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Mr.RC-Cam

Using the $50 Digital Scope to Measure Video Levels

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I've mentioned in the past that the video levels on some wireless systems are often poorly calibrated. Even systems from our favorite makers. Although a typical monitor or recorder can mask the problem, we just cannot achieve the best video performance if the levels are incorrect. So if you want the best dynamic range and colors from your video, don't rely on what was shipped to you. Get out the o-scope and check it!

I published some tips about video level adjustments a few years ago, as seen here: Lawmate Video Tricks and Tips.

The problem with this simple task is that not many hobbyists have access to an o-scope. The PC sound card based solutions just don't have the performance to do the job either. But, the $50 digital storage scope produced by Jyetech does a good job when it comes to measuring video levels. This new test tool is now distributed by dpcav.com. That's where I work and is how I got my hands on one.

Truth be told, I was not expecting this scope to work well with video signals. Although it does not have TV-Triggering features, I discovered that it's trigger functions do just fine. Honestly, I was very excited about that. Below is a photo of the scope I am talking about.

To get you on the right path, continue reading to learn some tips.

post-2-1223407793_thumb.jpg

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Let's start off with looking at the age old graybar pattern from a common video test generator. This is what it looks like to our eyeballs.

post-2-1223409501_thumb.jpg

Now let's see what the Jaytech scope show us. In the setup I have used alligator leads to parallel a 75 ohm resistor (the required terminator for video) at the input of the scope. The scope is setup for DC coupling, 0.2V scaling, and 10uS timebase. The scope's three slide switches are as follows:

SW2 = X2

SW3 = 0.1V

SW1 = DC

The trigger level has been set to midpoint of the horizontal sync. Trigger mode is Norm and polarity is Rising Edge. I used the H.Pos feature to horizontally move the waveform over so that one full video line was seen. The V.Pos feature was used to move it vertically so that the bottom of the sync was just touching the bottom of the screen.

Here is what the annotations on the photo are telling us:

S = Sync region. Trigger is set to mid point of this.

BLK = This is the Black region of the graybar.

WHT = This is the White region of the graybar.

63uS = Sixty-three microseconds is one horizontal video line, sync to sync.

post-2-1223409330_thumb.jpg

Now let's measure the video levels. Keep in mind that each vertical graticule is 200mV. Here is what the annotations are telling us:

V = Video level, 1.1V at full white area (spec = 1.0Vpkpk).

S = Sync level, 320mV (NTSC Spec = 285mV, PAL Spec = 300mV).

post-2-1223410352_thumb.jpg

In this case the video level is slightly high. A bit too high is usually much better than a bit too low. But, tweaking it for 1.0V is a fine thing to do to. By the way, you won't be able to individually set the sync level. But, if you see a sync level that is grossly wrong then that may cause problems in some applications. So, I always check it for sanity and if it is bad I do my best to find out why and solve it.

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Ok, so let's take what we learned so far, but adapt it so that you can use your existing video camera as the video test signal. This is a bit tricky, but with patience and practice it works out fine.

The main problem with using your video camera for the test signal is that cameras have an AGC (auto-gain-control) feature. The AGC prevents bright video from saturating the image. However, a saturated white image is exactly what we need. That is, we want to use a bright light source to create a full white image. However, the camera is going to fight us on this.

There are a couple solutions. If you have one of those fancy OSD cameras, then you can turn off the AGC. But, most of us aren't so lucky. As a workaround, I found that I can trick most cameras. What I do is slightly turn down the overhead room lights and use a flashlight (torch to you English blokes). I start with the flashlight pointed away from the camera. When I'm ready, I quickly aim it directly at the camera lens. For a short moment I can see the top part of the video go to its maximum level and sort of flat line there. That is the saturated video level, which represents full white. Within a moment the AGC will catch up and reduce the level, so you have to look fast. Using this cat and mouse game I can easily identify the peak white level.

The video level we need to measure is from the bottom of the sync to the top of the saturated white. Because this little 0-scope is a digital storage scope, you can press the scope's Hold button to freeze a waveform of interest. It takes practice (and luck) to catch the one you want, but is not impossible to do. Below is an example of a saturated waveform that I was able to catch with the digital storage Hold feature. This captured video level shows that mine is a tiny bit too high. But that's the way I like it. :)

Success with using this method will depend on your camera. Besides the issues with the AGC, it requires that your camera adhere to the video standards. If you have an R/C telemetry OSD then you must disable it while you measure/adjust the video. Do not allow the OSD telemetry text to appear during your measurements.

post-2-1223413586_thumb.jpg

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Nice. Great little tutorial. Please keep 'em up as you have time either here or on your website. This old TV repairmen & fledgling FPVer appreciates you reminding me about the things I've forgotten since Tesla was born.

My scope is in the mail. Should have it in two or three days.

Might I make a suggestion?

Another forum dedicated to tips like this with a topic for each tip/tutorial. No general chatter allowed. It would, I think, make it easier to find specific information without having to read through many forums or threads. RCG has a lot of useful info but it's a pain digging through each loooong thread trying to find the meat. Sort of like trying to find the chicken in a bowl of Campbells chicken noodle soup.

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Please keep 'em up as you have time either here or on your website.

Normally I would put this sort of How-to info on the main web site. But someone sent me a PM today asking if the scope could be used to adjust video. So, to quickly share the info I posted it here. When I have time (wishful thinking), I plan to create a fancy page on the rc-cam projects page.

... has a lot of useful info but it's a pain digging through each loooong thread trying to find the meat. Sort of like trying to find the chicken in a bowl of Campbells chicken noodle soup.

Creating more sub categories for a How-to is not really needed. Keep in mind that the rc-cam forum has a lot more chunks of quality meat, so the good stuff is easy to find. :)

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Yes. I agree about this forum. I was referring to the RCG forums.

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Nice. Great little tutorial. Please keep 'em up as you have time either here or on your website. This old TV repairmen & fledgling FPVer appreciates you reminding me about the things I've forgotten since Tesla was born.

My scope is in the mail. Should have it in two or three days.

Might I make a suggestion?

Another forum dedicated to tips like this with a topic for each tip/tutorial. No general chatter allowed. It would, I think, make it easier to find specific information without having to read through many forums or threads. RCG has a lot of useful info but it's a pain digging through each loooong thread trying to find the meat. Sort of like trying to find the chicken in a bowl of Campbells chicken noodle soup.

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Ron , I am also new to FPV using Electrostik to carry camera , goggles and recorder. Also an old TV Repairman, N5KBL is my call sign. I was interested in the small scope you mentioned. I am using OSD with a GPS attachement to give location, altitude, speed . I mounted a Hi-gain omni ant on a tripod on top of my RC plane trailer. Using the 2.4 Futaba . If you don't mind let me have some info on scope. I worked TV repair in Oregon and Texas. I could see the throw away stuff coming in and got out in time. I still miss working on the 6BK4 shunt regulators, heh,heh.

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That little scope is super (awsome as you American guys would say) ;)

Terry

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Hi Jaywalker. The link Mr RC posted at the top has all the details. http://www.dpcav.com/xcart/product.php?productid=16330

I bought one from them and still have it setting here. Bought a plastic 8.5 x 6 x 2.5 box to build it into (scope, pwr supply& test lead storage etc.). Need to build a ckt board for my two inputs, termination, & switches (75 & 50 ohms). Still trying to decide on input jax, BNC, F59, and a few other things. Don't have room yet for a test/work bench in the RV so haven't finished packaging it.

Great little unit judging from my preliminary look-see.

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This is what I found in my e-bay sony 1/3 ccd camera. Looks like it´s more like 2.2 Vpp. What should I do?post-8370-0-45955600-1304080397_thumb.jp

1 div = .5V / 10 uS

Edited by tascheri

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is it causing you a problem?

if so then adjust it if it has a pot you can adjust or make an attenuator with a pair of resistors.

Terry

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A 2.2V measurement is a sure sign your test setup is missing the required 75 ohm terminator on the video output of the Rx. So install the terminator on the Rx and use your scope to measure across the terminator.

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A 2.2V measurement is a sure sign your test setup is missing the required 75 ohm terminator on the video output of the Rx. So install the terminator on the Rx and use your scope to measure across the terminator.

This was measured with the camera hooked up to the Airwave 633 TX module and the measurement is the video signal of the camera. I believe the TX will supply adequate load to the camera´s output, or not? Funny thing is the camera works fine with the matching ebay TX/RX module but when I hooked it up to the Airwave module, full white screen when pointing the camera outside or to a bright light.

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Airwave Tx modules usually provide an approximate load of 50-80 ohms. You can use your ohmmeter to see what your module is providing.

Regardless, if you are getting 2.2V at the input of the Airwave module then either a mistake was made with the measurement or something is wrong with camera.

Most importantly, the video level out the Rx must be checked too. So you need to measure it and confirm you have 1Vpp with a 75 ohm load on the output of the Rx.

when I hooked it up to the Airwave module, full white screen when pointing the camera outside or to a bright light.

If your signal is really 2.2Vpp then this amplitude is much too high and over saturated video will occur.

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A lower value termination on the camera's video signal will reduce the amplitude going into the Airwave module. Just use a resistor value there that provides 1.0Vpp at the output of the Rx (when the Rx is terminated with exactly 75 ohms).

But before you do this I advise that you try another camera to see if the problem goes away. If it does not go away then there is something suspicious about your measurement or your circuit.

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Coming from me I would bet on the latter... but then, this camera hooked up to the airwave system gave me a VERY saturated video, like I could only see white when pointing the camera towards the window... I did check on the scopes calibration signal to validate my measurements ( I always do )...

Thank you very much sir and have an excellent Sunday. Tomorrow only flying, no lab stuff.

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Appologies for digging out a old thread, but the information shared here is great.

I spent some time reading it and another related to the tuners in the video RX's.

Question, would it be ok to use a generated video signal? Example use a simple arduino

to generate the required full white video signal?

Couldn't that produce better results as we know the incoming signal is the one expected?

Thanks.

Edited by Nandox7

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Using a calibrated video source is ideal and it will provide the most accurate results. I use a professional video generator, that has a variety of test patterns, and it is awesome for calibrating wireless video systems.

If you build your own video generator then pay very close attention to the NTSC (or PAL) video standards. The signal levels are very well defined and whatever you create needs to conform to these standards. Just keep in mind that many DiY microcontroller based video designs do not provide industry accurate video signals. BTW, to keep it simple you can follow RS-170 standards (set to 1Vpp), which is the equivalent to NTSC without the color (monochrome video).

If you don't have time to build a video generator then eBay sometimes has some good deals on used color bar generators. Or, just use the camera method that has been discussed here.

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I'll probably not buy a video generator, as I don't expect to do this much.

It must just as a curiosity and to try and tune my 3 RX's and squeeze every bit of performance I can get.

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Hi Mr RC-Cam,

I wrote you a PM a while ago on RCgroups, You encouraged me to ask my questions here. It took me a while because this topic is unfortunately a little corss-linked and I wanted to try catching Old Man Mike first to explain, but he hasn't been active for quite a while, the answers I got from other members are not to my satisfaction. Sorry for the lengthy post, it pays off to read through my post, otherwise I might get stuck again. Please, let me evaluate:

You probably know Old Man Mike, a pro-HAM guy. He has posted the following: http://www.rcgroups.com/forums/showthread.php?t=1147430&page=8 post #113

I'm particularly interested in the bold highlighted paragraph

I've been doing a lot of testing of various Chinese RX and TX 1280 Mhz systems for FPV. As part of that testing I built a test fixture which shows the FPV video quality as the signal levels is reduced from -80dBm to below -100dBm. Here is a video showing what it looks like with one of the better receivers in which I had replaced the SAW filter:

Saw3Play.jpg

For comparisons I use the lowest level where I can read all the OSD level as the defined sensitivity for the receiver. For this receiver I marked it as -95 dBm. Notice how the 1Khz reference audio signal drops out about the same time as the video.

By the way, it is not an easy thing to make accurate measurements at these low signal levels. I wanted to be sure that I could do this with an absolute accuracy of less than 1 dB. Here's a quick summary of the process:

1) A 60mw Video TX was used as the source. It was mounted inside an RF tight box with high quality chassis mounted bypass filters for the video, sound and power inputs. Copper Tape was used to reduce ground/RF leakage between the 60mw TX and the inside of the box. In-line attenuators were used inside the box to bring the signal down to -23 dBm at the SMA output of the box.

2) The -23 dBm level was verified with a well calibrated HP8594 Sectrum Analyzer. This was measured without modulation.

3) A PIC controller was used with a programmable step attentuator to provide the final -80 dBm to -100 dBm signal at the receiver input. Double shield cables were used for all RF connections. The PIC display was set to show the calibrated signal level at the receiver and setup so that the FPV camera could see it as the signal was reduced.

Notice that the video sync is maintained all the way down to -97 dBm where the signal where the image is just barely visible. To achieve this you must have the TX and RX video modulation and demodulations set to the correct levels. For that you need a good scope which can accurately show you the video levels.

One big discovery is that unlike the Lawmate receivers, all the Chinese transmitters I tested had a high input resistance rather than the standard 75 ohms for the video input. This really destroys weak signal reception because the TX will often be overdriven by the video camera expecting to see a 75 ohm load. That will cause the sync to drop out at higher signal levels well before major video degradation. For those of you that may have experience this, try adding a 75 ohm resistor in parallel with the video input of the transmitter.

So here is what I want to do:

1. CAM/VTX: adjust video levels correctly

2. Groundstation(GS)/VRx: adjust video levels correctly

I most appreciate almost everything that OMM writes, so I stumbled across his post which made me ponder if I do things correctly.

1a) take a 75ohm terminator, hook it to GND and VID of the cam, power the cam and check if I get 1V p/p, i.e. making sure my cam settings are fine. I use the camera test picture for white saturation.

1b) as there are no de-coupling caps in my VTx, I measure the impedance directly with a DMM. It varies from brand to brand, but I get something in the range of 250ohm to 1kohm most of the time. I connect cam/VTx, power everything and measure with my osci the video levels. They are too high. I add a resistor in parallel to GND to bring levels down to 1V p/p

2a)connect VRX to power and my display(s) [properly with video splitter due to multiple goggles n screens], measure VID again using my osci. If I get anything other than 1V p/p I'll adjust the poti inside my VTX to bring levels to my 1V p/p.

2b) I'm done setting things up

Questions:

a) the thing that confuses me is why OMM suggests to adjust levels when the signal reception is low. Why is he suggesting this procedure and where does he adjust the levels at the VRX or VTX?

B) I figured through testing that even though my VTx has a high input impedance, resulting in too high video levels between CAM and VTX, I can regulate the poti inside the VTx in such a way that at the GS/VRx I get 1V p/p. I guess this is not optimal, but can the sync be lost that way? Is that what he is meaning?

c) My fatshark goggles have ~40ohm input impedance, hence levels are too low. What can be done in this case? I can "mask" it by turning up the brightness in the goggles. Any better suggestions?

d) in your first posts, you measure with the osci in DC setting. I tried that as well, now not sure if I'm missing something but my DC offest is pretty high, 2V if I remember correctly. Is this a cause of concern?

e) sync problem (see att. pic): On a 2.4GHz boscam 500mw VTx (cannot adjust levels: not poti) in combination with a TBS69 pixim cam and a lawmate RX (adjusted levels with a resistor, looks good on osci). I get these curved lines occasionally. Normal?

post-25758-0-85477200-1382458416_thumb.g

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It sounds like your goal is to achieve perfection on a system that has design issues. The ideal situation would be if you had a lab full of test equipment (spectrum analyzer, low distortion sinewave generator, colorbar video generator, etc.). However, if you have access to a compatible vTx *or* vRx that has been carefully calibrated (by someone with the tools/skills to do it) then it can be used as a gold standard for validating your video system. Beyond that, all you can do is a best effort attempt of setting the NTSC composite video level to 1.0Vpp on the output of the existing vRx.

Comments to your general plan:

1a. Ok.

1b. Ok.

2a. Set vRx to 1.0Vpp (white level) while the vRx output is terminated by 75 ohms. Any special video level requirements your goggles/monitors may have should be accommodated on the outputs of your splitter/buffer-amp.

BTW, Lawmate has removed the video level pot from some of their vRx's. I'm not sure what they were thinking since this was a bad idea. If you don't have a video level pot in your Lawmate vRx then just accept that your only video level adjustment opportunity will be at the vTx.

the thing that confuses me is why OMM suggests to adjust levels when the signal reception is low.

What/where does he say that? In the quoted text you provided I can see that he warns that weak signal issues will be adversely affected if the video levels are wrong. I totally agree with that!

I figured through testing that even though my VTx has a high input impedance, resulting in too high video levels between CAM and VTX, I can regulate the poti inside the VTx in such a way that at the GS/VRx I get 1V p/p. I guess this is not optimal, but can the sync be lost that way? Is that what he is meaning?

Incorrect video signals can cause several problems, including: Sync distortion; reduced dynamic range (affects contrast and color); Corrupt audio sub-carrier; etc.

However, just because the vTx has the incorrect input impedance does not mean these things will occur. We don't know if they put "lipstick on the pig" and setup the modulation circuitry to expect the higher video levels caused by their poor design/manufacturing choices. Unfortunately, some cameras and OSD's will behave poorly with non-standard termination values, so the designer should never stray from the video standards.

If it was me, I would change the vTx video input impedance to 75 ohms, adjust its video pot to provide 1Vpp out of the vRx, then compare the flight performance to the stock system. After the comparison, I'd select the termination (original or 75 ohms) that demonstrated the best overall performance.

My fatshark goggles have ~40ohm input impedance, hence levels are too low. What can be done in this case? I can "mask" it by turning up the brightness in the goggles. Any better suggestions?

Time for another comparison. Setup the video level for 1Vpp into 75 ohms and do some flight tests. Then re-adjust the video level for 1Vpp into the 40 ohm goggles and compare the results. In both cases use the brightness/contrast controls on the goggles to obtain the best image. After the flight comparisons you can decide what solution works best for your eyes.

in your first posts, you measure with the osci in DC setting. I tried that as well, now not sure if I'm missing something but my DC offest is pretty high, 2V if I remember correctly. Is this a cause of concern?

A DC offset is odd because the output of a typical vRx will be AC coupled. Regardless, it would be good to verify that the DC offset does not allow the video signal to clip.

sync problem (see att. pic): On a 2.4GHz boscam 500mw VTx (cannot adjust levels: not poti) in combination with a TBS69 pixim cam and a lawmate RX (adjusted levels with a resistor, looks good on osci). I get these curved lines occasionally. Normal?

The video ringing could be "normal" for that configuration, I just don't know. It wouldn't be acceptable to me, but I've seen many FPV videos with worse video artifacts and the pilot was happy with it. So perfection can mean different things to different people.

BTW, like many others you've done a major sin by mixing equipment! The video modulation and pre-emphasis/de-emphasis techniques used by the various wireless video vendors are not standardized. So video performance will be subject to your tweaks and a LOT of luck. And as mentioned, perfection comes in many forms; if you can get it to work to your satisfaction then that is all that matters.

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Yep mixing or changing equipment can open a can of worms, you can get everything perfect then change one thing and it all goes to pot!

Terry

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Thanks Mr. RC-Cam. For taking the time to read through my post and evaluating on my questions. I really appreciated your thorough answer. Kudos!

question a) A few posts later in the RC groups thread, OMM was asked about the procedure, he explains (important paragraph in bold):

Another approach is to monitor the Video Camera input to the TX. Cameras are design to output 1 Volt Peak to Peak into a 75 ohm. You can verify this by monitoring the output voltage with no load and then with a 75 ohm load. It should be higher than 1 volt with no load and then drop down to 1 volt with the 75 ohms. So another way to verify the TX unit has a 75 ohm input is to monitor the camera voltage before and after you plug it into the TX input. If it does not drop down to something close to 1 Volt, then the TX does not have a 75 ohm input and you should install the 75 ohm resistor.

Finally, once you know the video is at 1 volt for the TX input, you can then adjust the TX video gain while monitoring the RX output for a 1 volt level. To fine tune it for the optimum level, one of the best ways is to adjust the TX video gain while monitoring the receiver when the signal is weak and snowy. This is a lot harder to do than it sounds since even with a sealed TX unit running into a dummy load, the receiver will still see a large signal when the TX is in the same room. As described in that -95 dBm video setup, it is not easy to get enough isolation.

Hope this helps some.

OMM

...here is my interpretation: CAM/VTX is adjusted for proper 1V p/p. He then uses everything called attenuator or dummy load on the VTx, takes the whole thing away until the picture almost breaks up. So far so good. Now that's me talking: He tries to get the AGC voltage of the VRx up till max, as the AGC may again distort video levels where they are most important: When flying real far, if too high/low the VRx may again get out of sync. This means fine-tuning the levels on the VTX poti so that VRX/GS VID signal measures again 1V p/p even a low RF-signal [far distance].

Comments? Thoughts?

question B)

Ok, I think it makes sense in this case to simply adjust everything to 75ohm. At least it appears as a good trade-off. Thanks.

c)fatshark goggles

Well, I didn't see a noticable difference when adjusting the contrast/gains, but my video levels were like 0.5V p/p. Not to my liking. If the impedance is too low, is there another way to increase them to proper 75ohm termination 1V p/p that is not bulky or out of the world expensive? I'm using the dominators. In fact they parallel two video modules a 75ohmm resulting in 37.5ohm roughly. If I remove the "RX' module inside the goggles, I'm slightly above 80ohm, which is easy to correct and since I don't need the Rx inside the goggles, I'll stick to that solution, still any suggestions on how to ""increase"" levels would be nice to know of.

d)Uhm, what I'm trying to say is that my 1V p/p signal is offset by about 2V, so the VID sync portion starts at about 2V when measured with DC setting, the white levels end at 3V absolute. will a cap in series help to play around? What value,type?

d)what is video ringing? how would it look like on a osci? any ideas to mitigate the effects?

thanks for your time, you're being an immense help here.

Sincerely,

Dio

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