Writing Quality - Page 11

EFM - Page 1

The data stream must undergo CIRC error correction and EFM modulation to reduce the possibilities of creating an error.

- Whats is EFM?

EFM (Eight to Fourteen Modulation) is a method of encoding source data for CD formats into a form that is easy to master, replicate and playback reliably. EFM modulation gives the bit stream specific patterns of 1s and 0s, thus defining the lengths of pits and lands. EFM permits a high number of channel bit transitions for arbitrary pit and land lengths. The merging bits ensure that pit & land lengths are not less than 3 and no more than 11 channel bits. This reduces the effect of jitter, distortions on the error rate, increases data density and helps facilitate control of the spindle motor speed.

- How EFM is performed?

Block of 8 data bits are translated into blocks of 14 channel bits. The 8-bit symbols required 2^8=256 unique patterns, and of the possible 2^14=16,384 patterns in the 14-bit system, 267 meet the pattern requirements; therefore, 256 are used and 11 discarded.

Blocks of 14 channel bits are linked by the three merging bits to maintain the proper run length between words, as well as suppress dc content, and aid clock synchronization. The digital sum value (DSV) is used to monitor the accumulating dc offset. The ratio of bits before and after modulation is 8:17.

The resulting channel stream produces pits and lands that are at least two but no more than ten successive 0s long. 3T, 4T…11T where T is one channel bit period. The pit / land lengths vary from 0.833 to 3.054 µm at a track velocity of 1.2 m/s, and from 0.972 to 3.56 µm at a track velocity of 1.4 m/s.

- Why using 14-bit system?

Using 14-bit symbols allows up to 2^14, or 16384, 14-bit combinations. This provides bit patterns that have a low number of transitions between 0 and 1. Using 8-bit symbols would require too many pits due to the large number of transitions. The 14-bit symbols used in EFM are taken from lookup tables, which are defined by the Red Book specifications. However, only patterns in which more than two but less than ten 0s appear continuously are used. If the size of the data gets 17/8 times bigger, the number of bits per pit is at least 3 times bigger. Thus at the end, more data can be recorded on the CD in spite of the 17 bits used instead of 8.

Below are some examples of 8-bit data symbols and their 14-bit equivalent.

Once the 14-bit symbols are put together, it is possible that the bits between the two symbols will create an illegal bit pattern. For example, joining the last and first bit patterns in the examples above would create an illegal bit pattern since a minimum of two 0s must be recorded continuously.

In such cases, Merge Bits must be used between the two 14-bit symbols in order to satisfy the specifications. The following picture shows a bit pattern and the pits that it would produce. The areas between the pits are called "lands".

If the source binary data were recorded without encoding in this way, the disc would frequently need to represent a single '1' or '0' requiring mastering and replication to reproduce very small artifacts on the disc. EFM encoding ensures that the smallest artifacts on the disc is three units long and the average artifacts is seven units long.

Within the EFM lookup table, it is possible for the 14 bit code to start or end with a '1'. In order to prevent a situation in which one 14 bit code ends and the next one both start with a '1', three merging '0' bits are added between each 14 bit code. So in reality, EFM is eight-to-seventeen modulation.