CHAPTER 13

700 Chapter 13 Optical Storage
time intervals (usually refered to as 3T) and the maximum spacing between 1s is 11 time intervals
(11T).
Because some of the EFM codes start and end with a 1 or more than five 0s, three additional bits
called merge bits are added between each 14-bit EFM value written to the disc. The merge bits usually
are 0s but might contain a 1 if necessary to break a long string of adjacent 0s formed by the adjacent
14-bit EFM values. In addition to the now 17-bits created for each byte (EFM plus merge bits), a 24-bit
sync word (plus 3 more merge bits) is added to the beginning of each frame. This results in a total of
588 bits (73.5 bytes) actually being stored on the disc for each frame. Multiply this for 98 frames per
sector and you have 7,203 bytes actually being stored on the disc to represent each sector. A 74minute
disc, therefore, really has something like 2.4GB of actual data being written, which after being
fully decoded and stripped of error correcting codes and other information, results in about 682MB
(650MiB) of actual user data.
The calculations for EFM encoded frames and sectors are shown in Table 13.4.
Table 13.4 EFM Encoded Data Calculations
EFM-Encoded Frames: 74-minute 80-minute
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Sync word bits 24 24
Subcode bits 14 14
Data bits 336 336
Q+P parity bits 112 112
Merge bits 102 102
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EFM bits per frame 588 588
EFM Encoded Sectors:
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EFM bits per sector 57,624 57,624
EFM bytes per sector 7,203 7,203
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Total EFM data on disc (MB) 2,399 2,593
B = Byte (8 bits) MB = Megabyte (1,000,000 bytes)
KB = Kilobyte (1,000 bytes) MiB = Mebibyte (1,048,576 bytes)
KiB = Kibibyte (1,024 bytes) EFM = Eight to fourteen modulation
To put this into perspective, see Table 13.5 for an example of what familar data would actually look
like when written to a CD. As an example, I’ll use the letters “N” and “O” as they would be written
on the disk. Here are the computer representations of these letters.
Table 13.5 How Data Is Written to a CD
Character “N” “O”
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ASCII decimal code 78 79
ASCII hexadecimal code 4E 4F
ASCII binary code 01001110 01001111
EFM code 00010001000100 00100001000100
ASCII = American Standard Code for Information Interchange
EFM = Eight to fourteen modulation