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- All the old TV standards adopted around the world have an odd numbers of lines in a "complete" picture, which is defined as a TV Frame. (25Hz Europe, c.30Hz USA)
- To conserve RF bandwidth, the signal is transmitted in sequential Fields, each of which contains half the lines (odd lines Field 1, even lines Filed 2 etc), as required to assemble the full TV Frame.
- This is known as interlace scanning, and gives rise to display artefacts such as line-twitter on strong horizontal black-white transitions.
- As the odd number of lines is clearly not divisible by 2, odd fields terminate (theoretically) half way through a line, and even fields commence similarly.
- The problem confronted by TV engineers is to get the 'scope to trigger stably (ideally) at the FRAME rate, which is half the field rate. This allows a 'scope with a good dual-time-base delayed sweep to select a particular line. (The line can be a bit dim, as the 'scope is only writing for c. 60uS in every c. 40mS).
- I have a 453 with TV triggering ( MOD127C ): Tek used to supply special graticules (which I also have) based around standardised TV test & measurement waveforms.
- broadcasters use a few of the "blank" lines in the field blanking area* to transmit the test waveforms so an entire transmission chain can be quality monitored

I can supply a copy of the MOD127C 453 handbook description to anyone who's interested.

John

* the "field blanking" lines contain synchronising pulses which are "broader" than on normal video lines, and the shift in DC level is used by the sync separators in the TV to lock the frame oscillator to field rate. They were kept free of video in order to allow the CRT scan time to return to the top of the screen without losing active picture content.