I have an old SDTV with a BD player (Panasonic DMP-BD80) connected. I noticed that when I play DVDs on this player, sometimes there is a judder. However, if I set the "Progressive" option in the playback display menu from "Auto" to "Video," the judder goes away.

This setting is described here in the manual. What exactly does it control, and why does setting it to video make the judder go away?

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This is because a CRT SDTV always runs in interlaced mode, and the "video" setting makes the player run in interlaced mode also. They probably avoided the terms interlaced and progressive because most owners would not now how to determine which their TV is.

The other modes sometimes (or always) run in progressive mode, which will work with some CRT HDTVS and most flat screen TVs. But, the result may be distorted depending on the combination of disk coding and and the TV used. Hence the instruction to adjust for least distorted mode.

Since it's outputting to an interlaced TV, it was probably deinterlacing interlaced footage then reinterlacing on output, which produced a judder.

I'll just leave it on video then.