Author Topic: Displays - Noise on video and how the sync works  (Read 4179 times)

Offline Nilla

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Displays - Noise on video and how the sync works
« on: March 08, 2017, 10:44:03 AM »
A video signal contains embedded synchronization pulses. The image is scanned from left to right, top to bottom. The horizontal sync rate is 15.734 kHz and the vertical sync rate is 29.97 Hz. 15.734 kHz corresponds to a period time of 63.6 µs and 29.97 Hz to 33.4 ms.

A disturbance in the ns range will probably not cause more than some noise in the image, the video decoder in the display can still detect the sync pulses correctly.

A disturbance in the µs range may result in that the synchronization is lost for one or more lines. The video decoder can tolerate the absence of synchronization for a few lines (~dead reckoning).

The video decoder will fail to keep track of the video signal position when the disturbance persists over a large number of lines or perhaps even frames. This instigate the need to re-synchronize the video decoding hardware and software. No video image can be showed while this happens.

When any of the issues above occurs, activation/deactivation of an inductive load may be at fault (or may be the root cause), e.g. a solenoid, large relay or electric motor. Inductive loads will send out large voltage spikes if the proper filtering is not used. It is not necessarily only the wire that goes to the inductive load that is affected. A voltage spike on one cable could be powerful enough to induce a disturbance on adjacent cables.

See attached pictures from https://en.wikipedia.org/wiki/Analog_television

It’s good to measure the video signal on the machine with an oscilloscope. Follow the procedure below:
  • Connect one probe to the video signal.
  • Check that video signal amplitude is around 1 V peak-to-peak.
  • Set the oscilloscope trigger to normal mode to assure the triggered capture stays on the oscilloscope screen.
  • Set the trigger voltage at a level a bit higher than the peak value showed on the oscilloscope screen.
  • You should now be able to see that the oscilloscope triggers at the same time as the display lose synchronization.
  • The oscilloscope screen will show how the video signal is affected by the disturbance.
  • Connect a second probe to the power supply of the camera.
  • The supply voltage should be at a stable voltage level.
  • The disturbance is most probably entering the camera through the power supply line if there are glitches on it happening simultaneously to the synchronization issues. Try to separate the power supply for the camera or improve filtering for the camera. You could also try to improve filtering at the point where the disturbance is originating from.
  • If the camera power supply is measured to be stable at all times the disturbance is most probably induced on the video signal from adjacent cables. The cable routing, cable length and cable/connector quality should be examined.
The best is of course to identify and address the origin of the disturbance to prevent from any new issues in the future.

Best regards
Nilla
PLUS+1 Helpdesk team