That's fair. Although a lot of BluRay rips are re-encoded and seem quite usable. Even the YiFY stuff at 1GB/hour or so.
Edit: Why can't a compressor perfectly re-compress the decompressed audio? It's obviously possible since the compressed data exists producing that specific decompressed data.
> Why can't a compressor perfectly re-compress the decompressed audio?
> It's obviously possible since the compressed data exists producing that specific decompressed data.
It's not granted that a compressor c1, that given A produces Az that decompress with d1 to A', can easily find any Ax that compress to Az, or equivalently can easily find Az given A'.
Formulated like that it doesn't seem quite so obvious: finding Az from A' amounts[1] to finding A from Az -- ie: lossless compression.
> Although a lot of BluRay rips are re-encoded and seem quite usable. Even the YiFY stuff at 1GB/hour or so.
Usable at any given viewing/listening set-up != actually remotely "good enough". I always say people shouldn't buy more expensive hi-fi gear than what they can actually tell apart -- the one problem with that (apart from people not being honest with themselves, optioning for the more expensive stuff anyway) is that when you're used to listening to crappy audio, you stop being able to tell the difference.
It's like listening to an FM radio that's slightly off station -- after a few hours, you probably don't notice anything wrong, until a new person walks into the room and adjusts it to be better.
Another point -- while BluRay certainly isn't lossless -- when you're talking the kind of compression/quality differences you mention (not sure what regular bluray films are, but if they max out at 48mbit/s for AV, that's by my calculations about 20GB/h) -- 1:20 -- I think you'd be hard pressed to notice any "additional" artefacts. It would be like comparing a raw/flac audio file compressed first to 320 kbps vbr mp3, and then compressed down to 16 kbs mp3, versus just doing the compression to 16 kpbs mp3 (well order of magnitude is correct, obviously this is going to be mostly cutting into the video data, but still). Just something to keep in mind.
With vanilla JPEG, you should be able to redo the DCT and find the quantized values exactly as they were, which means that you could losslessly reverse the JPEG compression not in the sense you get a compressed version that decompresses to the same lossy reconstruction.
With deblocking filters in MPEG2 and later, this is not necessarily the case, because you try to smooth things over in decompression and can't reconstruct the compressed version either.
Because the media uses lossy compression, the uncompressed version of the compressed file still contains artifacts of compression. If you the re-compress the file, so add even more artifacts.
A good example of this is the guy who re-uploaded the same video to YouTube many times.
The GP's question isn't if current re-encoders are capable of recovering the original compressed data, but rather if it's theoretically possible to write a decompressor that given the parameters to the psyco-acoustic model that's introducing most of the artifacts, is able to produce a compressed file that still has artifacts, but no new artifacts.
Yes, it sounds theoretically possible, but it may involve searching a huge search space and may be computationally infeasible.
If I'm being fair, audio encoders are more advanced than video encoders, and we can probably shave more off with psychoacoustics than we can with our current understanding of psychovisuals. On the other end of the scale, video encoders have to process orders of magnitude more data, in more dimensions!
For example, Opus's CELT encoder uses lapped transforms and keeps about the same level of constrained-energy. Combined with the (hybrid) voice codec in some very advanced ways, it makes for the most advanced audio codec around by quite some margin.
You look at video, and nothing's that good yet, not even HEVC. The only thing that leaps to mind is Xiphophorus's Daala project - https://www.xiph.org/daala/ - which is hoping to do for video what Opus did for audio and develop a royalty-free, awesome video codec (rather than being donated a royalty-free, okay two or three) - and that's I'd say one or two generations ahead of HEVC, but of course, still very very early work.
Edit: Why can't a compressor perfectly re-compress the decompressed audio? It's obviously possible since the compressed data exists producing that specific decompressed data.