I wish to know — in my own particular case — why the conversion of VOB files to mp4 files using ffmpeg results in files of only about one quarter the size, whether this reduces the quality and, if so, how to ameliorate the effect.
The commands I am using are of the type:
ffmpeg -i VTS_01_1.VOB -q:v 0 VTS_01_1.mp4
I understand from my research that “-q:v 0” specifies maximum quality.
I also gather from the answer to a related question that the bitrate and codec could be affecting the reduction in file size, but I have no idea how to experiment with these parameters in my case.
I am converting commercial DVDs produced by a friend who is a retired Hollywood technician. The DVDs are instructional DVDs for a particular textile craft and typically involve two, three or four discs, made on a Mac using an old copy of Final Cut Pro. With the decline in DVD drives on computers my friend has started also to distribute the files on thumb drives after I suggested I could convert the DVDs to mp4s. I had played with free software for my own purposes, although I know next to nothing about video, but I am familiar with the unix interface on my Mac and discovered that ffmpeg seemed better than the GUI software for converting and then concatenating the various VOB files. The reduction in file size seemed a bonus as it allowed him to use 8Gb thumb drives. For some of the DVD conversions he felt there was no loss in quality, but for the latest one (done exactly the same by me) he reckoned some loss was noticeable (size reduction from about 10Gb to 2Gb — a bit more than usual). The “Get Info” on the videos includes the following:
Kind: MPEG-4 movie Dimensions: 853 × 480 Codecs: AAC, H.264 Audio channels: Stereo