[bcn_246]

Widescreen NTSC DVDs are not quite 16:9 (1.77778:1). The standard NTSC DVD has a resolution of 720x480. It also has a signalled Pixel Aspect Ratio of 40:33. This tells the decoder to stretch the image out horizontally. This makes the actual Aspect Ratio 1.81818:1 (720/480 = 1.5, 1.5 x (40/33) = 1.818...).

480 x 1.81818 = 872, so 480p DVDRips should really 872x480. However, 872 is not mod16. Ideally clips should be mod16, however with newer decoders it seems far less important (I have yet to find any player that struggled with anything other than mod2). If you want to be safe, go with 864x480 (1.8:1) and crop to reduce error.

It is worth bearing in mind that some mastering facilities may have made the mistake of assuming a DVD to be 16:9 also. When I can I have a look through the DVD for squares or circles to check what the actual aspect ratio is (logos in the credits are always a good indicator). If you do find the DVD has been mastered at 16:9 then I would go with 852x480 (again, not mod16, so go with 848x480 if you want to be safe).

It is also worth bearing in mind that when muxing to MKV (with MKVToolNix GUI) you can manually set the playback Aspect Ratio (see screenshot). While I don't use this to encode at Aspect Ratios far off (not all devices read this header correctly) it is useful to get things exact. For example, if you encode at 848x480 and wish to have it (when possible) be resized to 1920x1080 on your display it is worth setting the flag (although an error of 0.63% is really not much of an issue).

Also, for reference PAL DVDs are also not exactly 16:9 (1024x576). They use an aspect ratio of 16:11, so 576p content should be really be resized to 1048x576 (or 1040/1056 if you wish to keep it mod16).

Hope this helps explain things,

Ben