Quote:
Originally Posted by peter_scott
Looking at your screen shot it does show the same lack of interlace that Hugo's does. This might be a peculiarity of the 905 but it might also relate to the Aurora. I found a similar effect with my Sony 9-90.
Peter
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I have looked at the interlace issue in the past. In every pre-WWII I have the interlace is poor, even with a video signal with equalizing pulses.
Investigation of this showed that the primary cause was excessive integration of the vertical sync pulse. The small 1/2 line timing difference gets swamped by an excessively long RC time constant.
( was not due to the vertical osc triggering from another source).
In fact in one of my American pre-war sets; the time constant was 10 times too long.
In the post WWII period, the Americans completely solved the issue. They realized that the filtering properties of a single stage R-C filter was not suited to a vertical sync pulse separator (that is if you wanted perfect interlace) it had to be a two stage filter, so as to get good filtering off of the H sync, but a fast rise time due to the shorter time constant. So they settled on a two section filter, Typically series 10k, 0.005uF to ground, then another series 10k and another 0.005uF to ground.
Of note, to calculate the time constant of this sort of filter it is:
R1(C1 +C2) + R2C2.
Some set makers went further to a three stage filter.
The typical recommended time constant for this application is around 50uS.
It seems in the post WWII period, British set makers were slower to catch on to this notion, but by the late 1950's, most sets had better vertical integrator circuits in the vertical sync separators, and interlace issues practically vanished, except for issues of triggering from a non sync related source.
Of note, to calculate for a three section filter, if you want to "go all the way" the time constant is:
R1(C1+C2+C3) + R2(C2C3) +R3C3.