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| Sep 26, 2008 - Why DVD+R(W) is superior to DVD-R(W)? -02 |
Pre-pits versus ADIP
To help its recording, a blank disc usually gives 3 kind of information to the drive : tracking (so that the pits are correctly written along a track), addresses (so that the drive can write at the good location) and speed (so that the disc is spinned at the correct velocity). With CD-R(W), tracking and speed information are carried by the wobble, while addresses are contained in the ATIP data (Absolute Time In Pregroove, a frequency modulation of the wobble).
DVD-R(W) format uses a slow wobble (140,6kHz) for tracking and speed, and the addressing (and additional) information is carried by the land pre-pits (pre-recorded pits between grooves). On the groove signal, pre-pits give amplitude spikes.
DVD+R(W) format uses a much faster wobble (817,4kHz), and the addressing (and additional) information is carried by a phase modulation of this wobble called ADIP (ADdress In Pre-groove).
As taught by signal theory, the phase modulation method has a better noise immunity than the pre-pits method, and therefore ADIPs are generally more robust than LPPs against all external disturbances (electrical noise, disc tilting, focusing problems, etc). Apart from the usual sources of noise in a drive, a particularly annoying example of this problem occur when you are burning data on a DVD-R(W) and try to read the pre-pits information at the same time : because the light emitted by the burning laser is interfering with the reading beam, correct pre-pits detection is much more difficult to achieve, which can jeopardize linking precision.
But the pre-pit technology is not only a weakness against noise, it also becomes a limiting problem as the speed of the disc increases, because at high speeds pre-pits are more difficult to detect than phase inversions. Indeed, on the wobble signal the pre-pit information only exist where the pre-pit is located in time, while the information of phase inversion is spread over the complete inverted period of wobble (or more, actually as long as the phase is not inverted again). According to the specifications, the minimum length of a DVD-R(W) pre-pit is 1T (1/26.16E6 s), while a DVD+R(W) wobble period lasts 32T, which makes it much easier to detect.
Another bad side effect of this pre-pits method is that DVD-R(W) mastering is made more difficult than with DVD+R(W), since a higher precision is required to cut both the grooves and the pre-pits between them. Special dual-beam recorders are usually required for - stamper mastering, although some manufacturers now also use single beam cutting.
Not only are the pre-pits more error prone than phase modulation, but data they carry are also less protected. In one ECC block pre-pits carry 192 bits of information (one pre-pit block). Out of these 192 bits, 48 are not protected by any error correction mechanism, 24 bits are protected by 24 bits of parity (A parity), and the last 56 bits are also protected by 24 bits of parity (B parity). All in all, this strange heterogeneous structure finally gives a pretty weak protection to the information bits carried by pre-pits.
On the other hand the corresponding DVD+R(W) structure is 4 times smaller : one ADIP word is 52 bits large, consisting of 1 sync bit, 31 data bits and 20 parity bits (which protect all data bits). One ECC block contains 4 ADIP words, so 204 bits of information in total. Also each ADIP word contains the full ECC block address, while 4 times this size are needed in the - technology to extract this address : this gives significant speedups when seeking uses this method.
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