Recompressing HD Movies

I’m (slowly!) working on ripping all my DVDs, converting them to VC-1 WMVs, and storing them on my computer so I can be lazy and have access to all my video entertainment on my 360 over the network. In so doing, I’ve been tempted by the recent offers on the 360 HDDVD that Amazon and places have been running. My question is whether or not HD-DVD’s “recompress”. A standard MPEG-2 DVD will give me about 50% compression when I run it through Windows Media Encoder choosing VC-1 (takes forever, but that’s a different matter), so the 500GB drive I’m looking at to hold things is roughly 100 DVDs worth of stuff: Close to my whole collection, and another 500GB isn’t that much more if I need more. However, HD movies start at 20GB raw images and go up. That’s fine if those 20GB are stored in an inefficient format. If, however, they’re already in VC-1 or something else which is essentially as compressed as it gets, then I probably want to wait until hard drive space is a bit more prevalent. A 20-1 ratio for the movies seems a bit extreme.

Anyone have any experience with ripping/re-compressing HD like this? I’m not particularly interested in the tricks that go on to put HD stuff up on the web (3/4 resolution, etc…)

HD-DVD movies are mostly already compressed with VC-1 or H.264. That doesn’t mean you can’t compress them even further just by varying the bitrate, but they would suffer a loss of quality beyond the expected slight loss from transcoding.

(I think MPEG-2 is still an option with both standards, but from what I recall reading, it’s mostly Blu-Ray titles that are still using it.)

Yeah, you can re-encode them with a lower bitrate and get a smaller file size. I think it’s fairly common to see HDDVD or Bluray sources get re-encoded with a low enough bitrate that the resulting video file can fit on a 4.38GB DVD (or 7.95GB DVD-9)

I thought they were supposed to stop using it?

What do they end up looking like? They’re aren’t a whole lot of HD/Blu-Ray rips online, but most of them are still 20 gigs.

Not easy to describe. The best I can manage is “What you’d expect a DVD movie to look like if DVD’s used h264 instead of mpeg-2.”

Not bad, I guess.

Well, except why rip a HD source then downsample it to DVD resolution/clarity? Just, y’know, rip the DVD and leave it in MPEG-2 at the same size and voila.

Sounds like there’s nothing to be gained here. My money’s safe for a while longer.

Maybe I’m not following, but wouldn’t the signal still be at DVD resolution? If you’re using a better compression algorithm, then ripping an HD source and compressing it down to DVD size (in terms of bytes) should look better than DVD signal using older compression.

If the ripped HD source is digital and you’re using the same algorithm that it was encoded in to compress it, it’s possible there might not even be any transcoding loss (depending on how the algorithms work, they might just drop more terms that are close to zero in the transformed space).

You’re not gaining anything by having higher resolution if it still looks like DVD quality. You just have to push more pixels through whatever is processing the video to get the same look (by definition). Now it’s certainly possible that the new compression algorithms are good enough to preserve some slightly better looking copy from a hi-res sample at that type of extreme compression. I haven’t seen things like that, so I’ll just take Roger’s word for it. But if it truly does produce DVD quality rips at (raw) DVD sizes, that seems like the very definition of futility.

You can get better-than-DVD quality using the newer codecs on HD content while remaining within DVD file sizes, if you want. .264 and VC1 both assume considerably more processing power is available for realtime decoding than what MPEG-2 required.

HD also has a wider color palette than standard DVD. I don’t know how compression impacts that. IMO I think the color depth improves quality as much as the resolution boost.


Well, that’s obvious. I don’t know why you’d assume it would look like DVD quality, though. It all depends on how much of a delta there is between newer coding schemes and MPEG-2. If there is a big difference, then you could afford to encode a higher resolution without the noise being too bad. Otherwise, the coding advantage would just go to reducing MPEG-2 artifacts, which don’t seem that bad to me on most DVDs.

Right. I’m obviously not doing this as I couldn’t be bothered, but if you’re at home and you want to back up your HD movie collection, it’s not bad.

Online, given the choice between a 4.7 gig HD rip that looks a tiny bit better than DVD, or a 700 meg DVD rip that looks fine, I’m going with the DVD rip.

Bah… now I want some comparisons of a HD source compressed with H.264 and the comparable MPEG-2 source.

While I can definitely see the difference between HD and 480P sources, it is reasonably subtle. I can’t help but wonder if that type of subtle visual difference wouldn’t be the first casualty of heavy compression. (Which, regardless of the algorithm, is what we’re talking about here. The MP2 equivalent of a HD movie at 20GB would presumably be 40GB. Going from 40GB to under 10 is a 4:1 compression ratio. I wouldn’t expect to compress my current 480P/SD DVDs 4:1 without having visible quantization.)

Honestly, both look like shit compared to a commercial DVD encode. I’ve downloaded a few 720p x264 and VC-1 movies that targeted a single layer DVD and they looked awful. While technically higher in resolution, the compression artifacts made the HD version much uglier than what you’d find on a regular DVD. Likewise, the “near DVD quality” divx films targeting a single CD look atrocious. I don’t bother anymore. I’d rather wait for stuff to work up my netflix queue than bother with that crap anymore.

Some older BDs were using MPEG-2. The newer ones I’ve seen all use VC-1 or MPEG-4.

They look fine to me. I watch them on my PC monitor, so they don’t get blown up huge.