Is it helpful to compress the DVDs with DVDShrink, Rcode, etc. before using RatDVD?
Absolutely No! These programs compress the DVDs already on cost of quality. You’ll get a RatDVD with a higher quality at a better compression/size ratio if you use the original DVDs as input material. Furthermore the resulting DVDs compressed with these programs are known to contain some structural errors that may cause errors when converting into RatDVD, playing a RatDVD or converting from RatDVD to DVD. See here for more details.
