Distortion at bottom of VHS captures--causes? solutions?

[mcpogue]

Caping from "clean" VHS source JVC SR-V101US with TBC/DNR enabled, S-Video Out to AVT-8710 S-video Out to ATI AIW 8500 128

Experiencing distortion at very bottom of frame, capturing at 704 x 480

Rest of image looks as good as VHS can

Does this with all tapes...I'm guessing this is normal and I should just crop or mask...but not completely sure.

[thecoalman]

If it's just a small area like 10 pixels that's normal. You won't see it on a TV as it's in the overscan area, if you crop the video you'll actually be losing viewable video for playbasck on a TV. If your going to view on computer the mask is the best alternative, you can just leave it alone for normal TV playback.

[lordsmurf]

http://www.digitalfaq.com/dvdguides/capture/understandsource.htm#overscan

[Cornucopia]

Head wrap / Interchange / Tape skew. That's the way it is with analog tapes, particularly VHS (BetaSP is much better). A TBC can help somewhat, but it's never completely gone.

My preference is to mask it out. On analog/interlaced it isn't visible (so it doesn't matter what's there), but it's annoying on digital/computer monitor. As most all of my caps go to DVD/VCD or WMV files, I want to use the available compressed bitrate absolutely as efficiently as possible, and that means MASKING. You've got a constantly moving segment down there that sort of resembles random noise (at least to the encoder), and you don't want that sucking up your quality with needless quantization.
Usually, I do a multiple of 2 or 4 lines(don't want to accidentally flip field order), depending upon the severity. Don't CROP without masking, because you'll often mistakenly be resizing the final image, which introduces blurriness due to interpolation.

Scott

[lordsmurf]

I don't see how masking would affect field order.

[Cornucopia]

Masking doesn't affect field order, cropping can.

The # lines of masking affect where the black falls on macroblock boundaries, so affects quantization/bitrate allocation.

Scott