Video Distortion and Quality

Using a San-Disk V-Mate to record video transmitted from a model plane I have long had a problem with tearing down the left side of the recorded video. It’s not seen on the monitor connected to the same video receiver.

Finally I investigated and fond that it only occurred around the horizon and when it was near horizontal across the picture. This is where the top part of the image was bright (sky) and the bottom part was darker (ground). It sometimes appeared in the top part (sky) but never in the bottom darker part of the image.

The solution was to build a new video amplifier-splitter board for the ground station.

Also to make sure the video signal from the camera is properly matched to the transmitter.

Technical

In the ground station is a 4-way video splitter that feeds the recorder and a couple of LCD screens. All connections are standard 1V-pp composite video signals. The old splitter uses AC coupling of the video signal; it passes through big capacitors in and out.

With the large brightness shift at the horizon there was a sudden DC voltage shift on the video signal that the V-Mate has trouble with and looses line sync. Any time there is a smaller white area in the image the V-Mate can loose line sync. but it is much worse when the white level is too high (the sky flares or is over-exposed) and/or the composite level is too high.

The solution seemed to be to DC correct the video signal. Some web browsing suggested that this was quite involved but I found a amplifier chip at a reasonable price that included DC correction. With a couple of these I built a new amp-splitter with level control on the input and three of the outputs. Some bench testing indicated that the amp with DC correction generally improved the look of the video so I went ahead and designed a PC Board.

I also found that most of my cameras output a signal that was well over 1V-pp into a 75-ohm load; most were 1.2 to 1.5 V-pp. As these are security cameras the high output is probably intended to overcome long cable losses. For FPV it does mean that the video into the transmitter is probably too high and may cause some problems with the transmitter or video quality. I have also found that some equipment has a high impedance input, not the expected 75-ohms. This can result in a video signal of 2-3 V-pp, or more.

New Amplifier-Splitter Specs

• 75 ohm composite video input with level control
• one buffered DC-corrected output (no output level control)
• three buffered DC corrected outputs with level controls
• the three outputs can be sync corrected **
• operates from 5 to 15V DC @ < 100mA
• small size : 59 x 42 x 7 mm
• four 3mm mounting holes
• solder pads for all connections

** sync correction may be added when/if I get around to writing some software for the micro-controller.

Sync Correction

While designing the PC Board I decided to include a micro-controller so that I could have a play with correcting missing or out of place frame and line sync. If this works out it should allow a corrected video signal to drive a monitor that would normally go blue-screen on a noisy signal. This would be a huge benefit to RC FPV pilots.

I chose a micro-controller that I already had that should be fast enough to handle the video; a PIC18F26K20 capable of operating up to 64MHz. The video signal is colour burst filtered and fed to the internal comparator. Along with the internal programmable voltage reference and considering that the video is DC corrected, will allow the PIC to detect the sync and provide an interrupt. The PIC can drive the video to 0V for sync pulses, and to black or white levels. The hardware is there for the PIC to modify video passing through or to create frames from scratch with the appropriate code.

I did write some code some time ago for a slower PIC16F88 to create PAL frames from scratch but I haven’t moved it to this PIC yet.

Using a timer or two it should be possible to track the incoming video and correct the frame and line sync so that the monitor thinks it is always seeing good frames.

Creating frame and line sync from scratch could be useful for filling in signal loss to keep other video recorders running. The V-Mate keeps recording on signal loss but I have tested a couple of recorders that pause and/or stop recording. This is no good if you are setting up and starting the recorder before starting the transmission or have periods of signal loss  for other reasons.

Testing and Level Settings

Testing and level setting proved to be quite difficult. This is how I started and the procedure and findings may change with more testing. Cameras and monitors are not all equal; swapping the camera or monitor will change the image and setup results. So much for a 1Vpp composite standard signal. For normal daytime FPV the setup should be done on a typical bright day. For darker or night use you can setup on a dark day or at dusk but it won’t be as good on a bright day.

With a camera connected to the input and a 75-ohm monitor on output 1 (no level -adjustment) set all monitor picture controls to 50% (brightness, contrast etc.). Mount the camera so that half the image is looking at a bright sky with clouds and the bottom half is viewing the ground and darker foreground. Turn the amp output level to minimum and adjust the input so that the brightest parts of the sky are not over-exposed. When increased slowly at some point the detail will go out of the clouds and the brightest patches will flare. Set the input level just below this point and leave it there. Swap the monitor to one of the level controlled outputs and check that it works over the full range of output level adjustment. The range of control is not large and the monitor should automatically compensate so you won’t see much change. Leave the output level controls at about 50%. The output level adjustment is mainly used to compensate for a device (monitor, goggles, etc.) that has problems.

Update – June 2016

Sync correction is no longer as important because monitors for FPV no longer quit to blue-screen at the slightest hint of interference.

Correctly coupling cameras transmitters, receivers, recorders and displays is still important if you want the best from your FPV system.

I sill consider it important to record any flight that goes more than a few hundred meters away or passes over scrub or ground that would make recovery difficult.