Hardware Reference
In-Depth Information
American physicist Harry Nyquist in 1928) states that the sampling rate must be at least
twice the highest frequency present in the sample to reconstruct the original signal accur-
ately. That explains why Philips and Sony intentionally chose the 44,100Hz sampling rate
whendeveloping the CD—that rate could beusedtoaccurately reproduce soundsofupto
20,000Hz, which is the upper limit of human hearing.
Subcodes
Subcode bytes enable the drive to find songs (which are confusingly also called
tracks
)
along the spiral track and contain or convey additional information about the disc in gen-
eral. The subcode bytes are stored as 1 byte per frame, which results in 98 subcode bytes
for each sector. Two of these bytes are used as start block and end block markers, leav-
ing 96 bytes of subcode information. These are then divided into eight 12-byte subcode
blocks,eachofwhichisassignedaletterdesignationP-W.Eachsubcodechannelcanhold
about31.97MBofdataacrossthedisc,whichisabout4%ofthecapacityofanaudiodisc.
Theinterestingthingaboutthesubcodesisthatthedataiswovencontinuouslythroughout
the disc; in other words, subcode data is contained piecemeal in every sector on the disc.
ThePandQsubcodeblocksareusedonalldiscs,andtheR-Wsubcodesareusedonlyon
CD+G (graphics) or CD TEXT-type discs.
The P subcode identifies the start of the tracks on the CD. The Q subcode contains a mul-
titude of information, including the following:
• Whether the sector data is audio or data. This prevents most players from trying to
“play” CD data discs, which might damage speakers due to the resulting noise that
would occur.
• Whether the audio data is two or four channel. Four channel is rarely if ever used.
• Whether digital copying is permitted. PC-based CD-R and RW drives ignore this; it
was instituted to prevent copying to DAT (digital audio tape) or home audio optical
drives.
• Whether the music is recorded with pre-emphasis. This is a hiss or noise reduction
technique.
• The track (song) layout on the disc.
• The track (song) number.
• The minutes, seconds, and frame number from the start of the track (song).
• A countdown during an intertrack (intersong) pause.
• The minutes, seconds, and frames from the start of the first track (song).
• The barcode of the CD.
