DVD vs XviD vs DivX ?

[richbell]

Hello Friends,

I am back to this forum from vcdhelp.com days . Great to see the site grown to this level. I am still a newbie in video formats, so bare with me and direct me if my questions are already answered.

1. I would like to know how we distinguish the video quality? i guess it's with bitrate. Is there any universal standard for video formats associated with bitrate? and tools used to calculate the bitrate for the video files we have?

2. I know DivX and XviD are two latest formats for better compression of DVD. My question is - Is there any image quality loss in these formats compared to DVD standard? All other feautures are supported like subtitle, menus etc? Also could we re-encode back to DVD formats from DivX or Xvid without loosing image quality or features of DVD?

Thanks in advance.

[Jeremiah58]

1. ...tools used to calculate the bitrate for the video files we have?

Avicodec, GSpot...

2. I know DivX and XviD are two latest formats for better compression of DVD.

h264 is probably more recent and advanced.

Is there any image quality loss in these formats compared to DVD standard?

I would say yes, but that depends on too many parameters.

Also could we re-encode back to DVD formats from DivX or Xvid without loosing image quality?

Image quality depends on a source file and every conversion between formats looses some quality.

[jman98]

Hello Friends,

I am back to this forum from vcdhelp.com days . Great to see the site grown to this level. I am still a newbie in video formats, so bare with me and direct me if my questions are already answered.

Not to state the obvious, but if you hadn't done what you and so many others do of joining the forum and then disappearing after you ask your one or two questions that you care about, you wouldn't still be a newbie after 3 years. It's not my intention to criticize, but the less you participate, the less you'll get out of it and you'll STILL be a newbie 5 years from now if you disappear after reading the answers to this post. And if you say that you didn't disappear shortly after you joined, then why on earth are you still a newbie 3 years later?

1. I would like to know how we distinguish the video quality? i guess it's with bitrate. Is there any universal standard for video formats associated with bitrate? and tools used to calculate the bitrate for the video files we have?

Generally the higher the bitrate, the better the video, but I can't give you what you want - a checklist that says if the bitrate is over value X, then quality = great. It all depends on what was used to encode, the source, and so on. For example, you could encode a truly awful quality old VCR tape to DVD with a video bitrate of 8500 Kbps and it would still look like crap because the source video is crap. However, it might be an extremely well encoded piece of crap. Encoders vary in their quality and options used by the person doing the encoding can make things better or worse, so there's no one size fits all guide I can give you. The links at the top left for "What Is" tell about various formats and anything with video bitrates towards the higher end of each format should generally look better than video with bitrates on the lower end of the scale.

Download and install the Gspot program and open your videos with it to see their bitrates.

2. I know DivX and XviD are two latest formats for better compression of DVD. My question is - Is there any image quality loss in these formats compared to DVD standard? All other feautures are supported like subtitle, menus etc? Also could we re-encode back to DVD formats from DivX or Xvid without loosing image quality or features of DVD?

Thanks in advance.

Actually, DivX and Xvid have been around so long that you are showing your "newbieness" by calling them "latest formats". Is there any image quality loss compared to DVDs? YES! Will you notice it? It depends. With higher bitrate encodes, you may not notice the qualty loss, but it's still there, it just may be too small for you to notice. Can you encode back to DVD without losing image quality? Nope. The more encodes you make, the more quality loss you will get. Going DVD -> Divx -> DVD is not a good way to preserve quality. Can you re-encode without losing features of DVD? No, because you lost all those features when you encoded to Divx in the first place.

When I say "Divx", it means Xvid too. I'm just too lazy to type both.

[jagabo]

You always lose some quality when using lossy codecs like MPEG2, MPEG4 ASP (Divx, Xvid), MPEG4 AVC (h.264, x264). If you start with a pristine source and compress with Divx and MPEG2 at the same bitrate, frame rate, frame size, etc., the Divx file will look better. If you start with a DVD MPEG2 file and compress it to Divx there will be some degradation. Exacly how much depends on what settings you use. The same will be true if you compress it with MPEG2 again.

Normal DVD players don't play Divx at all. Divx Certified players will play Divx/Xvid within certain limitations (not all MPEG4 ASP features are supported). Divx Ultra certified player will play Divx with menus, chapters, multiple audio tracks, and subtitles. These must be created with software specifically designed for that purpose.

[richbell]

Thanks for the reply guys. I have lot of quality DVD backups from original DVDs and would like to store them on single place to play back on my new RAID box hence the question on what method would help on better compression as well as able to play them back again on normal DVD players.

I have a Sony entry level NS50 player. So i don't think it supports DivX/XviD. So as you seem to suggest i will drop the idea of converting them to any format to save space.

And like jman said, i will participate more in the site activity Only that i have been on and off with the site to learn some things quickly. Thanks for insisting anyway.

[EddyH]

1. I would like to know how we distinguish the video quality? i guess it's with bitrate. (etc)

I'd say really it's more to do with it's perceptual quality, i.e. how it looks to your eye when you play it back. The quality you get per a certain bitrate can depend enormously on the source material, the resolution and framerate, and the efficiency of the codec used. Similarly you could say very roughly that the faster the engine is running in your car, the shorter your journey time; and there is *some* correlation, but you've also got the size of the engine, how advanced it is, what fuel it's running on, whether it has a turbocharger, the weight of the car & it's load, how it's geared/what gear you are in, the roughness and twistiness and hilliness of the road...
(my old brick-simple 1.1 litre VW wouldn't be able to charge up Shap approach on the M6 anywhere near as quick fully loaded at 5000rpm (3rd gear), than a lightly loaded, turboed, multi-point, 16 valve version of my current 1.6 GM would be able to at 3600rpm (5th)... just as a 640x304 DivX at 2000kbit will probably look a lot less 'compressed' than a 720x576 DVD at 3000kbit; but both are operating outside of their usual & ideal encoding environments there)

In any case, most video codecs will measure bitrate in kilobits per second, kbit/s, though there can be some dispute over whether this is 1000b/s or 1024b/s; others may use kbyte/s or even mbit/s for DV and HD. An internet video may be between a few tens and a couple hundred kilobits/sec, a reasonable DivX/XviD may run to a little under 1000kbit/s (1mbit/s), standard VCD is 1150, SVCD and DVD variable from ~1000 to ~9000kbit/s (2600 SVCD) depending on quality setting and source material, HD from 5mbit/s upwards, and DV ~25mbit/s

A better but still imperfect measure, for mpeg style codecs, is the average macroblock quantisation value for each frame (essentially a measure of how much the picture is corrupted by lossy compression at any point), which typically stays at a moderate level (potentially noticable but insignificant artefacts) if you use Constant Quantiser VBR, varies wildly between very low (super quality, imperceptable changes) and very high (awful - obvious mashing and blocking) with Constant Bitrate, with 2-pass VBR sitting at a semi-intelligently controlled level in between.

I don't know of any tools to measure / graph this quantity for MPG-4 (divx etc), but one little tool I'm trying to hunt out of my archive for use with MPG1 & 2 (VCD/DVD) is, helpfully, called BRView (BitRateViewer) EDIT! <---aha! Thank you, automatic tool link! *downloads* Used it to analyse XVCDs to great effect before, identifying bitrate pinch points, and whether encoding settings can be changed to improve poor-looking sections (lower overall VBR quality slightly so there's much more slack to spend on the few difficult parts, play with the minimum BR setting if it's evidently too much/too little), or if they're already maxed out bitrate-wise whether they should be pre-smoothed, whether the encoder is doing what it says, etc. There surely must be something similar for MPG4...

To calculate bitrates ahead of time, the simplest route (oddly) is really just to learn the maximum reliable capacity of your chosen media minus typical transport stream overheads, and how to take away the predicted audio size at a chosen rate in order to leave the space available for video, and to divide the length into this to get final video rate... Simpler than it sounds...
(e.g. VCD 800mb, -15mb rough TS size = 785mb... video length 90m (5400 sec), sound quality 192kbit/s (24kbyte/s), sound size = 126mb... 659mb for video, or in other terms, 125kbyte/s = 1000kbit/s... sorted in just a few seconds and a few button-taps, there!)

2. I know DivX and XviD are two latest formats for better compression of DVD.

In about 2001 they were...
(god damn, has it been that long already?)

My question is - Is there any image quality loss in these formats compared to DVD standard? All other feautures are supported like subtitle, menus etc? Also could we re-encode back to DVD formats from DivX or Xvid without loosing image quality or features of DVD?

Well, as a way of confirming your n00b status, that wins quite hard. Still I can't join in with JMan's complaint... like none of us have never done that? (Cmon, i bet even you have J, when pressed but didn't have the time or will to leap into yet another whole new online community). Ye without sin cast the first stone and all that cliched stuff.

Anyway...

Image quality loss - this debate could rage on, but basically, yes, in typical use - as they are normally seen, and indeed expected to be used in situations where the available bitrate is so much lower. They do a great deal of amazing work in the perceptual domain and other areas of compression to hide it and to minimise the loss, but they do deprecate the image further from the existing compression of DVD. Their strength is in making most of the loss hidden in areas which the eye doesn't notice, as well as simply better (if less robust..) encoding procedures. Similar to how MP3 overall chucks away more of the sound signal than MP2 (with one being at 128k and the other at 224k), but as the encoding is much cleverer, it actually sounds better in all but specialist cases.

MPG4 (in whatever guise; they're not whole new compression standards, simply different codecs with different additional capabilities within the mpg4 overall spec) will allow you to get basically the full length and full-resolution picture data of a super-widescreen (1:1.85, 2.25) movie, or a slightly reduced resolution full-frame (1:1.77 (16:9) or 4:3) title onto a single standard capacity CD, with similarly advanced mp3 sound. Naturally there's only about 1/6th of the storage space, and halving the sound contingent can only do so much, so the video itself has to be squashed down pretty hard. MPG4 does a fantastic job of this, but if you know where to look you can see the tell tales... softer edges in some cases, colour smearing particularly in dark areas (which can be hit pretty bad), background "wibbling", minor playback errors that would persist for a half second on DVD hanging around for upto 30 seconds (making the super-extended GOPs/gaps between index frames very obvious), etc.

DVD is perhaps a little inefficient in comparison, but it still uses the reasonably (compared to MPG1) efficient/powerful MPG2 compression standard, and relies more on the vast amount of storage available to it to keep compression levels low and quality high, rather than working hard to squeeze the video really tight (which is also quite computationally intense; mpg4 shifts a fair amount of the video data generation load from the disc storage space to the CPU... mpg2 needs a far simpler DSP chip, at least for playback), and is quite error resistant and suitable for fast, accurate searching, etc.

You can get subtitles - with the right media player / winamp / etc plugins and the titles either muxed into the AVI file somehow, or supplied as a seperate text file. Haven't really looked into it much; when I did, it was to install the subs as permanent features within the actual video stream itself, rather than overlays that can be turned on and off (as my level of understanding of any particular foreign language isn't likely to be improved to fluency for the sake of one or two films!).. So I can't comment any further on that. It's not simply pre-built into the stream as it is with DVD though, it requires extra bits.

Menus .... again, it's not really part of the DivX - AVI kind of standard. You can add an autoplay menu to the CD it's on (if you've somehow got more than just the one movie on a disc) - or I guess the DVD if you're using it - and as I understand it "DivX Ultra" compatible players and suitably authored discs can offer menus, but when they exist as one, two, maybe six seperate monolithic long-name files rather than something wierd as in the case of DVD file structure, there's not as much point to it all.

The quality is supposed to be quite good going from MPG4 (back) to DVD from everything I've read (all I've done in this field so far is using it for saving disc space during broadcast/VHS captures, which involved an artificially extra-high bitrate higher even than most DVDs, so I can't really comment), but there will be some difference from the original disc that, even if it's not immediately noticable in a solo viewing, will be obvious if you do a side-by-side compare, as - all other things aside - the material has gone through two additional mpg encoding steps, with non-identical encoders. If your title was originally interlaced, then it will likely now seem slightly jerkier and less well defined in the vertical direction due to the generally simple "blurring", field-combining way de-interlacing is applied. More than likely it will also seem a little softer overall anyway, both because of various anti-blocking measures applied in the encoder and decoder, and because most divx videos are downconverted to 640 (or lower) horizontal rez with a corresponding (often more severe, in order to maintain square pixels) reduction in vertical rez. There will also be evidence of the other problems mentioned above (wavering backgrounds, poor dark-scene performance, etc). Finally there will be a temptation to fit more than one video on a DVD, so the available bitrate for the new MPG2 file will be artificially restricted, when it should ideally be as high as possible to prevent any additional compression artefacts.

And of course, your subtitles will probably not appear unless they're either hard-coded to the picture, or you take steps to extract them from the AVI / additional file and add them into the video stream. Most other advanced features ("follow the white rabbit" type extras, trivia pop-ups, forced subs, alternate angles/storyboards, alternate languages and commentary voiceovers, fancy menus, games, featurettes, databanks, chapters, etc etc etc) will simply not exist unless you take pains to track them down and add them in. Basically, it's a return to a very VHS-like format. So long as you're happy with that (and a vaguely SVHS/Digital-8 quality of picture), then it's perfectly fine. But don't expect to download a DivX file, or make your own, and have an exact replica of the original disc in any way.

.... there, is that enough for the wikipedia article you're writing, or whatever this is in aid of?
(The guides to DVD, Divx, etc are up at the top left there, if you care to read thru' em..)

[richbell]

Lovely summary, thanks a lot eddy. I hope i will stick to DVD formats for now, until BD/HD takes over completely.