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EPE Online, February 2000 - www.epemag.com - 101

This project is the result of avisit by the author to a friendshome to see a video recorder andportable TV the friend hadinstalled in his sons bedroom thatwas giving trouble. He wascomplaining that the picture wasflickering and the brightnessvarying.

The brightness variation wasparticularly noticeable whenwatching at night with the lightturned down low. Both of the unitshad been obtained from second-hand shop, and they did not wantto know about any problems.

Using a known good tape andhead cleaner etc., the system wasduly checked out. After muchexperimentation it wasdiscovered that the problem laywith the tape a recently-purchased Block Buster because the good tape, alongwith his home-recorded tapes, allworked fine. You could see thatthe picture flickered and thebrightness was indeed wavering,but this did not happen with thesame movie on the main TV inthe lounge.

TV was having a struggle. Theresulting sync pulse waveform,showing details of theintroduced interference, canbe seen in Fig.1.

This simplified diagramshows the difference between aclean signal and theinterference pulses added toprevent tape-to-tape copying.VCRs use the amplitude of theblack level voltage immediatelyfollowing the Line Sync pulse toset their AGC level. They alsorely on incoming sync pulses onthe video information, unlikeTVs which have their own synccircuits built in. Because theadded pulses are variable bothin amplitude and repeat timingthe brightness of the picture canvary and the line and framestability can be affected to apoint where it is almostunrecognizable.

These interference pulses

Improve your PICture quality.

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