cptvideo@ihwpt.UUCP (D. Vlack) (10/16/85)
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About a year ago, I decided to upgrade my first VHS vcr to a new
stereo HI-FI version. The old one was one of the first models to
be sold in the U.S.(RCA VBT200) and had only 2 and 4 hour speeds.
I picked out a near to the top of the line RCA VKT650 and took it home
and hooked it up. I was extremely disappointed with the picture quality
compared with my old machine. Pre-recorded tapes seemed to suffer
the most. Since I liked the sound and the features, I decided to get
the service manual and see what could be done to improve the
picture.
After a lot of poking around and experimenting, I now have made a
set of changes to the machine which make a truly remarkable improvement
in the picture. The changes are in the following areas:
1) Disable the 1H noise suppression in SP mode.
This noise reduction isn't needed on standard speed and only
serves to reduce detail. It turns out the circuitry to do
this was on the PC board, but not populated with parts.
2) Balance frequency response of normal and 1H delayed video into
1H noise reduction circuitry.
On my machine the frequency, response of the delayed channel
did not match the main channel causing excessive coring of
the video. Matching these made a significant improvement
in the detail of facial features.
3) Change fixed video de-emphasis to a compromize between old
and new "standards".
The old VHS machines had smaller time constants and it appears
that most pre-recorded tapes conform to the old or something
in between the old and new.
4) Disable the @$#%$#$ high frequency coring circuit that follows the
Non-linear de-emphasis circuitry.
This circuit is like doing non-linear de-emphasis twice. It
eliminates the low level high frequency signals from the video
signal. This reduces noise, but also detail in just about
everything in the picture except for large luminence
transitions. The s/n specs may not look as good on paper,
but the picture sure looks better.
These changes collectively make a big improvement in the picture. The
slight mistracking between "old" and "new" pre-emphasis curves are
easily within the range of the sharpness control. Only 4 or 5 resistors,
3 diodes, 1 NPN transistor, and a half dozen capacitors made a huge
difference. By the way, this vcr was made by Hitachi.
<------D. Vlack [ihwld!cptvideo] ------>