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oxxyfx

Video signal Levels

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Hello,

I tested my receiver for video signal levels - as per description on the Rc-cam pages. With the white sheet in front of the camera I get around 1Vp-p signal - this is after I spent some time calibrating the transmitter.

Now, when I take out the white sheet and I let the camera see in color, the peak to peak signal will jump up to 1.35V.

Is this normal?

Thanks, Ox.

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Is this normal?

No. If I had to guess, the adjustment was not correctly done.

FWIW, I don't involve a piece of white paper. I use a flash-lite for the illumination source to help saturate the camera view. While I move it around in the camera view, I adjust the measured video level so that the max pk-pk I see out of the Rx is 1.0V when it is driving a 75 ohm terminator. I ignore the pk-pk's that are lower in amplitude (they are just lower levels of max brightness).

BTW, things like OSD's must not be enabled during the adjustment since some violate the white level limits. So if you have one, don't use it during the calibration.

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Thank you. There is no osd. I used a white sheet because the camera adjusts to the flashlight and the beam is not as wide to cover the whole screen. I illuminated the white paper from behind, so it is as bright white as it could be.

I will redo the adjustments to make sure I get the correct signal levels.

Thank you.

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I did some testing today, and it turns out that the video levels were as high as 1.35V because the 470uF condensers at the video output.

The shcematics is done like this:

Airwave Receiver Video Signal - grounded with a 75Ohn resistor. The video signal also enters the Max4417. The 4417's Vin+ are joined and the Vin- is grounded.

The outputs have a 75Ohm resistor in series and a 470uF capacitor in series. I replaced one ot the 470uF with a 100uF and the levels dropped to around 1Vp-p.

See this topic: http://www.rc-cam.com/forum/index.php?showtopic=2050

Also I checked the Max4417 datasheet today and it looks like they have a 24Ohm resistor from the output to the Vin- of each Opamp - which I didn't use.

Is this something I should use as well or just leave the Vin- grounded?

Thanks, Ox.

Edited by oxxyfx

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The outputs have a 75Ohm resistor in series and a 470uF capacitor in series. I replaced one ot the 470uF with a 100uF and the levels dropped to around 1Vp-p.

100uF is too small for good sync fidelity. I never go smaller than 330uF. 470uF is even better.

Also I checked the Max4417 datasheet today and it looks like they have a 24Ohm resistor from the output to the Vin- of each Opamp - which I didn't use.

You might have looked at the wrong data sheet. I believe you have the MAX4217, not the MAX4417.

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I do have 4217. I think I need some sleep, I can't see well tonigh... :)

So if with 330uF the video levels would be still too high, what could be done to bring it down to 1Vp-p?

The transmitter is already at the minimum possible value, one more micron rotation on that trimpot and it goes black.

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So if with 330uF the video levels would be still too high, what could be done to bring it down to 1Vp-p?

Add a 22 ohm resistor in series with the output of the Airwave module's video signal.

But, be very sure that the "high" amplitude is not caused by something else. It seems very odd to me that you have that problem.

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I think I know what the problem is. The 4217 has 2 opamps. Each of these has a gain of 2. In my circuit I link the inputs of these together, and I drive a separate video output from each of the outputs.

So technically speaking each of these outputs could drive 2 separate outputs.

Is this the problem with my circuit?

Terry - I tried to adjust the sould out of the transmitters, right now the transmitter's level is down so much that one more click and it goes black.

Thanks, Ox.

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I think I know what the problem is ...

I didn't see anything on the data sheet that would indicate that your implementation would be a problem. {But, if you think it is, then just disconnect one of the inputs and see what happens.}

It's time to make some basic measurements.

1. Connect a 75 ohm terminator on your U$6 video out jack. Install a monitor on the U$7 jack so that you can see the video to ensure it is ok. Turn on your video system.

2. Measure Max4217 Pin 3. What is the pk-pk AC level? What is the DC bias (offset) level?

3. Measure Max4217 pin 1. What is the pk-pk level? What is the DC bias level?

4. Measure the 75 ohm terminator on U$6. What is the pk-pk level? What is the DC bias level?

5. Measure the video signal at the U$7 jack. What is the pk-pk level? What is the DC bias level?

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I think I know what the problem is. The 4217 has 2 opamps. Each of these has a gain of 2. In my circuit I link the inputs of these together, and I drive a separate video output from each of the outputs.

That sounds normal to me. Video buffers typically have a gain of 2, in order to compensate the fact that once you will put the output 75Ohm series resistor you'll put the receiving device in the middle of a 1/2 voltage divider made by that resistor and the device's 75Ohm termination. The level is then OK again.

Haven't seen your schematic, do you have the output 75Ohm installed?

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Kilrah,

here, the last schematic on this thread shows how the max is installed, and what do I have after. There is a 75 ohm resistor and a 470uF capacitor in series.

http://www.rc-cam.com/forum/index.php?showtopic=2050

I will be doing the measurements tonight as proposed by Mr. Rc-cam, I will post the results later. For now I have to take my son to karate.

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Ok, here I am. I have not got yet to do anything on my receiver, because I decided to calibrate the tx first.

Here is what i did:

I set up a nice calibration station - see attached picture with white paper in front and also prepared a color paper with a nice color barcode called SMPTE color bars.

I also took one of my older Airwave receivers which is a very straight setup, vreg, receiver, dip switch, patch and connectors. Nothing else. This has not been modified in any way. I made sure that the video output has only a 470uF condenser on it - no 75 Ohm resistor in series - this is how the Airwave datasheet shows it.

I connected the output to the TV and made sure that the whole screen is covered in white.

post-6-1187832807_thumb.jpg

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Now, I connected the scope to the video output making sure that there is a 70 Ohm resistor from the output to the ground. As a matter of fact I took the following scope trace off the resistors leads. The original p-p value was around 0.7V and I rotated the trimpot inside the TX till I've got the value in the picture below. This I believe it is pretty close to the 1.0Vp-p value +/-5%.

post-6-1187833193_thumb.jpg

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Now, so nobody can say I am crazy - all I did is I changed the white paper to the color bars. Here is the capture from the scope after that:

post-6-1187833332_thumb.jpg

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As you can see, the p-p voltage now is 1.323V. This is still ok, I was expecting this. At this point I went back to the TX and tried to recalibrate it as close to 1Vp-p as I can. Here is the trace:

post-6-1187833541_thumb.jpg

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So again, I changed the paper, and I put the white sheet back. The value of the p-p voltage is down to 0.77 Vp-p.

Here is the picture from the scope:

So the question is, before I go any further from here and I start measuring my other receiver and the signals around the Max4217, please let me know which transmitter level would you want me to go further. The one where it is 1Vp-p for the white image or the one where 1Vp-p is on the color image.

post-6-1187833726_thumb.jpg

Edited by oxxyfx

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So the question is, before I go any further from here and I start measuring my other receiver and the signals around the Max4217, please let me know which transmitter level would you want me to go further. The one where it is 1Vp-p for the white image or the one where 1Vp-p is on the color image.

You need to use the one where you have fully saturated luminance. It looks like in your setup that is better achieved with the color image. So I vote for it.

As I have mentioned before, I use a flashlight that is directed into the camera. As an experiment, I tried your white illuminated paper procedure on a KX-131 camera. It does not work for me either. I cannot adequate illuminated the image to reliably force signal saturation. On the other hand, my flashlight method works perfect every time.

Below is a snapshot I took a few minutes ago. What I do is move the pocket flashlight around until I observe the maximum pk-pk signal level. I ignore any stray transients -- the clipped video region is the point of interest.

On an good analog scope with TV sync features, the results are easy to relate to. On entry level scopes, including digital types, it might not be so obvious. Besides other issues, some might not sync up, so you have to use a bit of oscope persistence to "see" the video signal silhouette. But the goal is the same -- adjust the video so that the highest observed whites are 1.0Vpk-pk when driving a 75 ohm load.

There is no need to nail this to any serious precision - close enough is good enough for our application. From my experience, if your are going to have a slight mis-adjustment, slightly high is usually better than slightly low. A reasonable value would be at least 1.0Vpk-pk but less than 1.1Vpk-pk. Aim for that range.

At this point I would be interested in the requested measurements. I don't really mind which way you illuminate the camera for them. The relative numbers will help tell me if your circuit's gains or offsets are odd.

post-6-1187839230_thumb.jpg

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Without cursors its hard to know hard numbers, but I would say that the raw output of the Rx is about 30% too high. That may be due to the camera, Tx, or Rx. High signal levels are a good problem to have (versus low levels) because that can easily be corrected with a simple attenuator on the video signal.

The observed MAX4217 gains and offsets are not out of line, which is good.

One last thing to check: I wonder if the signal level going into the Tx is high; Have you measured it?

BTW, what camera and A/V Tx are you using?

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The camera is a 12V ESC912CCD, here are the parameters:

Camera apparatus: 1/3,1/4 Picture sensor

Validity Pixel: CCIR: 500 X 580 EAI: 510 X 492

Sensitive area: 4.9mm X 3.7mm

Beaconage: CCIR/EIA

Horizontal Definition: 420 TV LINES

Lens: 3.7mm/F2.0

Min Illumination: F1.2 inch , 0.05Lux

S/N Ratio: >48db

Shutter speed: 1/50 or 1/60 - /100,000

Video output: 1.0Vp-p,75

Power supply: DC-12V

Power Consumption: 90mA

Size: 30mm X 30mm X 13mm

I haven't measured the signal coming out from the camera but I could do that. The TX is an Airwave 610 - 10mW transmitter.

btw, I ordered a kx131, I hope I'll get it in in a week, so I can test with a known camera.

Edited by oxxyfx

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