Video Comparisons (VCD / SVCD / DivX)

Its nice to know what kind of compression and qualiy we will get from the various video codecs. Thats one of the most common questions I get asked in fact. The three most popular compression formats are VCD, SVCD and Divx There are, of course, other formats that preserve the image quality better, such as uncompressed AVI or MJPEG. But these do not compress enough to store a movie on two 650MB CD's! And this is what we are interested in.

There are many issues to consider when choosing a storage format. For example, how much detail is lost during compression. How does it handle action scenes and low motion scenes. How much space does the final file take. How smooth is the playback and how much CPU power does it take to play smoothly. We must also consider how we want them played. If we wish to use only our PC's then any format may be fine. But if we wish to play our movie on a standalone video player, then we must restrict ourselves to a format that is compatible with that player.

Trying to figure out a fair way to compare them hasn't been easy. First I had to decide on the method I would use to encode them. After a while of testing, the programs that did the highest quality were: BBMpeg, TMPGEnc, Ligos LSX Encoder, Panasonic Mpeg Encoder, Xing Mpeg Encoder and Cinema Craft encoder. For Divx any encoder seemed to look basically the same quality, so in the end I choose VirtualDub because it made life easier. Anyhoo, after long deliberation I decided to go with TMPGEnc. The quality looks about the best and what is more it is a free program so anyone can use it. The only downside is it is painfully slow compared to, for example, Xing Mpeg Encoder which zipped through a VCD in seconds =).

HOW DID I COMPARE?

I took a DVD and extracted the Vob files to Hard Disk. It was an anamorphic DVD, which means the picture is slightly squashed, just in case you are wondering why. I wanted the best source video to use as a starting point. This means I didn't want to resize the video or add de-interlace filters etc. Luckily, since my PAL DVD was 720 x 576, I merely cropped the edges until they were 480 across. This gave me my final size of 480 x 576 which is supported by all the formats I am testing. I then used FlasK Mpeg to convert it to uncompressed AVI ready for encoding in the software encoder of my choice; I did not add sound at all!

For compression I used the highest quality settings I could find in TMPGEnc and turned them all up to full. As a result the VCD and SVCD's took an inordinately long time to compress. Please do not disregard these formats merely because it looks like they spent too long to compress. With lower settings they look almost as good and compress much much faster. For Divx you do not have all those fancy pre-filtering settings, so to keep it fair, I didn't mess with TMPGEnc extra filters either.

THE CLIP

I used a scene of 60 seconds (approx.) in length. I made sure I choose one that had both scenes of very low motion and also very fast motion. This was so I could see how the formats coped better.

THE SIZES

All three format specifications allow the encoding of the same size, namely, 480 x 576. But a VCD really needs to be 352 x 288 for best quality. So in the end I opted to do an xVCD (eXtended VCD) resolution and a normal VCD resolution so you could see both; all other settings were exactly the same. Just be warned that the xvcd will not play on a standalone VCD player.

THE BITRATE

The most vital vital setting for movies are the amount of bitrate we use to encode. Although SVCD's allow us to go as high as 2600 kbps, this is not a reasonable amount to use to fit it on 2 CD's. The VCD Mpeg-1 format can fit 74 minutes of video on a single CD. This requires a constant bitrate of exactly 1150 kbps. It doesn't matter what compression format we use, to fit it onto 2 CD's we must keep very close to this amount. So this is the touchstone I have decided to try and follow.

But the situation gets a little more complicated. The VCD format is a constant bitrate (CBR), which means it will always end up about the same size in the end. The Divx and SVCD specifications work best by incorporating a variable bitrate (VBR) which change the final filesize depending on the amount of action contained in the movie. How could I honestly compare a CBR movie to a VBR movie when the CBR movie, at 1150 kbps, turns out to be 10MB but the SVCD, because of its VBR, turns out to be 15MB?! Thats half the size again! If I encoded a movie of 650 MB and then another at 975 MB and said 'see, the SVCD at 975 MB looks much better than the 650 MB because I used such and such compression' you'd think I was crazy!

Anyway, to solve this problem I had to 'cheat' a little. I started off by setting both the VCD, Divx and SVCD with a bitrate of 1150 kbps and encoding them. As it turned out the Divx and SVCD's ended up slightly smaller than the CBR VCD, so I fiddled with the bitrates of the Divx and SVCD until it was almost the same filesize as the VCD. It was impossible to get them to match exactly but I managed to get it down to well within a megabyte. I also allowed the SVCD to have the full 2600 kbps as its maximum bitrate and gave it 300 kbps as a minimum bitrate to allow it enough room for variability.

To summarize, I used the same movie clip, the same resolution and highest quality settings I could for all formats. I encoded them so that the final movies were all almost exactly the same filesize! Because of this I did not use exactly the same bitrates.

CONCLUSIONS

As was expected the latest codec's seemed to perform the best and the earlier the worst. All final movies played back relatively smoothly. The VCD and xVCD's still looked very very slightly smoother but this is almost always CPU related and doesn't reflect badly on the other formats abilities at all. The VCD would obviously take the least amount of CPU power, then the xvcd perhaps slightly more, then the SVCD and finally the Divx For low motion scenes you cannot easily beat SVCD's! The colour matching and halftones are almost identical with the original. For action scenes Divx wipes the floor with absolutely everything! There are hardly any noticeable macroblocks at all!

Perhaps its something I did wrong with encoding, but I am a little disappointed with how the SVCD handles fast motion scenes. I set the maximum bitrate to 2600 kbps hoping that this would allow it get rid of the blocks but it did not help much. The xVCD's had the most problems with macroblocks and the Divx had the least.

WHEN PLAYED FULL SCREEN

My quality test is to sit about 5 feet away from my computer screen and play each movie full-screen to see which one I thought gave the overall best look. No one watches a movie from a foot away from the TV screen, so I think such a test is just as useful at determining overall quality as examining the picture up-close.

I'd have to say the Divx looked overall best full screen. The image was sharp, smooth and didn't break up into blocks too much. The main annoying thing about the Divx was the dark colours used! The SVCD looked bright and clear and would have been the best quality if it had not been for its bad macroblocks. As for the VCD, yes it also looked dark and slightly blurred, but you couldn't really notice any macroblocks like you could on the SVCD! So in some ways a VCD may be the better choice over SVCD for action movies and the SVCD for drama movies. The xvcd looked the worst of all! It was dark, blurred and blocky!

THE SNAPS

No comparison article would be complete without detailed examples, so here they are =). All images were captured with either VirtualDub or PowerDVD and have been saved as Jpeg's at the lowest possible compression. You may need to turn your monitors brightness up a little to see the full details of the samples.

HIGH ACTION

VERY HIGH ACTION

VERY LOW MOTION

THE FULL SIZE EXAMPLES

HIGH ACTION SCENE

xvcd

SVCD

DIVX

VERY LOW MOTION SCENE

xvcd

SVCD

DIVX

VERY HIGH MOTION SCENE

xvcd

SVCD

DIVX

MEDIUM / LOW ACTION SCENE

xvcd

SVCD

DIVX