Hello,
This question has some connections with the other one that I recently posted.
When data is stored on a CD, a straight-forward approach would be to store a 0 as a land and a 1 as a pit, or vice-versa. But the representation used is that a 1 is representated by a change from pit to land or from land to pit, and a 0 is represented by a pit or a land.
My questions is: Why is it better to use the transition/not-transition approach? I have read an explanation, but don't understand it. It is quoted below this post. The whole text can be found at
Thanks in advance! /Carl
To ensure accurate recovery, the disc data must be encoded to optimize the analog-to-digital conversion process that the radio frequency signal must undergo. Goals of the low level data encoding include:
- High information density. This requires encoding that makes the best possible use of the high, but limited, resolution of the laser beam and read head optics.
- Minimum intersymbol interference. This requires making the minimum run length, i.e. the minimum number of consecutive zero bits or one bits, as large as possible.
- Self-clocking. To avoid a separate timing track, the data should be encoded so as to allow the clock signal to be regenerated from the data signal. This requires limiting the maximum run length of the data so that data transitions will regenerate the clock.
- Low digital sum value (the number of one bits minus the number of zero bits). This minimizes the low frequency and DC content of the data signal which permits optimal servo system operation.
A straightforward encoding would be to simply to encode zero bits as land and one bits as pits. However, this does not meet goal (1) as well as the encoding scheme actually used. The current CD scheme encodes one bits as transitions from pit to land or land to pit and zero bits as constant pit or constant land.
To meet goals (2) to (4), it is not possible to encode arbitrary binary data. For example, the integer 0 expressed as thirty-two bits of zero would have too long a run length to satisfy goal (3). To accommodate these goals, each eight-bit byte of actual data is encoded as fourteen bits of channel data. There are many more combinations of fourteen bits (16,384) than there are of eight bits (256). To encode the eight-bit combinations, 256 combinations of fourteen bits are chosen that meet the goals. This encoding is referred to as Eight-to-Fourteen Modulation (EFM) coding.
If fourteen channel bits were concatenated with another set of fourteen channel bits, once again the above goals may not be met. To avoid this possibility, three merging bits are included between each set of fourteen channel bits. These merging bits carry no information but are chosen to limit run length, keep data signal DC content low, etc. Thus, an eight bit byte of actual data is encoded into a total of seventeen channel bits: fourteen EFM bits and three merging bits.
To achieve a reliable self-clocking system, periodic synchronization is necessary. Thus, data is broken up into individual frames each beginning with a synchronization pattern. Each frame also contains twenty-four data bytes, eight error correction bytes, a control and display byte (carrying the subcoding channels), and merging bits separating them all. Each frame is arranged as follows:
Sync Pattern24 + 3channel bits Control and Display byte14 + 3 Data bytes12 * (14 + 3) Error Correction bytes 4 * (14 + 3) Data bytes12 * (14 + 3) Error Correction bytes 4 * (14 + 3)
TOTAL588channel bits