White streaks behind OSD text - Page 1
Forum section: electronics / projects

author: mackrc1
author_rank: Contributor
timestamp: December 05, 2015, 06:10:30 pm
content: Hi, different topic than usual but basically I have an OSD for my quadcopter. The data being presented on the video is legible but the image of it seems to taper across the length of the line that is on. Sorry its pretty hard to explain so Ive attached an image of someone else with the problem.Someone mentioned somewhere it has to do with an impedance mismatch between OSD and video transmitter? Looking to the analog video guys out there, what could cause something like this to happen? I really no nothing about analog video so I am clueless as to what might be causing it. So far I can confirm that the problem is not a result of the OSD (the device is behaving as expected).I have tried putting a 470uF cap in between the OSD and the video transmitter (no idea why, someone suggested it!) this did absolutely nothing.Would normally go to an RC forum for something like this but most of them with the same question are down/not loading grrr so I thought Id see if any engineers have seen a problem like this.Cheers

author: amyk
author_rank: Super Contributor
timestamp: December 05, 2015, 07:46:01 pm
content: Impedance mismatch would cause reflections and show up as "ghost images". Your symptoms are more the fact that the lines with OSD have a slightly higher signal level.

author: bktemp
author_rank: Super Contributor
timestamp: December 05, 2015, 08:18:34 pm
content: It looks like something is wrong with the dc level when the OSD is active. That's why somebody suggested the capacitor. But it is too large to be effective. And a smaller cap would distort the signal, therefore it does not help.If you have a scope, look at video signal. Maybe the OSD generator uses AC coupling and pulls the output to a lower value during the blanking interval after it has been active. Because the blanking interval is beeing used as dark reference value the active image appears brighter.

author: mackrc1
author_rank: Contributor
timestamp: February 09, 2016, 12:17:31 am
content: Quote from: amyk on December 05, 2015, 07:46:01 pmQuote from: bktemp on December 05, 2015, 08:18:34 pmHi again, Thanks for your replies, I've been really busy so haven't had a chance until now. I fired up the scope and looked at a video frame with a cap over the camera (black screen).I've attached two images one of the scope and one of the actual video received. The Scope shows the 6 lines very clearly (3 at the top and 3 at the bottom). I do not know enough about analog TV to interpret whats going on but if these images show the cause of the problem to anyone that would be a great help.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 09, 2016, 04:35:37 am
content: Seems like the OSD's white level is too high, skewing DC recovery.Is the OSD a separate chip or device? I wonder what would happen if you simply reduced its supply voltage slightly...Tim

author: mackrc1
author_rank: Contributor
timestamp: February 09, 2016, 12:04:02 pm
content: Quote from: T3sl4co1l on February 09, 2016, 04:35:37 amHaha if only it was so simple, unfortunately it seems the OSD has a fairly well engineered DC-DC converter which works right down to 5v or so, there is effectively no reduction in the white lines until it cuts out.I tried the same on the receiver, no effect.Thanks

author: mubes
author_rank: Regular Contributor
timestamp: February 10, 2016, 01:01:23 pm
content: Is it possible to put a series resistor into the OSD output before its mixed with the video? Do you have a circuit diagram available? Attenuating the whole of the OSD output will reduce the black level and have no effect on white if its being overdriven.Dave

author: mackrc1
author_rank: Contributor
timestamp: February 10, 2016, 02:35:55 pm
content: Quote from: mubes on February 10, 2016, 01:01:23 pmUnfortunately no circuit diagram, however I opened it up and it seems everything is being done on aPIC32.I connected the output straight to a television and behold no white lines, so the problem is definitely a mismatch between the transmitter and the OSD. I've ordered a new transmitter but I'm still curious as to what exactly is causing the problem on this particular transmitter.Thanks

author: cvanc
author_rank: Frequent Contributor
timestamp: February 10, 2016, 02:42:56 pm
content: Is it possible to simply reduce (with resistors) the signal level going into the transmitter?

author: mackrc1
author_rank: Contributor
timestamp: February 10, 2016, 04:41:56 pm
content: Quote from: cvanc on February 10, 2016, 02:42:56 pmJust tried that and it worked a treat, so made a little board with a 2K pot to allow me to adjust the signal level and not lose too much actual signal.What do you think is actually causing this? Transmitter got a bad reference voltage regulator or something?Thanks!

author: mubes
author_rank: Regular Contributor
timestamp: February 10, 2016, 04:59:26 pm
content: Your 'black' level is too high I suspect. It cant get whiter than white so you don't see symptoms in the text itself, but it will probablybe overdriven. Either due to a termination mismatch or a dodgy gain (eg wrong resistor or duff joint or something).Anyway, its fixed now....on to the next niggle.Dave

author: mackrc1
author_rank: Contributor
timestamp: February 10, 2016, 05:20:47 pm
content: Quote from: mubes on February 10, 2016, 04:59:26 pmI suspect the latter, it is cheap Chinese stuff after all. Thanks a bunch! Haha yeah it never ends...Cheers

author: mubes
author_rank: Regular Contributor
timestamp: February 10, 2016, 09:48:53 pm
content: Actually, didn't notice your flag. First thing to check would have been for water in the circuit. Hope you've dodged the weather in your neck of the woods.Have fun.DAVE