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Has anyone noticed that the input impedance (at least measured using a DVM) on some recent Lawmate transmitters is around 340 ohms?

Older gear measures around 70, as one would expect.

Strange stuff...

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That's not good -- some video sources won't work properly with that. Perhaps you just got a bad Tx?

A high input termination is not bad on its own, especially when driven with short cables. However, some video sources derive their drive bias currents from the expected 75 ohm termination. With exceptionally high terminations, the awkward biasing may cause clipping. The popular cameras we use are very tolerant to such things, but I suppose the video outs on some digital still cameras might be fussy about it.

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That's not good -- some video sources won't work properly with that. Perhaps you just got a bad Tx?

A high input termination is not bad on its own, especially when driven with short cables. However, some video sources derive their drive bias currents from the expected 75 ohm termination. With exceptionally high terminations, the awkward biasing may cause clipping. The popular cameras we use are very tolerant to such things, but I suppose the video outs on some digital still cameras might be fussy about it.

I'm not sure its a bad Tx, 4 transmitters of various types have been tested this evening (in 3 countries), and all show this. One was an older range-video unit, two 500mW lawmates, and a 100mW.

Kilrah kindly popped open his 100mW unit and traced it out. There is a 270 ohm resistor in series with the input, which then terminates to ground through a 70 ohm resistor (or there abouts).

Strange stuff, I've no idea why they would do that.

I did try to find a full datasheet for any of these LawMate units, but no luck so far, at least not with anything that shows the input impedance.

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It's too bad they are doing that. I just checked the 200mW Lawmate that is sitting on my desk, and it measures 75 ohms. All my Lawmate gear is several years old, so I expect it will all be safe from this recent change you're reporting.

I'm curious -- are the latest Lawmate systems' video levels properly calibrated? That was an issue on all that I had checked in the past (the levels were always too low).

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That was an issue on all that I had checked in the past (the levels were always too low).

Funny, I had too high ones :lol:

I measured all my "old" Lawmate TXs I had around (2x200mW, 1x600mW, 1x100mW) and they were all 70 Ohm.

Then I measured the "new" 100mW and got 350Ohm too. The "new" series are the 100,500 and 1000mW you'll find on their website now, the ones we're used to aren't made anymore.

So I opened that new 100mW and had a look at it:

http://i18.photobucket.com/albums/b105/Kil.../L100mW_TOP.jpg

http://i18.photobucket.com/albums/b105/Kil...0mW_BOT_ann.jpg

If you follow the video trace you can see it goes through what Anthony said, then into a lot of stuff (filtering?), and even possibly a transistor amp, but it's hard to follow as the there seems to be vias under components.

My guess is that the TX is fine with this, and has enough active parts to use that video well, and actually when I've tried that TX it was working fine with normal levels, even with a receiver from another brand.

However, there could be problems with some devices like new OSD's that go inline between camera and TX without having a fully buffered input/output and who just "tap" into the existing line. They'll see a higher level and could be disturbed, which seems to be the original problem that brought that to light...

Edited by Kilrah

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My guess is that the TX is fine with this, and has enough active parts to use that video well,

I agree, but that is not what will cause a problem. A high value termination can bugger some video sources, as mentioned earlier.

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Still no reply from Lawmate, but I can't wait to find out why they did this.

One thing I'm wondering is whether these modules are designed to be externally terminated. No idea why they would want to do this though...

Puzzling stuff.

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One thing I'm wondering is whether these modules are designed to be externally terminated. No idea why they would want to do this though...

It is possible they did this mod for a big OEM customer and somehow let the modules slip into their normal inventory.

If they were expecting external terminations then you should find that a Lawmate Rx's video level, with the factory adj, would be too high without it. If the old and new Tx's provide the same Rx video level, then they have purposely accommodated the termination change in the video chain. What does your oscope show?

I'm curious, has this odd termination affected your video sources? I would suspect that some digital still cameras (with video out) will not like it, as well as some OSD's. Fortunately most installations won't show any problems, but will benefit from a Rx video level tweak to ensure proper dynamic range.

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It could be that it was a mod, but if affects several models (100mW, 500mW), so its a bit surprising.

I just ordered a 500mW and a 1W unit, to run some tests, exactly the video level test that you mention, plus some flatness tests, I'd be surprised if the impedance mismatch looks good in the frequency domain.

This odd termination did affect my OSD, I have a workaround, but it frustrates me that it is neccessary. I am still amazed that these things ignore such a well established standard.

When the KX131 is hooked directly to one of these transmitters the pk-pk composite signal is 1.6v, instead of the 1v expected.

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When the KX131 is hooked directly to one of these transmitters the pk-pk composite signal is 1.6v, instead of the 1v expected.

In the old days the factory adj video levels were often too low. I guess they "fixed" it. :) Sadly, too high is not good either. In a perfect world, Lawmate would restore the 75 ohm termination values and correctly adjust the video levels to the industry standard. Otherwise some installations will not achieve the best possible video quality.

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Is there any way around this? I have one of the new 500mW transmitters, and am planning on getting an OSD at some point. I want to make sure I don't have any problems.

BTW, why is this in the for sale section? I almost missed this discussion.

edit: Thanks for moving it.

Edited by twinturbostang

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Is there any way around this? I have one of the new 500mW transmitters, and am planning on getting an OSD at some point. I want to make sure I don't have any problems.

If you run into problems with your OSD, then you should contact the OSD supplier for a solution. Sounds like Anthony will be able to help if you are using his product.

I'm more into the basics. If it was me, I would add an external termination to the input of the Tx so that it showed the same video brightness/contrast as an older Lawmate Tx. Then I would calibrate the Rx's output for industry standard 1V video. That would ensure the best dynamic range, which will give you good contrast and brightness. I'd do this long before an OSD was installed.

It will be interesting to hear if Lawmate has a explanation for their recent change. I'm leaning towards the high value termination being the result of some confusion on the production line.

edit: Thanks for moving it.

Thanks for reporting it. Because I use the "Assistant" feature to view new posts, sometimes I don't notice oddly placed discussions.

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Then I would calibrate the Rx's output for industry standard 1V video. That would ensure the best dynamic range, which will give you good contrast and brightness. I'd do this long before an OSD was installed.

How would you calibrate rx´s output for 1V Peak-Peak. I don´t see it in that link ;)

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How would you calibrate rx´s output for 1V Peak-Peak. I don´t see it in that link 

It's all there in "Tweaks: Level With Me "

Terry

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Still no response from Lawmate... frustrating, I can't wait to hear their explanation for this.

Don't expect much. You need to call them and harrass them for a few weeks before they'll get back to you most times i've found.

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It's all there in "Tweaks: Level With Me "

Terry

Hehe, English is not my mother language but what I understand from there is that you have to adjust TRANSMITTER POT. Now I have 1 transmitter + 2 receivers with different levels coming out the receivers, how would you adjust each receiver? If you touch transmitter pot you would be better in 1 receiver but worse on the other.

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Now I have 1 transmitter + 2 receivers with different levels coming out the receivers, how would you adjust each receiver?

That is a common problem when mixing Rx brands/models. Even identical Rx's may be different because the factory often does not pay attention to these details.

If the Rx's level differences are excessive, then adjust their internal level pots if they have them. If they don't have a pot, and you are happy with they way the image looks, then just set the Tx's video level for the best compromise.

The goal is to get the signal that is sent to the video monitoring equipment as close to the 1Vpk-pk standard as you can. The measurement doesn't need to be obsessively perfect. For sure, the 0.6V I have found on some Lawmates, and the 1.6V reported by Anthonyrc, are too far off from the composite video standards. For most applications, within 5-10% of ideal is good enough. But, getting it perfect is not hard to do either.

This problem is not specific to Lawmate -- other A/V brand users will propably benefit from a video calibration too. In the case of Airwave modules, some have a level pot and some don't. The Airwave modules are a OEM component, for use in RF A/V products. So, the "host" board may need to provide an external level adjustment if the module's internal pot is too hard to access or is not available.

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So, the "host" board may need to provide an external level adjustment if the module's internal pot is too hard to access or is not available.

Thanks, MR.RC-CAM your replies are always clear.All you´ve said here we´ve discussed in my Airwave rx issues post. But what I´m trying to know is how would you install an external level adjustment. I´m not an electronic engineer, I did mechanical engineering, so here I´m a bit off.

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... how would you install an external level adjustment

In the case of a personal Airwave DiY project: Install a resistor in series with the video output of the Rx that has the *highest* video level. The resistor value that is needed will depend on your installation. Perhaps start with a 50 ohm pot, then switch to a fixed resistor once you find the best value.

This is getting a bit off topic. So it might be best to start a new discussion (or revive an old one if this was being discussed elsewhere).

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Finally!, a mail from Lawmate, everything is clear now, this problem just doesn't exist..... :angry:

Dear Sir,

Sorry to reply you late.

According to your urgent question. Its seem your client have some mis-understanding for this technical issue.

1. All our TX & RX are all adjusted well with their I/O impedance parameters to be able to work perfectly with the general CCTV cable or device under the standard 75 ohm impedance parameter. There is no any concern the problem you mentioned will be occured.

2. The 340 ohm you mentioned is the impedance parameter for the part of wireless transmission inside our wireless system. It doesn't concern at all to harm the compatibility for our TX module working with any general CCTV system & device (75ohm). The two parameters mentioned above, they don't relate with each other at all for your ref.

Should you have any further question pls feel free to contact me directly at your earliest convenience.

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Try to get back to them with some precise facts about how the video line level will be affected, by how much voltage, and how it harms some devices in a video chain... we never know. But indeed, it seems they did that on purpose with no apparent reason other than maybe making it easier for them without caring about the devices that are connected... :(

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