[Argus25]

The problem really is that good interlace cannot be guaranteed unless the vertical sync is correctly filtered off the main sync pulse train, this is especially important when the signal has equalising pulses.

These are the facts: one stage of RC filtering , even with the correct time constant of around about 50uS is only just adequate. Two stages of filtering using the formula I mentioned earlier in this thread is much better, but three stages is perfect.

The reason is that with only one stage of filtering, by the time the serrations from the H & equalising pulses are properly filtered the vertical pulse is low in amplitude with a slow rising edge, asking for interlace troubles an poorer vertical look. A three stage filter does not have these issues and is a much more effective BPF for the task, enabling a fast rise V sync, free of serrations too.

RCA settled on a standard 3 stage filter, it had three 10k resistor's and three capacitors (I could post the values if anyone is interested). This "standard network ultimately appeared in many monochrome valve TV's and later was built into hybrid modules used in many Japanese TV's and VCR's.

The thing that is odd, despite in the USA, most set makers using this (on RCA's advice ) and most television tech training manuals teaching it as standard practice, British manufacturers paid little if any attention to it and most just stuck with one stage filters (and occasionally two stage filters were used which were better). Also, there appears to have been a complete absence of the knowledge of the the fact that poor interlace results from a poorly designed vertical sync filter, none of my British text books refer to it, or mention it as the primary cause of trouble and it appears that nobody on the forum knew about it either and had invoked all other kinds of causes for faulty interlace, when the topic of poor interlace came up. So it might have not been standard practice to teach this to TV techs in the UK.

So many old vintage UK TV sets can be improved for perfect vertical pulse separation and perfect interlace, just by modifying them to a simple three stage filter.