CD ROM FAQ Techical info

Information in this page updated: 07 Jan 96


1. Technical info

1.1 Layers of a CD

This 1.2mm thick polycarbon-disk is produced using a spray-print- technique and a negative matrix. A CVD process is used to apply the reflecting 100Å (0.1µm) aluminum layer. To protect this layer from scrating and altering by mistake 5-10µm of protective coat is applied. On top of this the disk description and labeling is done with conventional printing technique.


          ###### #### ## ##   ### ####          Description, Label
        --------------------------------        protective coat
        ================================        reflecting aluminium layer
        ································        information dots
        ||||||||||||||||||||||||||||||||
        ||||||||||||||||||||||||||||||||        polycarbon-carrier
        ||||||||||||||||||||||||||||||||
        ||||||||||||||||||||||||||||||||
        ||||||||||||||||||||||||||||||||


What does this mean:
· The CD is scanned from the down-side not from the top like a
  conventional music album (LP). Scratches, holes or other damage on
  the polycarbon-carrier will probably prevent the laser from reading
  the information. Some scratches can be eliminated using a special
  polish-kit for CDs.

· The information layer is only protected by the thin reflecting aluminium
  and the protective coat. Deep scatches will destroy the information dots
  immediatly.

· Do not use any kinds of pens that are labeled "permanent" or "waterproof"
  for custom labeling the disk. These pens may damage the coat layer and
  then the reflecting layer.

1.2 Data format

The data on a CD is stored on a track winding from the inside to the outside of the disk. This makes it possible to have disks with different dimensions as the 8cm single-CD, the normal CD and the 23cm LaserDisk for video. The holes in the surface are called 'pit' and represent a '1' opposed to the 'land' which reprsents the '0'. Pits and land cover a great deal of the synchronisation and therefore the data can not be stored as in the digital memory of the computer, but is transformed (like MFM or RLL on harddisks) into a code know as 8-to-14-modulation or EFM. This means 8 bits are coded to 14 bits on the cd track.

The first dataformat is caled a "short frame" and converts 24 bytes:

Name:   Sync    control data    Data    Parity  Data    Parity
-------------------------------------------------------------------
Bit:    24      14              12×14   4×14    12×14   4×14

The error correction is done by parity bits and is called CIRC (Cross- Interleaved-Reed-Solomon-Code). An audio CD player rreads 7350 of this short frames in a second which results in 1764000 bytes for two (stereo) channels with 44100Hz each. This is defined in the first book: "The Red Book". On a CD-ROM 98 of this short frames are grouped togehther as a "long frame" with 2352 bytes of data according to the first CD-ROM book: "The Yellow Book".

Name:   Sync    Header                  Data
                Sektor-Address  Mode
------------------------------------------------
Bytes:  12      3               1       2336

The mode byte now defines how to interpred the following data:

Mode-1:
-------
Name:   Data    EDC     free    ECC             ECC
                                P-Parity        Q-Parity
--------------------------------------------------------------
Bytes:  2048    4       8       172             104

Mode-2/Form-1:
--------------
Name:   Subheader       Data    EDC     ECC             ECC
                                        P-Parity        Q-Parity
----------------------------------------------------------------------
Bytes:  8               2048    4       172             104

Mode-2/Form-2:
--------------
Name:   Subheader       Data    EDC
--------------------------------------------
Bytes:  8               2324    4

EDC = Error Detection Code
ECC = Error Correction Code RSPC (Reed Solomon Product-like Code)

Mode-2/Form-1 does not differ from Mode-1, but it is illegal to switch between modes on a track, so Mode-2/From-1 is defined to "emulate" Mode-1. This is because the new XA-standard holds important program data in error corrected M2/F1 (aka M!) frames, but the unimportant audio/video data is kept in the "longer" M2/F2 frames. This offers more space for the AV data without sacrifieing the security of program data.

Each short frame hold 14 control bits called P to W. Since every long frame hold 98 P bits (and 98 Q bits, and 98 R bits ...) this bits are grouped together and they form the subchannels:

· P - start of a track
· Q - directory, timecodes, catalognumbers
Subchannels R to W are currently free, but used on a CD+G for storing of graphical data or on a CD+MIDI for the MIDI data.

On the data sheets of your CD-ROM you may find data transfer rates in Mode 1 and Mode 2. This is not a mode of your SCSI or IDE bus but is the above data mode. You will see e.g. 150kb/s for Mode 1 (or M2F1) and 176kb/s for Mode 2 (M2F2). And since M2F1 holds this ~26kBytes of additional ECC data, these data rates are identical!

1.3 Filesystem

The data format of the CD is equal for all CDs. But what really matters is the filing system, that keeps togehter the files and programs on a CD-ROM.

There are proprietary formats such as the MacHPFs which is equal to the one used on Macintosh' harddisks. But the rest of the world uses the High-Sierra (named after the Hotel where the convetion of the developers took place) or its successor the ISO 9660 formats for storing the data. Be aware, that there is really no operating system, that internaly uses a ISO 9660 filing system. All OS therfor must use a driver to convert the ISO information on the files to native OS filing informations. ISO 9660 is known to be a "least common" filing system and nearly all other operating systems store more info for there files than ISO can offer them.

So the history of the CD filing system is as folows:

High Sierra - (only MS-DOS kompatibel characters and file identifiers 8+3,
               limited directory tree)
ISO 9660 Level 1 - same as High Sierra with marginal changes.
ISO 9660 Level 2 - filenames can hld 31 characters¹.

Since al of the user/group and status bit information is gone under ISO, the RockRidge interchange protocoll RRIP standrad expands fully transparent to Level 1 the filing informations. A driver that is RRIP unaware will not see the additonal information, but a drivers which does see the RRIP, can display lots of more information on the files and go near the OS file system. This includes mixed case, gokal names, status bits, corrected directory tree etc.

¹On early amiga specific CD-ROMs some developers have tried to rebuild the AmigaFS. These disks are no real ISO9660L2 disks and useless on other than Amiga systems. This applies for older CDTV titles.

1.4 The future of the CD

not yet :-(
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