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Old 10th October 2003, 15:09   #1  |  Link
alexnoe
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XCD FAQ (Myths and Reality)

1) What is XCD?
XCD means that you store normal data files in the same way as VCDs, meaning that one of the 3 layers of error correction of normal data CDs is removed and the free space is available for data
The correct name of the recording format is Mode 2 Form 2.

2) How much data can i store on a XCD?
XCDs can hold (2324/2048) times more data than normal data CDs, i.e. a 700 MB disc can hold 795 MB. If you combine it with Gigarec e.g. on 1.2x setting, you get 960 MB, and so on.

3) How can i create XCDs?
You have to use the Mode2 CD Maker Tool (Doom9 Guide)
some cd burning progs, like cdrdao, can burn any data as mode 2 form 2, but i am not sure if the results would be playable without problems

4) How can i play XCDs?
You need a CDXA directshow filter:
there are actually two available: from avih and gabest

5) Which files does XCD work with?
With all files for which XCD-capable/Directshow players exist, such as AVI, MKV, OGM, MP4 or RM/RMVB.
Be aware of that AVIs on XCDs can be unplayable when the header (at the beginning of the file/middle of the disc) or the index (normally at the end of the file/border of the disc) gets b0rked; MKVs, MP4s and OGMs also when the header gets b0rked (so handle your XCDs with care!)

6) Is there any error correction available on XCDs?
XCDs have as much error correction as audio cds, meaning that scratches that do not destroy more than 2mm of consecutive data can be corrected. Note that this length is smaller if you use Gigarec.

7) How can i check the status of a XCD before data gets b0rked?
Do a Cx-Scan, for example using the PlexTools with a Plextor Premium drive. If you see E22 errors reported, the disc is short before getting b0rked. If you don't have a Plextor Premium drive, use Nero CD Speed -> Quality check. Unfortunately, this check reports different errors for different drives, but many drives will report errors there before any data gets b0rked.

8) Are CRC elements in containers useless on XCDs?
Mostly yes, each sector of 2324 bytes of user data gets an 32 bit CRC value on XCDs (although it is not mandatory, the space it consumes cannot be used for any other purpose, so any Mode2Form2-Authoring-Software should write this CRC value). Because of the CRC values stored on the XCD itself you will know already if any data gets b0rked due to too bad scratches or due to having vampire discs exposed to sunlight. Another CRC value (ie from the container) won't tell you anything new.

Since there is a CRC over each 2324 bytes on the XCD already, and since data blocks in a MKV file are usually larger, a b0rked sector will most likely span only over 1 or 2 blocks. Since the physical organisation of sectors on a CD makes sure that errors are spreaded (and not concentrated in one part of a sector), most likely both blocks will be corrupted. The container CRC will thus not tell you anything more, even if you waste 6 bytes per block for that CRC (EBML element + size + CRC). In some cases, if there are really lots of small blocks, a sector of 2324 bytes can span over more than 2 blocks.

With OGM, it is even more useless: OGM uses one CRC per 4 kByte - page. That means, some sectors span over 1 page, and some span over 2 pages. -> Same situation as with MKV, with the difference that a sector spanning over more than 2 blocks is not unlikely, but impossible...

That means, if anyone tries to tell you that you should not use AVI on XCD just because of the lacking CRC, point him to this FAQ

9) What happens if i try to play a b0rked XCD?
Normally the CRC would detect the b0rked parts on the XCD and avoid passing them to the decoder:
Note that currently neither the available XCD directshow filters nor the available container parser/splitter filters take the CRC values stored on the XCD/Container into account during playback!
Because of that the b0rked data will get passed to the decoder filter and it depends on the decoder how to react on it (some might crash, some might display garbage but will continue playing...)

10) Compatibility of XCDs?
Except for a few b0rked drives, like the LG 16x DVD-ROM, most PC drives can read XCDs.

11) Is it possible to make X-DVDs?
DVDs cannot be trifled with in the same amount or way as CDs. Something like X-DVD is not possible with the current specifications.

12) Windows sees larger files on the disc than the source was?
Although there are only 2324 bytes of data in a sector, there are 28 more bytes of overhead (sector header, crc). If you access files on XCDs on M$ Windows, it will read the entire set of 2352 bytes, just as if they all belonged to the file! The application has to throw out those 28 bytes! (This is what XCD-Filters do). It is also up to the app to check the CRC in those 28 bytes for correctness.

13) How can i copy data from XCD to my harddisk?
Never ever copy XCD files directly to hard disc! If you do that, those 28 bytes of overhead will stay in the file, and the file is still an XCD-File. If you then make a new XCD from that file, you get more overhead, the file grows again, and the XCD-Filter must be run twice to read the file! The same is happening of course with VCD and SVCD.
To copy your XCD to hd use the dat2file programm bundled with Mode2 CD Maker, Extractor, XCDextractor, AVI-Mux GUI or CD-R eRRoR coRRectoR

14) What can i try to be save with scratched XCDs?
try especially protecting/extracting the files with XCDextractor, CD-R eRRoR coRRectoR or XCDBackupCreator as they offer options for repairing damaged sectors or protecting important sectors

15) I want to read more about XCD! Were can i find more infos?
try the official Sourceforge Project page and ReferenceDivX's Docs about MCF-CD and of course doom9

16) Can I safely use XCD then?
After reading and understanding this FAQ, you know enough about XCD to decide if you consider the risk of loss of data too high or not!

Last edited by bond; 12th June 2005 at 18:16.
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Old 12th October 2003, 11:04   #2  |  Link
bond
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thanks a lot to alexnoe for this FAQ

If you have any suggestions, ideas which questions to add to make the XCD FAQ better or found any broken links, feel free to post below!
I will do my best to keep the FAQ up-to-date!

Please don't use this sticky to discuss any XCD issues, this thread is meant for info only!
If you have any questions plz search the forum, perhaps they already have been answered and only then start a new thread

thanks,
bond
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Old 14th October 2003, 23:59   #3  |  Link
DeXT
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Awesome work! I'm too lazy to write a FAQ so it's a wonderful thing that someone has taken the time to do it I'll instantly add a link to it on my homepage.

Just a couple of corrections: the XCD DS filter is not made by me but avih (I just contributed to it) althought I host it on my homepage. And the MCF-CD information page is not mine but ReferenceDivX's. He also wrote a cdxa extraction tool ("extractor") which works like dat2file, so you can add it to the list if you wish.

About the file formats which are compatible with the XCD filter(s), you might mention MP4 and RM, now that there are directshow filters to handle them. But I suspect you did not pretend to be exhaustive here.

Great work.

DeXT
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Old 15th October 2003, 09:06   #4  |  Link
bond
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thanks for your additions
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Old 2nd November 2003, 21:15   #5  |  Link
avih
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nice work alexnoe
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Old 28th November 2007, 23:42   #6  |  Link
PsychicHigh
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Been reading up on XCD for a while now and trying to figure out if there would be a hack for it to allow MODE2 data and not use it just as a video container.

But great work on the FAQ as it did kinda give an idea ^_^
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Old 29th November 2007, 00:30   #7  |  Link
jethro
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You can put any file (not only movies) on XCD in MODE2. But it is of less use as in case of read errors file is going to be unreadeable (no correction). And who uses CDs nowadays?? XCD was fun...years ago.
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