CD-ROM Storage Capacity
26-Mar-93 18:23:12
Sb: CD-ROM Storage Capacity
Fm: Matt Seitz 75300,1721
To: David Hayden 71311,2732
Actually, using CD-I or CD-ROM XA can affect the storage capacity of your disc.
You see, there are basically two types of CD-ROM sectors: Mode 1 and Mode 2.
Both contain a total of 2352 bytes per sector. However, only 2048 bytes of a
Mode 1 sector can be used to store user data. The rest of the sector is used
for syncronization, addressing, error detection and error correction.
Mode 2 sectors do not have error detection and correction codes. They allow
you to have up to 2336 bytes of user data per sector. This allows you to store
more data on the disc.
CD-ROM XA and CD-I use Mode 2 sectors. However, they define two types of Mode
2 sectors: Form 1 and Form 2. The Mode 2 Form 1 sectors have error
detection/correction codes that are almost identical to those used on Mode 1
sectors. Mode 2 Form 1 sectors only allow you to store 2048 bytes of user data
per sector. Mode 2 Form 2 reserves four bytes for error detection codes to be
used during disc manufacture. Mode 2 Form 2 allows 2324 bytes of user data per
sector (the other 8 bytes are used for special audio/video features).
Why all the formats? Well the original idea was that Mode 1 sectors (with
error correction) would be used for data where you needed zero errors (like
programs or spreadsheets). Mode 2 sectors would be used for audio and video,
which need a lot of speed and disc space, but not need zero errors. The
problem is that the CD-ROM specification makes it very difficult to mix Mode 1
and Mode 2 sectors on the same disc. So CD-I invented the idea of using all
Mode 2 sectors, but storing error detection codes in some sectors (Mode 2 Form
1) and not storing error detection codes in other sectors (Mode 2 Form 2).
CD-ROM XA borrowed this idea.