You are really read a lot and do some research.
The first thing is to check the TV aspect ratio setting on BDP and AVR. In the BDP and AVR the aspect ratio can be set for the target TV. The aspect ratio setting can be original, letterbox, pan & scan. TV will also have the aspect ratio setting such as 16:9, Just, 4:3, 14:9, various Zoom's, etc.
Original means the source material original aspect will be shown as is.
Letterbox means adjusting the widescreen original material for standard 4:3 TV. It is a method of presenting widescreen images on a standard screen television. In order to preserve the aspect ratio of the original video content, the picture is scaled down so that it fits the available width of the television screen. Since the picture will not fill the screen vertically, dark bars are displayed above and below the picture.
Pan and scan is a method of adjusting widescreen film images so that they can be shown within the proportions of a standard definition 4:3 aspect ratio television screen, often cropping off the sides of the original widescreen image to focus on the composition's most important aspects.
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The second is source material aspect ratio.
Movies are recorded in different format 1.85:1, 1.78:1 (standard 16:9), 2.35:1, 2.41:1, etc. Only 1.85:1 and 1.78:1 will fill the widescreen TV. The rest will have black bars on top and bottom. Some old movies are in 4:3 format also.
For TV, the SD video is in 4:3 format while HD video is in 16:9 format.
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Now depending upon the aspect ratio setting and source material's original aspect ratio, you will get different formated output.
The whole description is too large to explain and quite elaborate too. Please read the user manuals of BDP and AVR to understand. Also search on web, wikipedia, etc.