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DVD Design Considerations
Three objectives had to be resolved to make the DVD a financially viable medium.

First:....... The linear velocity of a DVD must be held constant and be able to reproduce a vertical frame rate of 29.97 frames/second to meet RS-170A specifications for sync signals to maintain compatibility with the rest of the video world (at least in the case of creating a Video DVD). Consider that if the rpm is held constant, then the linear velocity will be quite different from the inner tracks compared to the outer tracks. Thus there must be a mechanism to measure the linear velocity and accurately adjust the disk rpm to maintain a constant linear velocity.

Second: ....... Every DVD player had to have absolute tracking accuracy to insure the extremely narrow laser beam would scan exactly in the middle of the track where the data was recorded. Consider that the track width on a DVD is only .74 um (microns) in width - which is much smaller than a single hair which is typically 50 microns in diameter. Approximately 67 DVD grooves would fit rather nicely in the width of a single human hair ! Add to that, the vagaries of rotating mechanical hardware, fluctuating power line voltages etc, and it became obvious this was not going to be an easy inexpensive task.

Third: ....... The real engineering "killer" was that the DVD player had to be made affordable (read: Cheap !) if it ever was to be a viable product. Building sophisticated tracking electro-mechanical mechanisms into each DVD player and have them remain compatible with high repeatable accuracy across different manufacturer's product lines and media offerings, was not an option. No way that was ever gonna work at an even semi - reasonable price tag. Add to that the mechanism being jostled about in shipping etc, and it was a real engineering challenge. So some clever engineers dreamt up a system whereby each "blank" DVD was to have what is known as pre-grooves. Thus a blank DVD disk isn't really blank at all. The disk already is pressed with the track grooves accurately pre-cut and encoded with a constant bit rate frequency.

There are notable differences between DVD-R and +R formats

DVD-R
The pre-grooves in the case of DVD-R and DVD-RW discs, are not perfect spirals. Instead, the groove is modulated with a constant frequency of 140.6 kHz, known also as the wobble frequency (since the groove actually wobbles !) Much like a lateral cut phonograph groove, groove wobbling means that the grooves wander back and forth in sinusoidal fashion at a fixed amplitude. This constant frequency allows accurate tracking by the laser as well as provides a highly accurate timing signal to which the write clock frequency is derived.

Between the grooves are the pre-pits. The pre-pits contain the sector addressing information.

DVD+R

The +R format pre-groove also uses a wobble frequency, but at a much higher frequency of 817 kHz. Instead of pre-pits, the +R formats convey the sector addressing information by frequency modulating the wobble frequency.

Thus the + and - format DVD's are quite different in their approach to encoding sector addressing and other low level information.

There are a wealth of technical articles on the web delving into the differences in much greater detail, but this is the simple gist of it all !



Anyways, these pre-grooves solve the two most perplexing problems. First of all, the position of the tracks are now accurately controlled at the time of DVD manufacture, allowing a simple and inexpensive linear tracking mechanism to be used in the player/recorder. All the DVD player now has to do, is to read the constant frequency in the pre-groves and keep the signal peaked...... a simple task. The frequency of the recorded "tone" also provides highly accurate timing pulses which are used as a highly accurate timing reference.

Thus much of the impressive accuracy required to make the system work was done when the "blank" DVD was pressed.

Just released newer DVD technology, employs a blue laser with a much shorter wavelength. Data recording densities employing the blue laser will offer up to 27 GB on a single layer side - that's greater than 5 times the capacity of a standard DVD ! (and multi-layer burners are already here !)

Also added is the DL (Double layer) recordable media and DL capable burners - allowing up to approximately 8.5gb per side. So far the DL recordable technology is limited to the +R format. Prices for blank DL media is still a bit "steep", but will undoubtedly drop as the efficiencies of mass production increase.

5. Burning Speed

Although your burner may support 2x, 4x or even 8x burn speeds, the media itself may not. There is a fine balance between the dye layer formulation, response to the laser, laser power levels and speed of the burn. They are all inter-related.

It's somewhat akin to passing your finger thru the flame of a candle. A quick pass thru, and you'll barely feel it.... Stop over the flame however, and you'll end up with a charred finger (assuming you can stand the pain).

Burning a DVD isn't conceptually much different. Upset that delicate balance between burn speed, laser power and dye layer formulation, then skips & freezes will be the likely result. When in doubt, burn at a slower speed.

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