Since first building Darius' converter both he and I have
been experimenting with various improvements and simplifications
to the original design.
http://www.scottpeter.pwp.blueyonder.../newscheme.jpg
gives a schematic for a simpler version of the current design.
Darius' converter basically translates three 625 lines into two 405 lines.
One consequence of this is that the position of the discarded line ripples up
through the frame because 625*2/3 has a remainder. An effect of this is that in fine
detail such as small text etc you can see a rippling movement. Darius substantially
reduces this effect by having an interpolator.
The simpler proposal here uses a different approach, namely to stop the
rippling by resetting the line counter at the start of each frame. The downside of
this can be that there is some disturbance of the first few lines at the top of the
picture.
This proposal limits the line sync disturbance by synchronously resetting
the line counter for the duration of one 625 line.
Adding an interpolator improves the presentaion of diagonal picture content
but for those who want a simpler build then this proposal gives good results
and a very stable picture.
http://www.scottpeter.pwp.blueyonder...newschpict.jpg
The schematic is simpler is various ways. Apart from not using a third CXL
for an interpolator the synchronous 74HC163 counter is used for both
the clock division to obtain 2.9MHz and for the 405 line sync.
The two JKs in the vertical sync path simply generate a reset pulse that
is one 625 line in width.
The back to back monostables (bottom right) generate the line pulses
within the frame syncs.
Peter.