How to choose a bitrate when converting between formats?

Question

I have a number of video files I need to convert to "regular" .mp4 format to view on my computer. The format they are in is not supported by my device (the driver has not been backported to 18.04). I have a VM running a later edition of Ubuntu that can convert them, but this raised a question about the bitrate.

I don't want to "worsen" the quality any more than the conversion would already do, so how do I choose an output bitrate that will contain all the information of the input file (especially as I'd imagine that the bitrate is also a function of the compression efficiency rather than the "raw" datarate)?

Am I correct that the bitrate is not the same measure when the formats are different, and if so, how do I ensure that my output quality contains the same information without increasing the file size too much? Is there a tool for that (for Ubuntu, of course)?

Also, VLC seems to automatically pick a bitrate which is larger than the input value and appears to vary between files (with a corresponding size increase after conversion). Would you know if it is doing this calculation automatically, or is it just padding my files?

Answer 1

Bitrate is not equal when formats differ. Some codecs are more effecient and thus less 'lossy' in terms of how they compress the data.

Secondly, the software you use is a factor in terms of output quality when the source codec and bitrate rates are equal. Simply put, h.264 1080p29.97 at 25mbps CBR (constant bitrate) may look better when encoded using one software over another. VBR (Variable Bit Rate) is another factor. It's more effecient, but it's more susuptible to issues if the software you use doesn't handle the temporal analysis between side by side frames.

Regarding you last comment on padding; yes you can certainly bloat your file size for no good reason, and adding that additional data will be all but un-noticable to even the best trained eye. Even on a scope the file which has more padding may be a near identical match.

So it's a matter of trial and error.

If you're starting with 444 Raw Source, you can certainly compress down to H.265 at 100 mbps CBR for 1080, or 200mbps for 4k, and you'd have a hard time, if even possible, to see the difference.

But your new file would lose a lot of bit depth data in terms of the RGB values and the way that data is stored. Your source file would allow for a high lattude of color correction, exposure correction, etc.

Your output, while visually indistinguishable from the source... would not allow much color correction at all.