DVD DL implies the definition; two layers of standard DVDR-5 are pressed together with a transparent spacer and a thin reflector between the two.  The bottom layer (L0) is read and written to in exactly the same manner as DVDR-5.  Reading and writing to the second layer (L1) is surprisingly simple; the laser simply focuses a fraction of a millimeter farther beyond the first recording layer - in focus with the L1.  Since the laser is powerful enough, and L0 is transparent enough, L1 can be easily read. 

Copyright 2004 Sony

L0 is written in the same manner today's DVDR-5s are written.  Whether through CAV or Z-CLV, data is written on L0 as the laser moves from the hub of the disc to the outer rim.  Writing to L1 is then done from the rim of the disc back towards the hub.  This is identical to a DVD-9 read.

Every DVD reader available is capable of the dual layer read operation.  Applying the exact same principle to the write mode has only been a matter of perfecting consumer media and simple modifications on the pickup/laser.  In fact, those who watch our Optical Storage news closely already know that Pioneer previously demonstrated their DVR-106 capable of burning dual layer discs during CES 2004 (January).  The drive only burned at 2X, but considering the DVD-R DL disc was burned using only a modified firmware, the future of existing DVDRW devices looks slightly promising.  Most likely vendors will choose not to release firmware that would enable DL burns, but it is nice to know there is a slim chance they might. 

The first thing everyone should know is that DVDR-9 (as well as DVD-9) are not capable of 9GB of storage.  Reading some of the questions in our forums, it became evident that this is one of the largest preconceptions about the technology.  Both DVDR-9 and DVD-9 are only capable of 8.5GB of storage.  Any more would render them incompatible with the DVD standard.  Tri Layer drives would not be compatible with the standard either, so do not anticipate seeing DVDR-14 in the future either.  For more storage, we will have to look toward BluRay and HD-DVD. 

Just like DVDR-5, DVDR-9 (DVD DL) does not have a uniform write standard. The first implementations of DVDR DL will show up as DVD+R DL (which, as you probably guessed, is backed by the DVD+R Alliance).  DVD-R DL seems possible, as demonstrated by Pioneer, but the DVD-R Forum has been much more conservative with their dual layer strategy.  You can read the official announcement by Philips concerning the DVD+R DL Format specification here.

Don't get too excited (or disappointed) anytime soon.  DVD DL is ready, but it isn't here yet.  Sony believes we will have the first drives ready before July, but the larger issue of media might give DVD DL a difficult jump out of the gate.  It has been six months after the first 8X DVD+R drives debuted, and we are still having problems finding reliable media.  From what we have heard from DVD media and recorder manufacturers, DL media can be extremely low yield and picky.  Even worse, according to manufacturers when media shows up, $5 to $8 per disc could be the norm! 

You may have to be a die hard Dual Layer fan to anticipate 2.4X burn times on $5 media.  A fortunate saving grace for DVD DL is the drive price itself will remain relatively predictable - arriving around the $200 mark and probably dropping very quickly over the next 6 months after that.  But again we ask, is it worth it?  Unless you anticipate backing up your DVD movie collection, burning data onto two DVDR-5 discs (at $0.50 a pop) under 6 minutes a piece may be a better solution.  We already have announcements for 12X DVD+R drives from Plextor, and 16X drive samples should be here before July. Now if we could only get some media...