Hi Brent - thanks for the compliment! I must say you take great care to make sure you post accurate information, and you spot many errors in other people's postings too.
To answer your question about me, no I don't regularly post elsewhere, although I have occasionally done so, on discogs.com for example.
Correcting for stretched or squeezed imagesI know almost nothing about ripping DVDs and Blu-rays, but I guess with the right ripping and burning software which gets around copy protection, coupled with the right video manipulation software, it would be possible create discs with corrected aspect ratios. A simpler solution would be to use hardware that has fine user control over the aspect ratio. My own setup is quite basic (Panasonic Blu-ray player + Panasonic TV), and the only AR changes I can make on it are to stretch a 4:3 image to 16:9 or squeeze a 16:9 image to 4:3. These options can be used for example when a disc's anamorphic / non-anamorphic status is incorrectly flagged. But I think some TVs have more comprehensive options, although I can't name any particular models. Certainly some software media players have the option for variable stretching, if you are able to watch movies on a computer.
Why DVDs might have vertically stretched imagesI can't say I've noticed this issue myself, and in general I don't really notice this kind of thing unless the change is quite large, but you might be correct. A possible reason for this is that mistakes might often be made during the process of creating the DVDs, perhaps during the transfer process when creating the digital master, or when authoring the DVD. Alternatively, or additionally, mistakes are perhaps being made in the software/firmware of players/TVs/applications, such that the image is not displayed correctly. But why might so many mistakes be made? A likely answer to this is that digital PAL and digital NTSC formats are more complicated than most people would imagine, plus there does not seem to be universal agreement about standards in this area.
The main issue is that with both PAL and NTSC, the pixels are
not square, but rectangular. It becomes obvious that this must be the case when you consider the dimensions of the image. For example with digital PAL, it is usually 720x576. This is a ratio of 5:4. Yet the image is displayed in a 4:3 area. Now you might think that the pixels are therefore simply wider than they are tall by the exact amount required to compensate for this. The amount necessary would be (4/3)/(5/4) = 16/15 = 1.0666... But in fact the truth is even more complicated than this. I don't know the exact reasons, but I think that some of the 720 pixels horizontally are considered to be outside of the 4:3 image area. It is beyond my knowledge, so don't take this as solid fact, but I think with analog TV, when the electron gun (which scans more or less horizontally across the screen from left to right) reaches the end of each line, it needs time to get back to the left hand side again. Additionally, due to variations with different analog TV models, in order to ensure the image fills the screen there needs to be a bit of extra image around the edge (this is known as 'overscan' - see link below). Therefore the image as displayed on a TV is in effect slightly cropped compared with the original signal.
An analog PAL signal has 625 lines in total, but only 575 of these are part of the actual image. In addition, each frame is sent as 2 fields, with one field containing the odd-numbered lines and the other containing the even-numbered lines. The electron gun scan is actually not quite horizontal but very slightly diagonal. The first line (or in fact half a line) of the image section of the first field starts halfway across the top of the screen and goes to the right. The next line of the first field starts at the left of the screen and moves right at a slight downward diagonal. The rest of the field is then complete lines, every other line, with gaps in between. The first line of the image section of the second field then starts at the top left of the screen, and the lines of the second field fill in the gaps left by the first field, with the final half line of the second field ending halfway across the bottom. Starting halfway across the screen with this slight diagonal direction allows the fields to interlace with each other (much like the red and white stripes of a barber pole, only with a much shallower gradient). The image section of each field is 287.5 lines, creating a total of 575 lines. For digital PAL, the 287.5 lines of each field is rounded up to 288, creating a total image of 576 lines.
I am not sure there is even a true industry standard regarding video pixel aspect ratios, but one standard that I have heard is that PAL pixels have a width:height ratio of 59:54, while for NTSC pixels it is 10:11. Assuming these are correct, then if we use the PAL pixel ratio and assume that in a 4:3 image we can see all of the 'scan lines' (in other words the vertical resolution is 576 pixels), the number of pixels horizontally will be 576 x (4/3) / (59/54) = approx. 703 . Since the total image is 720 pixels across, this means that to display the image correctly in the 4:3 area, about 17 pixels need to be cropped off. But suppose for example that instead the whole 720 pixels were placed in the 4:3 display area. Then the image will have been slightly squeezed horizontally (which is equivalent to being slightly stretched vertically).
The above is only an example for illustrative purposes, but given the complications of dealing with non-square pixels, plus the overscan issue, plus the fact that there seem to be different competing standards of video pixel aspect ratio within the industry, it is perhaps not surprising that the end result is sometimes inaccurate.
Wikipedia has a couple of good pages:
video pixel aspect ratios:
https://en.wikipedia.org/wiki/Pixel_aspect_ratioTV overscan:
https://en.wikipedia.org/wiki/Overscan