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Distortion on TV picture

 
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daa



Joined: 18 Jul 2007
Posts: 6
Location: Hastings

PostPosted: Wed Jul 18, 2007 4:34 pm    Post subject: Distortion on TV picture Reply with quote

The analogue image on my TV is perfect but when coming from my Daewoo SV-900 digital VCR the image will creep to one side (always to the left, in the middle) depending on movement in the image itself or a change in camera angle. It also happens to a lesser extent when coming via my PVR.

As current TVs do not have horizontal and vertical hold knobs anymore how do I correct this fault if possible and what causes this problem?

Any help appreciated.
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Mon Jul 30, 2007 12:12 pm    Post subject: Reply with quote

Begin with trying alternatively CVBS (composite) and RGB signal from your digital sources and check if the you have the sync problem in both cases.

In some cases, TVs have badly designed input stages for the video signal which cause clipping of the sync pulses if the signal source has a DC offset that the TV manufacturer did not cater for. In order to assess a such problem properly, one must study the circuit diagram for those parts of the TV, measure the incoming video signal by oscilloscope and check DC offsets etc, and possibly make some kind of test hookup to change the DC offset of the signal.
I have once tried using a bench power supply connected in series with a 1 kohm 2 W resistor in parallell to the video signal, then it is easy to change the DC offset by adjusting the output voltage and polarity of the power supply.

According to official standards for video signals, the black level should be at 0 V, sync tips at approximately -0,3 V and white level at +0,7 V. But in order to save a negative supply voltage, much consumer electronics have sync tips at 0 V, black level +0,3 V and white level +1,0 V. In really weird cases, several volts of positive offset have been found.
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