Little problem, out of sync i think.

[shaped]

Before i ask for help, i would like to say thanks for the schematic and eagle files for the video overlay. Its perfect for my needs.

I have made a homebrew board for the video overlay, it is working to some degree. i get the data on the screen but its in the incorrect place, it is only showing half the data on the screen. I cannot seem to get it to go center screen and be stable.

Saying that i can stable the overlay and center it if i test the resistance of the 680k (R1) resistor with the multimeter while it is in circuit. For some reason doing this test centers the overlay.

I have been over and over the board to see if i have gone wrong somewhere and everything seems to be in the right place. The only thing different on my board is the capacitors, nootropics caps are dipped ceramics i think, the caps i am using are standard 100v ceramic disks. (0.1uf)(104 europeon markings), i dont have any of the dipped type to try.
By the way R4 does not seem to doing anything, tested it and the reistance is there, maybe it a design fault with my PCB.

Anyone got any ideas?

THanks
Anthony.

[Michael]

What is the resistance you measure when you measure the 680K resistor (with no power applied, circuit not active)?

Our kit uses a 680K resistor with 1% precision because it needs to be very close to 680K to work. A 5% precision resistor can be WAY off and the circuit does not work.

Did you adjust the 100K potentiometer? If you did not include one in your circuit, now you know why. It is on the Video Experimenter to finely adjust the 680K resistance in the circuit.

[shaped]

Hello Michael
The 100k potentiometer is in circuit, and does not seem to affect anything on the overlay.

The 680k is a 5% and i think it was measuring about 674k in circuit. I will double check this and exhange it if needed.

I will keep playing the the resistance at the 680k point, As i said in my first post when i check resistance on the 680k while in circuit it centers the overlay and works perfect, (Most of the time). So i think you are right, the problem is going to be that resistor.

I should just buy one of yours, but i allways do it this way. I spend 5 times as much trying to re-produce something and after i fail 5 times i then end up buying a orginal.

Thanks for replying.

[shaped]

Just tested it while powered down, i am getting a 1000k reading with the potentiometer all the way over to the right and 1130 k reading with the potentiometer all the way over to the left. This is tested with the 680k resistor in circuit with no Uno board connected.

I have also tested the 680k out of circuit and i am getting a reading of 680k to 679k so i think that resistor is ok.

Just to add, i have done my own PCB for it, the design is more or less identical to yours except i have linked the +5 with tracks not using the whole top copper like your design does. I wanted to use your eagle files but i having problems printing it as the tracks and ground plane are to close together.
Do you think this could be a problem?

[shaped]

I cannot sync it correctly no matter what i try. Maybe its my TV allthough i have tried 2 tv’s both are modern flat screens. I have tried 2 cameras.

It is a timing issue, i tried a 1M pot from Rset to ground and adjusted it and managed to get the overlay center screen but it was still a little jittery.

I now have 8 spare lm1881’s .. on to the next project.

[shaped]

Update.

It looks like my wireless mouse in the workshop was the problem. I have a computer wireless mouse on my worktop where i build everything, it turns out that this was causing interference to the Uno or the Lm1881, not sure which yet, while it is on i was getting jumpiness on the overlay and it got worse as i moved the mouse.

Just in case anyone else has a problem with sync, it might be worth a look at your wireless kit just to rule it out.

[Michael]

Thanks for that update. I think the LM1881 chip is sensitive to RF noise, and you mouse must have bothered it. Interesting!

[rickl]

I purchased a Video Experimenter CCA to overlay simple alphanumerics on a camera output. The software for my project is similar to the overlay demo project software. The alphanumeric display worked great, but I had a problem with a line of extraneous stuff on the bottom of the display frame. After hours of trying to debug the software, I found the root cause to be the Arduino Uno serial interface that I was using as a debug tool. Apparently the serial interface was interfering with the LM1881 chip. After I commented out the serial interface code, the garbage on the display disappeared.

[Michael]

Yeah, actually you can’t use the Arduino serial library with TVout (or any other library that uses interrupts). The TVout library comes with a polling version of serial communication “pollserial” that does not use interrupts, so you CAN use that while using TVout. See the TVout examples of pollserial.

[Author]

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