FACTOIDS: LASER & MEDIA ISSUES (DVD9 Added)

[charlie_ps2]

General Information
As with CD-R, DVD-R/DVD+R (DVDR) uses an organic dye polymer that reacts to around 10 milli-watts recording power. Because DVDR holds around 7 times CD-R capacity the laser wavelength is shorter (650nm red against 780nm infrared) and the DVDR recording lens has a nominal aperture of 0.6 against CD-R of 0.5. These two factors combine through the higly focused red laser beam to produce opaque burned marks as small as 0.44um as compared with 0.83um on CD-R.

The dye polymer is coated onto an optically clear polycarbonate substrate that has a microscopic groove (known as the pre-groove), 650nm deep, formed as a spiral track across the surface. Behind that a thin layer of reflective metal is sprayed and then optically clear protective layers are applied to form the disk that we see.

For recording, the tightly focused (on the pre-groove) laser beam heats the dye polymer, permanently altering its state (to opaqueness) such that microscopic marks are formed in the pre-groove, the length of which depending on the time during which the laser was turned on. These marks correspond with the raised portions of a pressed DVD and are thus non-reflective or insufficiently reflective (provided that laser burning power was adequate) to register upon reading.

For reading, the areas between marks, not being opaque, are reflective when a tightly focused laser beam of much lower power at 650nm wavelength is sent to the pre-groove.

Data is then streamed by noting the transition from reflective to non-reflective and the length of time to the next transition. A digital-to-analogue decoder then converts this 0/1 stream to data as we know it

So the critical factors that enable a DVDR to be accurately read (assuming a clean lens) are:
a Sufficient power to burn opaque marks
b Sufficient precision to burn marks of the correct length and with sharp edges
c Good quality dye polymer to enable the above characteristics to apply
d Sufficient precision in the reader to correctly stream what was correctly written
e Sufficient power in the DVDR read laser to meet the reflectivity characteristics of the DVDR

The relevant ECMA standards require the read laser to accommodate reflectivity in the range 45% to 85%. If the media doesn't make it into this range, then there s no guarantee that the DVDR will be readable. Reflectivity is a function of the metallic layer and the optical clarity of the protective layers.

Competing Standards
DVD-R
This format sort of "came first" governed by the formal standard ECMA 279 (http://www.ecma.ch/ecma1/stand/Ecma-279.htm). All DVD-ROM drives are supposed to be able to handle DVD-Rs burned to this standard and thus within the correct reflectivity range. Sony would calibrate their lasers to work within the 45% to 85% range. DVD-R can burn at a maximum of 2x.

Without going into too much detail, DVD-R uses pre-set sector address information that synchronises at write time. This is the reason why burning speed is constrained to 2x because this has to be read and synchronised.

DVD+R
This is a competing standard backed by Philips, HP, Sony, Yamaha and Ricoh. The essence of DVD+R is its first incarnation as DVD+RW (ECMA 274 http://www.ecma.ch/ecma1/stand/Ecma-274.htm). DVD+RW uses a wobbled groove the count of which replaces the DVD-RW sector address synchronisation during the burn process. DVD+R is the write-once version, also with a polymer dye so that the burned result is narly identical with DVD-R. The "nearly" term reflects the fact that newer read laser mechanisms are more accurate and can resolve the small differences between DVD-R and DVD+R arising from the fact that DVD+R can be burned at 2.4x because of the wobbled groove.

This means that DVD+R can theoretically be read on any PS2 with a good quality/condition laser, such as the v5 and it has been reported as successful on a v3 - though not many v3s these days have new condition lasers!

As ever, this information is provided at your own risk to use.