As far as 2.4Ghz wireless video gear goes, the 200mW Lawmate system is a popular choice. It is sold by BlackWidowAV and other suppliers. It offers tiny size, 5 Volt operation, a convenient built in microphone, and ham legal operation on selected RF channels. Combine that with its reliable airborne range and it sums up to be a great system for the typical video camera equipped R/C model pilot.
I must first say that I have several other Lawmate video systems that were modified for my needs. Some have been hacked to include digital readouts that show Relative RF Signal Strength (RSSI), some have extra video outputs on the receiver, and others have been updated with additional audio output jacks.
The tweaks and hacks shown here are for the 4-channel 200mW Transmitter and matching 8-channel Receiver. Keep in mind that there are several versions of this system, but as of Oct 2005, the one shown here is the latest. With luck, these concepts can also be used on other video system brands. But, you will have to roll up your sleeves and determine how to adapt these tricks to your particular system.
Tweaks: Level With Me
I'm a stickler about video signals. I spent over a decade in an industry that was driven to achieve good video quality. So, it is only natural that I check each new video system for proper signal levels. Even if the image looks reasonable on the monitor, the system still gets checked. Why? Well, VCR's, monitors, and Camcorders will mask significant adjustment issues.
A properly adjusted video signal will optimize brightness and contrast, which is important for good video quality. However, I often find that the signal levels are incorrectly set by the manufacturer. A recent Lawmate system had a video amplitude that was half the accepted industry standard for NTSC signals. Don't be fooled by thinking that the other wireless video manufacturers are better at this. Unless you check, the video system's dynamic range will probably be under utilize.
Adjusting the video levels requires an oscilloscope that is rated at 10Mhz or better. As far as o-scopes go, that is not asking much. But, it is definitely well beyond the abilities of a cheap (often free) PC Sound Card scope. Sorry, but you will need to use the real deal for this task.
Besides the o-scope, you will need a 75-ohm resistor (1/4 watt 5% will do) and a female RCA connector. Start by soldering the resistor across the RCA connector. Congratulations, you just built a standard 75-ohm video terminator. Next, unplug the Lawmate receiver from your monitor/camcorder/VCR and attached the terminator to the video output. At this point the only thing attached to the receiver will be the 75-ohm terminator, the video cord, and the battery supply.
Next, connect the o-scope to the terminator. Set its vertical scale to 0.2Volt/Div and the Horizontal tiembase to 10uS/Div. Position the video transmitter about ten feet away and power everything up. Now here is the important part: place a bright light in front of the transmitter's video camera. I use a Maglite flashlight since it is convenient.
The bright light source is needed to establish a full (saturated) white level on the video signal. Failure to fully illuminate the video camera's imaging sensor will result in improper signal levels. To prevent camera damage, do not use the sun; use a bright *white* incandescent light such as a strong flashlight. Place the light source a couple of inches in front of the camera lens, aimed directly into it.
You will see video
on the o-scope that consists of syncs and some full white levels. Unless
you are a video pro, it might not look at all like the clean waveforms you
see in the text books (like the one shown below). However, do your best to
sync to it using the o-scope's horizontal timebase and sync
What is important to observe is the overall voltage level of the waveform. That is to say, from the bottom of the sync tip, to the top of the highest white level you can find, the maximum observed voltage needs to be measured. For both NTSC and PAL video cameras, the level should be 1.0Vpk-pk. The AV (active video) region should be approximately 715mV for NTSC and 700mV for PAL systems.
There is a POT inside the 200mW Lawmate transmitter that is used to set the required signal level. Just pop-off the metal sardine can lid to expose the POT. It will have some anti-tamper glypt on it which will probably interfere with your tweaking tool (I use a plastic screwdriver designed for this task). Just be very careful to prevent damage to the POT, otherwise you will forever be stuck with the factory setting.
Power is on during the adjustment. So, do not be careless if your tool is
metallic. Otherwise you will be left with a sad face and perhaps an interesting
Jack Hacks: Two Are Better Than One
Besides the video level tweaking, you can also perform a couple of useful hacks to the Lawmate receiver. I use a camcorder and monitor at the same time. In order to do this I need two video outputs from the video receiver. That is because simply connecting two video devices in parallel will severely impact image quality. Instead of an external video buffer, I modified the Lawmate receiver so that it had an extra isolated output. And, the new video output deserved its own audio output signal as well. So, that will be addressed too.
Step 1: Video Jack Modification
The new video output involves adding a resistor to the receiver. Begin by removing the circuit board from the receiver case. Next, turn the PC board upside down and identify location "Pad-V" (see photo below) on the daughter board. This location is one end of a tiny SMT resistor. Solder a four inch piece of 28 gauge insulated wire to it. This requires good skills since the wrong technique will liberate or damage the SMT resistor (both of which are not good things to do to it).
Add a 75 ohm 1/8 watt resistor to the end of the wire. The free end of the new resistor will go to your new video output jack. The jack's gnd wire uses any convenient ground on the receiver's PC board; I suggest the same ground pad that is used on the existing A/V jack.
Step 2: Audio Jack Modification
The new audio output requires adding a capacitor. Identify location "Pad-A" (see photo above) on the daughter board. This location is at one end of a SMT cap. Solder a four inch piece of 28 gauge insulated wire to it. Add a 4.7uF to 10uF electrolytic cap to the end of the wire (+ end towards the PC board). The negative end of the cap goes to your new audio jack. Audio Gnd can be any convenient ground pad on the receiver's PC board.
Step 3: Install the A/V Jack
The stock A/V jack
is a 1/8" phone jack, so I decided to simply duplicate that style for the
new A/V output. Fortunately, a stereo mini-phone jack is easy to find at
the local Radio Shack store. That, along with some 3-conductor cable, is
all that is needed to finish the job. See photo on the
Don't Goof Up the Antenna
Last but not least, you need a good antenna on the receiver. I think you will quickly discover that the stock whip aerial is not ideal for our R/C video application. Instead, a patch antenna is a much better choice. A very popular DiY patch antenna is the rc-cam Goof Proof Patch. Or, you can also spend a lot more for a commercially made patch antenna. The choice is yours.
The Small Print:
All information is provided as-is. I do not offer any warranty on its suitability. That means that if you perform these tweaks and hacks, you will do so at your own risk.