Why DVD+R(W) is superior to DVD-R(W)
|Article||Why DVD+R(W) is superior to DVD-R(W)|
Since the war between DVD+R(W) and DVD-R(W) started, several comparisons of the two formats have been published, but none of those I read did contain really accurate technical information. Instead, journalists relied on public press releases and white papers provided by vendors to write their articles, resulting in the end into superficial explanations and sometimes contradictory conclusions.
The main problem here is that getting DVD+R(W) and DVD-R(W) standards require money and NDAs, so that little detailed technical information about these formats can be found on the Internet. This is very unfortunate, as it prevents both journalists and techies from all over the world to independently review, compare and choose the best format by themselves.
Because they cannot access the technical details (or simply due to laziness), some people prefer to carefully conclude that there’s no real technical difference between the two formats, and that if you forget all the marketing propaganda + and – are equally good. To me this is an evidence that such people have not understood (or more probably not even read) the format specifications.
After having studied the two formats I found that there are several fundamental differences between them, and I concluded that + is superior to -. The goal of this paper is to present the technical details from the specifications that led me to this conclusion, so that anyone with basic engineering knowledge can make his own opinion. This article is by no mean exhaustive, and some exclusive + and – features (e.g. CAV writing or copyright management) are not tackled here because I considered them less significant. Note that this article is pretty technical, so some understanding of optical storage technologies is required.
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.
Defect management and recording quality
Another major advantage of DVD+RW format over DVD-RW (although no drive support it yet) is hardware defect management provided by the DVD+MRW standard (Mt Rainier for DVD+RW). On a DVD+MRW disc, when an error occur during a read or write access to an ECC block, this block is flagged as bad and the drive will not use it any more. Instead, when writing to the disc, data which should have been stored in this bad block are relocated elsewhere ; likewise, when the drive is asked to read these data again, it will fetch them from the new location. These operations are completely transparent for any software (whatever operating system, driver or burning application is used), and while the initiator is requesting consecutive sectors the drive will actually read data from various locations : this new abstraction layer is called Logical to Physical address translation.
As a side note, popular belief is that defect management is only useful when burning new discs (where data which are being written can also be read back, checked and moved to another location if needed), but that it is helpless on discs which get damaged after they have been burned. This is wrong, because when an ECC block is partially damaged and requires several retries to be read (or for instance give too many PI/PO errors), these data can proactively be moved to another clean location on the disc before media wear-out makes them unreadable. Of course, if an ECC block is damaged beyond error correction capabilities, data are definitely lost ; however, only very serious damages can make such a thing happen, as PI/PO correction can handle burst errors larger than 6 mm.
As with formatting, DVD+RW standard enables background verifications, i.e. the disc is checked for defects when the drive is idle. Of course, at any time the user can still read or write to the disc, or eject it from the drive ; background verification would then resume later from where it was stopped. Combining these features together gives a very powerful system which can continuously try to improve the longevity of discs : while the user performs its usual operations, the drive can check in the background the complete surface of the disc and move data from damaged locations to clean areas. Such advanced use of defect management are already described in the DVD+MRW specification, for instance with the Self Monitoring Analysis and Reporting Technology (SMART, a technology inspired from hard discs). Finally, it is important to mention that DVD+MRW provides full read-only compatibility for players which don’t understand MRW.
Although DVD-R(W) also support some defect management (Persistent-DM and DRT-DM), it is mainly software based and actions must always be initiated by a specific program. Furthermore, since DVD-RW format lacks the needed structures, address translation has to be performed also by software, and translation tables have to be stored on the user area of the disc according to a higher level specification (for instance in the sparing tables of UDF 2.0). This makes DVD-RW not well suited for simple file storage or image burning, as it requires a complete file system to benefit from defect management. Note also that although DVD-RW cannot use +MRW technology (due to technical differences), DVD+RW can very well use UDF 2.0.
Also a DVD+R(W) disc allows a drive to achieve better writing quality (independently of media quality), because it gives more information to a drive than a DVD-R(W). Indeed, just like with CD-R(W), the best writing settings for a given disc are found at startup during the OPC (Optimum Power Control) algorithm, which use data contained in the pre-pits blocks/ADIP words. And regarding OPC, a DVD+R(W) gives not only more information (e.g. power dependency on wavelength) but also more precise ones (e.g. startup laser power). Moreover, all these information are available for 4 different speed ranges (primary and upper speeds, normal and 4x+ mode), while – format only provides one set of data. This is very important because optimal writing settings are very sensitive to burning speeds. Also the OPC test area of DVD+ is 32768 sectors in total, compared to 7088 sectors for DVD-.
When for any reason writing on the disc has been stopped and is resumed, new data have to be linked with the old ones. Linking is a very important and tricky task, which can cause various problems both at physical and logical level. First, a short overview of the linking methods used by the two formats is required.
With DVD-R(W), 3 different linking methods can be used : 2K linking, 32K linking, and loss-less linking. In all cases recording has to stop 16 bytes after the first sync of the first sector of an ECC block, and new data are recorded starting between the 15th and the 17th byte of this same frame. The precision of the linking is therefore 2 bytes and the space waste either 2KB, 32KB or nothing (note that loss-less linking method does not work with DVD-R for Authoring). With DVD+R(W), linking is performed in the last 8 channel bits (4 data bits) of an ECC block. Linking precision is therefore 4 times higher and loss-less linking is the only method allowed by the standard, which guarantees no space waste.
Even when loss-less linking methods are used, the pits are not perfectly contiguous on the disc, and therefore some PI/PO errors will always occur : to minimize this effect, the location of the linking region is very important. With -RW, the linking region is in user data, and therefore useful bytes will always be corrupted there. Also since the linking occurs after the first sync, the second sync frame (and possibly the third one) will also be lost, since the sync words will not be correctly spaced in the ECC block. With +RW, the linking region is in the last byte of PI correction, which leaves user data bytes untouched. Also the linking position guarantees that all syncs in the next ECC block will be corrected spaced, which gives at least one sync frame less to correct for the player compared to -RW. Note that with +RW, corrections due to the linking region and corrections due to the sync shift are split between two ECC blocks, while they must all be performed by a single ECC block with -RW.
Linking can also cause various troubles at physical level, and when looking directly at the HF signal read by the PUH, the linking region looks like the following:
The slicing level is the digital threshold which separates zeroes from ones, and therefore it must always be centred in the HF signal for good reading quality : when the slicing level deviates too much from its perfect position, the run-lengths (3T to 14T) are wrongly recognized, which causes decoding errors. But as explained previously linking is not perfectly accurate, and therefore a gap will always exist between the two recorded sessions, and the longer the gap, the further the slicing level can drift. Furthermore, between the two linked regions the slicing level can also differ, because of various physical parameters which could have changed between the two recording sessions (laser power, media properties, recording speed, etc) : when this jump is too high, again errors appear. So the smaller the gap and the jump, the better quality and compatibility we get : -RW allows a 32T large linking gap and does not care about this slicing level jump, while +RW allows a 8T large linking gap and a maximum limit for this jump under any condition. This makes +RW loss-less linking also more powerful at physical level.
Multiple recording sessions and compatibility
If you want to partially record a DVD-R(W) disc and use it immediately, but also want to be able to record more data later, border zones are used, which are meant to make this partially recorded disc compatible with standard DVD-ROM players. So every -R(W) recording session has to start with a border-in area (except the first one, which starts with the lead-in) and stop with a border-out area.
However, the size of these border zones is quite amazing : 32 to 96 MB for the first zone, then 6 to 18 MB for the next ones. This means that a disc containing 3 recorded sessions can require up to 132 MB (more than 2% of the complete storage capacity) just for separating these regions. Furthermore, the border-out and border-in areas have to be linked together, using one of the 3 methods (and the possible associated problems) explained previously. Note also that for some unknown reason a border-out is needed before the lead-out, while the first border-in is replaced by the lead-in.
On the other hand, when multiple recording sessions are used on a DVD+R(W) disc, Intro and Closure zones are used (the counterpart of border-in and border-out), but they are always 2 MB large : therefore, with + format, a 3 sessions disc always uses only 4 MB to delimit the regions (the lead-out replaces the last Closure zone). Also a nice feature of multi-sessions implementation on DVD+R(W) is that one can use a session to reserve space, i.e. sectors which are left unrecorded (this blank area is called Reserved Fragment). Thus, additional data can be recorded in next sessions while the first one will only be written later : this can be useful for instance when a precise location on the disc has to contain file system tables, which can only be filled after all the files have been written to the disc.
Compatibility is a very sensitive topic when comparing the two technologies, but independently of media, recorders and players quality, some logical causes of incompatibility can be noted. Indeed, both recordable formats use values in the lead-in structures which were forbidden or reserved in the first DVD-ROM specification (e.g. disc structure, recording density, etc) and which can cause compatibility failures on some old or particularly picky players.
A famous example of such logical incompatibility is the “Book Type” field, which indicates the type of the disc. The first DVD-ROM specification only allowed 0 (i.e. read-only) to be written there, but later each recordable format defined its own value to identify itself ; unfortunately it turned out that some players simply refuse to read a disc with a non-zero value. To handle this problem, the latest DVD+R standard specifically authorizes to write a zero Book Type for compatibility reasons, and nowadays many drive manufacturers have made this bit programmable. But this cannot be done for DVD-R(W), as the Book Type (with several other information in the lead-in) are pre-embossed (i.e. pre-recorded) on blank media. Note however that although it reduces compatibility, this pre-recorded Book Type also improves copy-protection security, as it enables any player to easily identify a DVD-R(W) disc.
An additional compatibility risk exist with DVD-(W) in the user data area and is introduced by the 2K/32K linking methods. Indeed, the linking sectors used with these methods must use a special data type to be differentiated from normal data sectors, and this value was not allowed in the original DVD-ROM specification. There’s no known study about the impact of this field on compatibility, but sector headers are a vital part of the decoding process, and therefore it is always safer to keep them fully compliant with the strictest DVD-ROM standard.
During my study of rewritable DVD formats it seemed very clear to me that DVD-R(W) standard was not as well designed as DVD+R(W) (or even DVD-RAM). And although some serious efforts have been put in the latest revisions of the – format to fix some of the original problems (at the cost of a much increased complexity), it still remains technically inferior to +, due to some intrinsic weaknesses (e.g. pre-pits). This is not very surprising, as Sony and Philips have a much longer experience at defining standards than Pioneer (and several key patents), and they also had the advantage to publish their standards after their competitors.
Although the arguments presented in this document might look like technical details to most readers, disc format is what defines the limits of what drives can do with a given medium, both in terms of performances and features. Therefore, the technical advantages of the DVD+R(W) format will with time turn into faster, more powerful and more reliable drives for end users. This is already the case today, and the gap will continue to increase as DVD+R(W) drives will exploit more and more of the advantages of the + format. However, as history showed, the best ideas are not guaranteed to win on technology markets, and only time will tell which format becomes the new standard.
I would like to thank the engineers from Pioneer Japan and Philips Netherlands who reviewed early versions of this article for their most useful corrections and comments (note that this only means that these people kindly contributed to the technical correctness of the article, not that they – or their company – agree with my conclusions of this article). Also many thanks to J.W. Aldershoff for having suggested and organized these reviews, and for hosting this article.
This article is meant to be a list of technical arguments showing some of the advantages of the + format over the – format, and it reflects only my personal opinion, and not the one of CD Freaks. It is not a detailed comparison of the two formats, and it does not take into account specificities of drives, media or third party software : only the format differences are compared here, as described in the DVD-R 2.0, DVD-RW 1.1, DVD+R 1.1 and DVD+RW 1.1 standards. Corrections are welcome by email, but questions and contradictory opinions should be posted to our forum, so that everyone can benefit from an open discussion.
Michael Spath – firstname.lastname@example.org