Author Archives: Multimedia Mike

Important Strides In Compression Technology

After completing gcfuse and xbfuse, I am hungry for more game-related filesystem hacking. I set my sights on another format — I wondered what lay in hiding on those universal media discs (UMDs) that the Sony PlayStation Portable uses.


Universal Media Disc (UMD) for the Sony PSP

You probably don’t have the tools or the know-how to rip sectors off of these discs by yourself. If you happen to come across a pre-ripped image in the shadier portions of the internet, you may find that the rip has an extension of .cso. This stands for compressed ISO. Would you be surprised to learn that there is not a lot of documentation about this format out there? There are some closed but free Windows tools out there to convert between ISO and CISO and some other formats. What knowledge is out there indicates that the format offers 9 different compression levels. So does stock zlib.

Internally, a CISO format begins with a ‘CISO’ signature and eventually starts in with an enormous table of indices. These seem to be absolute offsets into the file. My first theory was that they are individual files. Combined with the above zlib speculation, I figured that each file was zlib compressed. That’s when I enlisted the help of the fellow who recently implemented native zlib functionality in FFmpeg — Mans Rullgard.

What he managed to figure out was that each index in the table actually references a 2-kilobyte sector. Further, the offset has its high bit cleared to indicate that the sector is the last of a file (a UMD has a max capacity of 1.8 GB which can be expressed in only 31 bits). Thus, the deltas between the indices imply the length of the sector, which is almost alway 2048 bytes, except for (statistically) the last sector of the file. So this leads us to the revolutionary compression technology on display here–

Do not store the final sectors of individual files with implicit zero-padding. (Not true; read further analysis in the comments.)

Hey, it counts as compression. The technique exploits a characteristic inherent in a specific type of data. It seemed a bit silly at first, in a “would that actually make a difference?” kind of way. But the math does work. At least for the few representative samples observed, the CISO image is about 75% of the size of the “uncompressed” image. If a disc uses its full 1.8 GB, then 1,800,000,000 bytes / 2048 bytes/sector = 878907 sectors. 878907 sectors * 4 index bytes/sector = 3515628 bytes. It’s 3.5 MB, but not nearly enough to blow the 2 GB limit necessary to make this format work in the worst case scenario.

Honestly, I haven’t had this much fun collaboratively cracking a file format since a bunch of us got together back in the day and worked out Nullsoft Video (NSV)… and then the official spec was released anyway.

There are still a number of mysteries:

  • None of this explains the advertised compression levels feature. I am starting to think that perhaps the entire index table can be zlib compressed, but that it simply wasn’t done for the observed samples.
  • There is no real filesystem data present. It stands to reason that the original UMD must have some notion of a filesystem since some utility knew where the files lived and how long they were. The Wikipedia article claims that these discs use an ISO-9660 filesystem. I generally know what ISO-9660 filesystem data looks like, and I’m not seeing it here. There isn’t much familiar, except for some PNG files which are difficult to interpret, leading to…
  • Is the filesystem fragmented? That would be highly uncharacteristic of a filesystem built to live on an optical, read-only medium. But cursory investigations have indicated that there might be some fragmentation.
  • Exactly what is the usage model for these images? Real-time access from a memory stick, I hope? Because Mans determined that stock bzip2 performs better than CISO on a raw image.

Here are the format details we have worked out so far. If anyone knows the original author of the CISO format, send him over here. In my experience, people who design these types of formats are not necessarily trying to keep anything secret; they just haven’t gotten around to writing up and publishing a formal description, and have no compelling reason to release their tools’ source code into a Windows world where most people can’t compile anyway.

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Video Coding Concepts: YUV and RGB Colorspaces And Pixel Formats

If you have any experience in programming computer graphics, you probably know all about red/green/blue (RGB) video modes and pixel formats. Guess what? It is all useless now that you are working on video codec technology!

No, that’s not entirely true. Some video codecs operate on RGB video natively. A majority of modern codecs use some kind of YUV colorspace. We will get to that. Since many programmers are familiar with RGB pixel formats, let’s use that as a starting point.

RGB Colors

To review, computers generally display RGB pixels. These pixels have red (R), green (G), and blue (B) components to them. Here are the various combinations of R, G, and B components at their minimum (0) and maximum (255/0xFF) values:

R G B color notes:
0x00 0x00 0x00 absence of R, G, and B = full black
0x00 0x00 0xFF full blue
0x00 0xFF 0x00 full green
0x00 0xFF 0xFF
0xFF 0x00 0x00 full red
0xFF 0x00 0xFF
0xFF 0xFF 0x00
0xFF 0xFF 0xFF full R, G, and B combine to make full white

YUV Colors
If you are used to dealing with RGB colors, YUV will seem a bit unintuitive at first. What does YUV stand for? Nothing you would guess. It turns out Y stands for intensity. U stands for blue and V stands for red. U is also denoted as Cb and V is also denoted as Cr. So YUV is sometimes written as YCbCr.

Here are the various combinations of Y, U, and V components at their minimum (0) and maximum (255/0xFF) values:

Y U/
Cb		 V/
Cr		 color notes
0x00 0x00 0x00
0x00 0x00 0xFF
0x00 0xFF 0x00
0x00 0xFF 0xFF
0xFF 0x00 0x00 full green
0xFF 0x00 0xFF
0xFF 0xFF 0x00
0xFF 0xFF 0xFF
0x00 0x80 0x80 full black
0x80 0x80 0x80
0xFF 0x80 0x80 full white

So, all minimum and all maximum components do not generate intuitive (read: similar to RGB) results. In fact, all 0s in the YUV colorspace result in a dull green rather than black. That last point is useful to understand when a video is displaying a lot of green block errors– that probably means that the decoder is skipping blocks of data completely and leaving the underlying YUV data as all 0.

Further Reading:

DRM Hits Home

This is positive news when viewed from a certain perspective: MLB rips off fans who bought DRM videos. So Major League Baseball (a big deal in American sports) had a media download service that was protected by iron-fisted digital rights management. It was the type of DRM that called home for authorization to check if you still had permission to view the downloaded content. And the mothership had decided to go with a different DRM system or provider.

I wonder if this is high profile enough to set a serious example for the implications of consumer media DRM? I know it’s a wake-up call for me to remember to crack all of Apple iTunes Music Store purchases while the keys are still somewhere on my computer.

According to the discussion linked above, MLB may have relented somewhat and offered existing consumers a means of re-downloading their purchased content under the new system. So maybe this episode won’t provide much of an object lesson after all.

Wii Motion JPEG

Did you know that the Nintendo Wii can play user videos? I didn’t, but the Wii can apparently play videos through a user’s SD card. I also didn’t know what format they have to be converted into in order to be played back — Motion JPEG.


Nintendo Wii

There is a Windows program based on — what else? — FFmpeg called Wii Video 9 that handily converts anything into the accepted multimedia format. From the Sony PlayStation 3 and PSP to the Apple iPod and now the Nintendo Wii, the trend is for consumer electronics devices to be able to play user videos. But they usually support more advanced formats like MPEG-4 and H.264. I’m guessing that part of the Wii’s comparably low price is directly attributable to lower multimedia technology licensing costs.

I don’t know why I find it so humorous to read the Wii Video 9 forum posts, such as “Why are the converted files so HUGE?” I think we all need to respect that the deeper aspects of multimedia technology are not second nature to the general populace. One day, this stuff may truly be transparent and ubiquitous. Until then, Monsieur Scandragon has a point when he rants, “It doesn’t matter how bad it is at playing compressed video.. 10 seconds at BAD quality should NEVER be 25MB!!”

I have been too lazy to try the program myself. I wonder if it’s normal MJPEG data, or if it is unescaped data like that seen in the THP format common on the Nintendo GameCube. I also wonder about the audio format. Presumably, if they were too cheap to license MPEG-4 or H.264 video codecs, they’re not going to license MP3 or AAC either. I am guessing either straight PCM or their own custom ADPCM format.