Same device serving different field orders?

[bigass]

Hey, gang.

I've been on a roll with capturing, processing and encoding lots of VHS and 8mm/Hi8 in recent weeks and months, and thought I was really getting the hang of my workflow.

Part of the process most of the time has been deinterlacing with Hybrid's Vaporsynth implementation of QTGMC.

I've been going Camcorder -> via Svideo -> TBC1000 -> Svideo -> Startech USB3HDCAP into Virtualdub2 in MagicYUV 422.

In pretty much every case that I've used this capture device, forcing top-field-first in Hybrid gave me the right results.

But my latest batch had two tapes that didn't look quite right in the end ... so I went back and re-processed the raw captures with bottom-field-first, and everything suddenly sat right.

Could someone help make sense of this? Same camcorder, same capture device, same all along the line except it's a different tape giving the opposite field order. If this is just "how it is", is there a reliable field order check I can try before deinterlacing in order to save a re-do? Or is TFF the rule, barring surprises?

[jagabo]

It's unusual that a capture device would change field order with different caps. You occasionally see a few frames that are wrong when sync errors cause problems.

An easy way to check field order in VirtualDub is to apply the Bob Doubler filter. Then step through the video field-by-field with the right arrow key. If the field order is wrong (as set in the Frame Doubler filter dialog) you will get jerky motions (two steps forward, one step back, two steps forward, one step back...). The motion is easiest to see in a horizontal panning shot.

[bigass]

Thanks, as always, Jagabo. I spotted that kind of stuttering as I was relearning my way through the tools ... I wasn't sure that what I was seeing was wrong field order, a framerate problem, or some other thing I set wrong. But when I did an a/b test of field orders, bottom was the right one this time on these two tapes, while top was the order of the day every other time. I dunno. Maybe the client had a quirky camcorder back in the 90s. Just glad I dug further instead of delivering something broken.

[jagabo]

A frame of digital interlaced video contains two half pictures, called fields, one in all the even scanlines, one in all the odd scanlines. The two fields are supposed to be seen separately and sequentially. The field order tells you which of the fields is supposed to be seen first. If the field order is wrong every pair of fields will be swapped temporally -- instead of seeing fields in the order 0, 1, 2, 3, 4, 5... they will be seen 1, 0, 3, 2, 5, 4... leading to jerky motion. At 50 or 60 fields per second it usually looks more like a flicker. But when slowed you can see the obvious back and forth motion.

Note that the analog signal doesn't contain frames. It's just an alternating sequence of top and bottom fields. It's the capture device that weaves pairs of fields together into frames and decides whether to start with a top field (creating top field first frames) or a bottom field (creating bottom field first frames).