Philips CDD522 Operating Instructions Manual page 29

For video/audio data the first two error correction layers (C1 and
C2 as defined by the Red Book) are sufficient. Sector type 5 is
used here (Type 3 is-extremely rare). It can hold more data. The
audio can not be CDDA quality since the amount of data
available is fess than in case of an Audio sector (Remember: the
audio sectors contain 2352 bytes of data and the Type 5 sector
only contains 2324 bytes of data. In the first case the data rate is
176 400 bytes/sec, in the second it only is 174 300 bytes/sec).
The CDROM-XA (XA stands for eXtended Architecture) type of
disks are Yellow Book Extension types. The CDROM-XA was
introduced to allow a so-called Mixed Mode in one track .
Remark: Mixed Mode is a somewhat confusing term, since the
Sector Mode does not change within one track (this is not
allowed by the Standard). It remains Mode 2. What does
change, is the Form. One track can contain Mode 2
Form 1 Sectors for Computer Data and Mode 2 Form 2
Sectors for Audio/Video.
So do not confuse this Mixed Mode with the Mixed Mode,
described in Chapter 7 p 35, Disk Types.
Mixed Mode is necessary to make the interleaving of Computer
Data Sectors with Audio/Video Data Sectors possible. With
appropriate interleaving, reproduction of reai-time Audio or Video
is possible while processing Computer Data.
Although the CDROM-XA sectors have the same configuration as
the Green Book sectors, the disks are not the same. CDROM-
XA is a disk defined by the Yellow Book, that only defines a disk
layout. The Green Book disks use the same sector layout, but
the Green Book not only defines a disk but also the Operating
System.
The header information is used to search a specific sector. The
header contains 4 bytes:
Byte 1: Minutes (copy of the AMIN value from the Q channel)
Byte 2: Seconds (copy of the ASEC value from the Q channel)
Byte 3: Blocks (copy of the AFRAME value from the Q channel)
Byte 4: Mode (8 bits), value decimal between 0 and 2.