To : ATV@WW
By G8MNY (CQTV 1994, New to packet Feb 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This circuit was developed to limit the large negative pulses from radar QRM on
23cms ATV that are recovered video from a NE564 demodulator (or symular). These
pulses had been effecting the video amp DC levels, resulting in short periods
of no sync. The use of negative clipper after the de-emphasis network was not
used as that would allow the radar pulses to damage the video syncs, by
integrating them with the syncs before the clipper.
Using a complementry circuit (Inverted e.g. a T1=PNP, diode reversed, & the
330ê & 220k to the other power rails) will give a peak white clipper circuit.
2k2 ³ 22k ³ ³
ÃÄ´ÃÂ´ÃÄ´ ³ 10n SYNC ³_ ³
Centre_ ³ u1Áu1 ³ ÃÄ´Ã¿ CLIP ³/| ³
Freq |\ ÚÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄ¿ Á LEVEL 220k ³
1-10pÚ´ÃÄÄ´13 2 1 10 ³ RV1 / ³ ³
ÀÄÄÄÄ´12 NE564 FM DEMOD ³14 ³ ³/
IF>Ä´ÃÄÄÂÄÄ´6 7 3 11 8 4 5 ÃÄÄÄ1kÄÄÂÄÄÄ)ÄÄÄÄ´T1
38MHz 1n 1k ÀÄÄÂÄÄÄÄÂÄÄÂÄÄÂÄÄÂÄÄÄÂÄÙ _³_ ³ ³\e____\ Video De-emphisis
ÀÄÄÄÄÄÅÄ1kÄÁ´Ã´ ³ ³4n7³ D1/_\ ³ R1 ³ / & Sound
n47=== 1n³ ³ === ³ +ÃÄÄÄÁÄ10kÄÄ´
³ ³ ³ ³ === ===C1 330
2 470 ³ ³ ³68p ³10u ³
Clipping in the buffer before de-emphasis was more effective. The effect of
loosing the sound subcarrier during radar clipping was un-noticeable.
The circuit takes the average buffered DC video level with R1 & C1. RV1 is used
to just get D1 to clip the high impedance base signal, when a white picture is
present by adjusting VR1.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP