Occasionally I've had reports of some televisions having a symptom of a white vertical line on the left, right, or both sides of the picture on premium channels. Mostly on CATV. This may appear worse on some channels. To complicate matters the symptom may only appear on certain televisions or models. The reason depends on the amount of horizontal over scan the particular television has. TV's that have a 5 to 7% of overscan, the vertical lines are benign. They occur off the screen. But if the set is designed with < 5%  horizontal over scan, the lines will be visible. Another possible effect with this line if it occurs on the side of the picture is doming. (The overheating of the shadow mask causing and uneven expansion creating purity error).

The symptom  is caused by-product from a commonly used form of scrambling called Gated Sync Suppression or (GSS). This is a form of scrambling is used by various cable systems to prevent theft of their service by suppressing the video synchronization pulses. If the scrambled channels are viewed without a decoder, the video will have sever sync instability and distortion of the picture. The following waveforms show how GSS is applied and what the effect is on the reconstructed video waveform.


The exact width of the reconstruction pulse cannot be perfectly matched by the decoders. This is due to the fact that each decoder has slightly different tolerance characteristics. Because of this, all GSS decoder manufacturers provide a slightly narrower reconstruction pulse than the original suppression pulse. The difference between the original sync pulse and the reconstructed pulse results in a vestigial signal that creates narrow white vertical bands at the extreme right and left sides of the television picture..

Video waveform indicating locations of
the GSS pulses



 
 

The remedy for this problem varies from one chassis to another. Some have a horizontal width and/or centering controls, which may be adjusted with a hardware control, or through a tech menu. Others may require the repositioning of the yoke or the front most rings. These rings will move centering but may also effect purity if moved too far. So be careful.

Horizontal width can also be influenced by the retrace capacitors. They are generally connected in parallel with the horizontal output damper diode. If one of the retrace capacitors became defective, by slightly decreasing in value, the picture will become slightly narrower. This action may cause XRP to trigger and shutdown. However, if shutdown does not occur the GSS artifact may very well be seen.

Special Note: DO NOT ALTER THE VALUES OF THE RETRACE CAPACITORS UNLESS YOU HAVE WRITTEN INSTRUCTIONS BY THE MANUFACTURE TO DO SO.
 

by Dennis Viereck
wa6ati@mindspring.com