Writing Modes
Building upon the advances made in CD-R and CD-RW technology writable DVD performance has progressed rapidly in a relatively short time. DVD recording speeds (data transfer rates) now surpass even their quickest CD-R and CD-RW counterparts. And, as with CD-R and CD-RW, DVD recorders employ a variety of writing modes to operate reliably and efficiently at both low and high speeds including Constant Linear Velocity (CLV), Zoned Constant Linear Velocity (ZCLV) and Constant Angular Velocity (CAV).
Constant Linear Velocity (CLV)
DVDs initially operated using a CLV mode to maintain a constant data transfer rate across the entire disc. The CLV mode sets the disc’s rotation at roughly 1400 RPM decreasing to 580 RPM (1x CLV) as the optical head of the player or recorder reads or writes from the inner to outer diameter (ID to OD). Since the entire disc is written at a uniform transfer rate it takes, for example, roughly 57 minutes (excluding lead-in/lead-out) to complete a full 4.7 GB disc at 1x CLV. As recording speed increases the transfer rate increases correspondingly so that at 4x CLV writing a disc takes approximately 14 minutes. Recording time as well is directly related to the amount of information to be written so partial discs will be completed in proportionally less time. But writing at higher speeds requires rotating the disc faster and faster (e.g. ID 8400 to OD 3480 RPM at 6x CLV) which places escalating physical demands upon both media and hardware. Manufacturers meet this challenge by moving beyond the CLV mode to obtain even higher performance.
Zoned Constant Linear Velocity (ZCLV)
In contrast to CLV which maintains a constant data transfer rate throughout the recording process, ZCLV divides the disc into regions or zones and employs progressively faster CLV writing speeds in each. For example, an 8x ZCLV DVD+R/+RW recorder might write the first 800 MB of the disc at 6x CLV and the remainder at 8x CLV speed. DVD-RAM (1x, 2x, 3x), on the other hand, uses a different form of ZCLV that divides the disc into many more regions (1.46 GB disc/14 zones, 2.6 GB disc/24 zones, 4.7 GB disc/34 zones). Here, rotational speed is kept constant within each zone but varies from zone to zone resulting in a roughly constant data transfer rate throughout the entire recording process.
Constant Angular Velocity (CAV)
The CAV mode spins the disc at a constant RPM throughout the entire writing process. Consequently, the data transfer rate continuously increases as the optical head writes from the inner to outer diameter of the disc. For example, a 5x CAV DVD-RAM recorder begins writing at 2x at the inner diameter of the disc accelerating to 5x by the outer diameter of the disc.
Verification and Defect Management
In addition to simply writing data, some recording software and hardware perform data verification or employ sophisticated defect management techniques, which can double the total amount of time to write the disc. Typically, data verification takes place after all data is written while defect management occurs during writing, actively verifying sectors and skipping over or relocating problems to a spare area of the disc. Data verification, a feature found in some recording software, works with most disc formats and often can be switched on or off. Both hardware (DVD-RAM, DVD+MRW) and software (UDF 2.0 formatted DVD-RW, DVD+RW and DVD-RAM) defect management is available and typically cannot be deactivated (consumer electronics DVD-RAM recorders typically do not implement defect management while, under certain circumstances, some computer DVD-RAM recorders may be able to partially disable this function).
Can writable DVD discs written at different speeds be read back at any speed?
The speed at which a disc is written has nothing to do with the speed at which it can be read back in a recorder, player or DVD-ROM drive.
How might DVD recording speeds increase in the future?
Product manufacturers have publicly discussed plans to increase DVD recording speeds to as high as 16x CAV. Achieving data transfer rates beyond this may be possible but is generally thought not to be practical given the technical challenges and cost considerations involved for minimal increases in real world performance. This is analogous to what happened with CD-R recording which, facing similar design (rotational speed capabilities of commercially available spindle motors) and market issues (high vibration and sound levels), effectively peaked at 52x/54x CAV speed.