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Comment by holy_city | Hacker News Reader

Having worked a bit in the field (on the audio side of film) "perceptually lossless" means that in a double blind study, an audience would not be able to discern between the lossless and the lossy encoded versions. The only way to verify is by testing with real people, and due to the nature of the business, it's easier to do it with professionals who have a higher bar for perception.

There are a lot of reasons to prefer lossless over lossy, but there is always the "good enough" point with storage media. Film is not lossless, so it doesn't matter if the digital storage is. What matters is if the lossiness in encoding is good enough to work with at the same level as film.

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The industry also prefers to use people with "golden ears", so they can be pretty confident that the target audience cannot tell the difference.

There's a weird bar for confidence, to be frank. Most often I've found that people disregard studies with people who have "golden ears" because they're not average ears, or that studies find that only those with "golden ears" can find a difference, and the audience tends to feel they're in that range.

For example, there was a study a few years ago at McGill with trained listeners on the effects of bitrate with mp3 and m4a audio encoding that found that only a slim number of mastering engineers preferred lossless over lossy encoded audio (interestingly enough, some professional mixers and musicians found lossy encodings preferable to lossless even for jazz and classical music). But trying to convince audiophiles that those codecs are comparable to lossless is a losing battle.

As someone with minor audiophilia, there is still some difference; when you have a lossless version, you can reencode to any lossy version without loosing quality over time.

I can transcode FLAC to Mp3 fast enough to stream it on my phone without issue, if I decide to use OGG or Opus or anything else, I don't have eventual reencoding issues.

For archival purposes, it makes no sense to store anything less than lossless.

But what happens when you make multiple roundtrips with this perceptually lossless codec? Is it still passing the test or does it degrade over time?

Quality degrades, just as it would if you did the same with film.

It's not a given though I imagine. For audio for example you could discard inaudible parts of the spectrum and compress the rest losslessly. In this case only the first round is lossy and it doesn't degrade over time. But I'm not sure the same exists for images.

In the VFX industry you don't roundtrip lossy codecs; first step is to decode to something lossless like EXR or TIFF, and those are what you pipe through your workflows. (With maybe lowres JPEG "proxies" to make iteration a bit quicker.)

Film/TV more generally I think tolerates slightly lossy codecs like RED or Pro Res, as there's often only 1-2 intermediate steps that could cause extra loss. (E.g. the part of editing that is just 'cutting' is pass-through, but color-grading would require a second encoding step.)

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