The RP91 has the following scaling (in progressive mode).
1. 4:3 - This takes the full image and scales down horizontally only, just enough to place the image in the center 4:3 area of a 16:9 TV. This can be used for material that is truly 4:3. The RP91 then adds vertical left and right gray side bars.
2. Zoom - This takes the full image and scales up vertically only, just enough to make 2.35:1 non-anamorphic titles have the correct aspect ratio (circles look like circles, not ellipses). This is the main scaling feature that people like about the RP91. It is KEY if your TV locks into full mode when receiving a progressive signal. Without this feature, in that situation, you could either (a) watch squashed-looking people or (b) switch to interlaced and let your TV's scaling adjust (generally much worse than the RP91's scaling quality).
3. FULL - This manually forces displaying the entire image (720x480) into the 16:9 visible area for non-anamorphic titles. Note that on ANAMORPHIC-flagged titles, the scaling is DISABLED - you always get FULL, which is exactly what you want for proper aspect ratio.
4. AUTO - This mode will select between 1, 2, and 3 above automatically based on content and flags. It works very nicely. I have only had to manually select on a couple of DVD titles.
The RP91 has no other scaling features, and none for interlaced mode. This will not allow you to selective zoom just enough to eliminate the smaller top/bottom black bars on 2.35:1 films shown on a 16:9 TV (~1.78:1). As with all DVD players, if you set your TV type to 4:3, then it will scale anamorphic titles vertically only to properly show the aspect ratio. This results in top/bottom black bars, which is required to get the aspect ratio right.