Control Bytes (Subcode Channels P through W)

Up to this point, this paper has been giving details on how the basic 2352 byte area is laid out. In addition to the 2352 bytes of information included in each sector there is 98 bytes of data called the Control Bytes.

These control bytes are produced by summing together 98 separate 8-bit fields. The 8-bits are called a control byte because the first two bits contain timing information that is used by audio players to position the drive head. The remaining 6 bits are available for User Information (see subcode channels R-W section).

We have been talking about a 2352 byte area, where each byte contains 8 bits. In order to discuss the subchannels that make up the control bytes, we must talk about bits.

The 8 bits in a control byte are named P, Q, R, S, T, U, V, W; where each bit represents a subcode channel. For example, if you look at the first bit of every control byte, this stream of bits is called the "P" subchannel. Likewise, all of the second bits in every control byte are called the "Q" subchannel. (See the following sections for information regarding how the subchannels are used.)

Subcode Channel P

The "P" subchannel contains a flag that signals where the music or data starts on a track

Subcode Channel Q

The "Q" subchannel contains the running times from both the beginning of the disc and from the beginning of the current track. Audio players use this information to display the music playing time.

98 control bytes, Layout for bit position "Q":

2 bits - part of the synchronization for the Control Bytes
4 bits - control flags defining what type of information is in this CD track.
4 bits - control flags for the following 72 bits of data
72 bits - Q sub-channel data (During the disc Lead In, the Q channel data contains the discs Table of contents. For the rest of the disc, the Q channel data contains the current playing time. Both track relative time and total disc time.)
16 bits - CRCC (Error Detection Bits) (Cyclic Redundancy Check Code)

Subcode Channels R Through W, ( CD + Graphics)

The Red Book not only defines how to put audio on a CD, it also defines a way to add graphics information to the CD. The graphics information is added in the "R through W" subchannels and is commonly referred to as a CD+G disc, or "CD plus graphics" disc. Currently there are only a few of these discs in the market and very few players can play the graphics.

Today there are only a very few audio CD titles that use the "R through W" subchannels for Graphics, most audio CD's have this area empty.

About 99.9 % of the CD players today ignore the R-W subchannels, and play a CD+G disc just like a normal audio disc. The presence or absence of R-W graphics does not effect the audio, which can be played on any audio CD player.

In addition to graphics, several types of information can be stored in the R-W subchannels:

    Line Graphics
    TV-Graphics
    Extended TV-Graphics
    MIDI
    USER defined

Technical Details for R-W Subchannels:

Because R-W is only 6-bits of information, this section will discuss the number of 6-bit words. (Instead of the number of 8-bit bytes.)

There are 98 Control Bytes in each sector on the CD. The R-W bits in the 98 control bytes are laid out as follows:

    2 words - part of the synchronization for the Control Bytes
    4 groups of 24 words
    (Each group of 24 words is broken down as follows:)
    1 word - defines Mode and Item
    1 word - defines Instruction
    2 words - EDC / ECC code
    16 words - User Data
    4 words - EDC / ECC code

98 control bytes, Layout for bit positions "R thru W":

The Control Bytes are not protected by the same EDC / ECC code that protects the 2352 byte area. A separate set of EDC / ECC code is generated and placed in the R-W subchannels as shown above.

R-W subcode can hold 64 6-bit words of User Data per sector.
(16 words User Data per group) x (4 groups) = 64 words
(CD-ROM Mode 1 can hold 2048 8-bit words (bytes) of User Data per sector.)

User Data can only be written to R-W subcode channels for Red Book tracks, (CD-DA tracks). The Yellow Book specifies that the R-W subcode channels will be zero for data tracks.