floating.

Keith.

 

[kristaps_r]

Re: Composite video switching

Thanks. I will try adding 50Oms to GND to unused camera pin on switch to see if it helps.

I am also waiting for other suggestions maybe someone has done similar things before and maybe there is better IC to use. I really can not find any good circuit.

Kristaps

 

[keith1200rs]

Video is normally 75 ohms.

Keith.

---------- Post added at 15:23 ---------- Previous post was at 15:07 ----------

If your video is 50 ohms, have a look at Analogue Devices RF switches - they do quite a lot with built in termination for the unused input.

Keith

 

[betwixt]

Adding to what Keith stated, remember that normal 1v p-p video will double when the termination is removed. Your diagram shows 5V supply which is fine but if you run those devices at the lower end of their supply range the input signal may well exceed the supply rail and cause similar problems.

Brian.

 

[kristaps_r]

Didn't had much time. I tried 68ohm resistor on floating video source (had no 75ohm) and not directly on switch input (some 3cm off). Did not helped. I don't know if exactly 75ohm and proximity to switch is so important.
When I connected 68ohm to GND for selected video then was better but video become dimmer.
Maybe I will order some other switches that are named exactly video not analog switches. Also read somewhere that less ON state resistance could help.
What do you say about solution that I draw in diagram attached?

 

[keith1200rs]

Do you know if your video is capacitively coupled? If so, that could be the cause of your problem.

Video inputs should already have 75 ohm termination so by adding another resistor you will reduce the signal.

Have you an oscilloscope to look at the signals?


Keith

 

[kristaps_r]

Video sources are unknown factor in this situation. I am using Samsung LCD TV as display one camera is Sony Handicam and one camera is Chinese noname. For end users who will use my circuit it will be without cameras and display and users in lot of cases will connect cheap chinese equipment like:
https://www.hobbyking.com/hobbyking/store/__15295__5_8g_200mw_FPV_Wireless_AV_Tx_Rx_Set.html
https://www.securitycamera2000.com/...WDR-Color-Board-Camera-with-OSD-Menu-DNR.html
...

I can get osciloscope but it is no constantly at home where I am working on this.

It would be good to have solution that works good for all kind of devices even if not properly designed.
What about circuit I proposed? Will it reduce cross talk?

Kristaps

 

[keith1200rs]

Without knowing the cause of your problem you cannot easily say whether an alternative circuit would also have the same problem. If you need it to work with unknown sources then you need to assume that the source could be AC coupled in which case you need a circuit that will work with AC and DC coupled camera outputs. I am guessing that in your circuit the video is turning on the protection diodes in the analogue switch and causing the problem. So, you need a relay, physical switch, or active video switch. You can buy active video switches or make your own circuit with an opamp and analogue switch. To make your own active video switch you would AC couple each input to a fixed bias point (e.g. mid rail) and then witch through the analogue switch, then amplify the signal by 2. You would need to include correct (75 ohm) termination for each input and include a series 75 ohm resistor on the output.

Keith.

 

[betwixt]

Take a look at this web site: www.batc.org.uk In the magazine archive you will find many examples of video amplifiers and switches. BATC is an organization of television and electronics enthusiasts, some professional, some hobbyists. The archive is free to view (over 230 magazines and other publications) but does not contain the last few copies, you have to join to get those by mail or download. The joining fees are very small.

In general though, on almost all commercial and domestic video equipment outputs are capacitor coupled with a 75 Ohm series resistor to match the impedance of normal 75 Ohm co-axial cable. At the switch end, you would normally connect a 75 Ohm resistor to ground at or as close as possible to the end of the cable. This not only terminates the cable correctly to minimize reflections 'ghosting' it ground references the signal again. The video level from the source will normally be 2V peak-to-peak so the series and terminating resistors (ignoring cable resistance which will be very small) form a 2:1 potential divider giving you the 1V standard level again.

Brian.

 

[kristaps_r]

Thank you very much. I will try your advices and hopefully will solve this issue.

 

[kristaps_r]

I have ended using special video multiplexer with built in amplifier BH76330FV.