As every superhero knows it's the second half of line 23, field one, that active content starts in a PAL signal yet come the start of field 2 line 336 starts as a full line with the corresponding half line at the end of field 2 on line 623. Consequently without a reference signal the only way to tell the difference between field one and two is by the half line at the top of field 1 (well, the broad pulses at line 3 vary but most equipment is field-locked by the time you get to that point in the scan).
We've been testing an ingest/archive solution at the workshop for an African state broadcaster who have a large analogue archive (SVHS and BetaSP). Capturing off the SVHS deck they'd provided for testing (a Panasonic AG-7550) we got some very strange effects. The route is this;
Analogue VT -> AJA FS/1 processor -> SDi into Content Agent uncompressed AVI
this is then compressed to 50 Mbit/s MPEG2 transport stream (I-frame only) and mux'ed into an MXF OP-1A and onto the shared storage. The file is then QC'ed on another machine with a Decklink SDi o/p running OpenCube MXF playback software. Both the input to the CA and the output of the QC are displayed on a JVC DT-V24 video monitor and Tek WVR5000 waveform.
All the clips captured off the SVHS were field-reversed by the time they got to the QC and so we assumed that there was some problem with the capture. After a lot of testing and head-scratching I discovered that the internal TBC on the Panasonic was removing the half-line at the start of field 1 and from then on the capture was marking F1 as F2 and vice-versa.
If you look at the output of the QC machine you can see the half line at the top of the frame, but the motion of the replaced video suggests the fields are reversed. In fact the fields are in the correct order but the capture card has marked them incorrectly and so by the time they are multiplexed and played out they are in the wrong order. The Tek shows the missing half-line at the start of field 1 (on input) and since there is a half line on replay it seems it must be there on field 2; hence the confusion by the capture card.
Turning off the AG-7550's TBC and relying only on the FS/1 showed the half line return to the start of field one and the problem disappeared.