Infinite window TBC

[DVtyro]

What does it mean? That is, what does it do and how does it work. Take this statement:

A time-base corrector, or TBC does one job: It corrects inconsistencies of an analog playback VCR. It accomplishes this by sampling one video scan line at a time (not an entire frame), corrects any instability by stripping the scan line of it’s old sync signal and replacing it with a brand new sync signal created by the TBC’s own built-in precise timing clock. Modern day TBCs, with their vast digital memory, can correct up to an entire field (262.5 scan lines) of incoming video. These are known as “infinite window” TBCs.

How come an field-size window becomes infinite window? Is there a qualitative jump from fewer than a field to a whole field? What exactly is infinite? Is it just a colorful figure of speech, or is there substance in it? I am going through Broadcast Engineering issues starting from 1972, but so far haven't found an explanation of the term.

[old_tv_nut]

I would take it to mean that the incoming signal can have ANY time offset with respect to the local plant. If you have less memory, you can't delay the incoming enough to phase it with the vertical sync, only the horizontal.

[DVtyro]

Originally Posted by old_tv_nut

I would take it to mean that the incoming signal can have ANY time offset with respect to the local plant. If you have less memory, you can't delay the incoming enough to phase it with the vertical sync, only the horizontal.

But then a TBC would have to drop or insert frames, right? So "infinite" does not mean no data is lost. It means that data is lost or inserted as complete frames without disturbance to the sync?

[Electronic M]

Originally Posted by DVtyro

But then a TBC would have to drop or insert frames, right? So "infinite" does not mean no data is lost. It means that data is lost or inserted as complete frames without disturbance to the sync?

Not really. They probably had to have 2 fields of memory minimum to do 1 field of TBC so one could be read in as another was being read out, or write over the beginning of the last frame as it's end was being read out and possibly adjust the windowing to prevent different fields from interacting. Broadcast VTRs with a TBC could be out of phase with master sync at the station causing a need for a fixed partial frame delay to match phase, but if they were off frequency to the point that frames had to be dropped or doubled periodically the station would consider the VTR defective and send it to be fixed. Prior to field duration TBCs all broadcast VTRs HAD to electromechanically sync up their servos to the station vertical sync and would do their best to match horizontal (single line TBCs were needed to make that work completely consistently). There probably would be some provision for field doubling/dropping for decks being used for trick play to air operation like pause, slow motion and reverse play where the deck could be changing speed away from standard play rates.