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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.
3,& 9,'(2 &/($1(5 E\ 0,.( '(/$1(
