DVD media reviewed

DVD media: who's who?

Recordable and rewritable DVD media business is booming. DVD recording is rapidly gaining popularity and everyone out there seems to want a part of the lucrative media business.

Previously we published an article on the differences between the media types. This time we dug a bit more into the technology and found some interesting things.

There are currently 4 types of DVD media (DVD-RAM not counting).
For both of the DVD-recordable and the DVD+recordable there is a rewritable variant (DVD-RW and DVD+RW). Difference is in the recording technology and recording method. The discs themselves look almost identical to the naked eye. But there is more then meets the eye …

Yep. You read that right. Blank DVD recordable media has data on it. Concealed information is stored on a blank disc that is used by the recorder to find out the optimum recording speed, power setting to use and much more. This data also stores physical address information that is used by the recorder to find a given track both during recording and playback.

And here is the catch. This information is not only differently formatted between the + and – standard, but is also physically stored in a different way.

An empty recordable disc ( whether DVD or CD doesn't matter they both use this technology ) has grooves pressed in it. This is done during manufacturing of the disc. These grooves are used to guide the recording laser when burning data.

Data is stored on a DVD or CD in concentric circles. When a CD or DVD is stamped ( the technology used to manufacture prerecorded discs ) these circles are nice and concentric. A playback device has no problem following the track.

An empty medium would have no guide for the optical head to follow. If you were to record to an empty disc that has no optical guide for the laser head it would be very hard to burn concentric circles. The slightest mechanical tolerance in the recording drive would lead to possible overwriting of previously recorded information. A slight vibration is all there would be needed.

To solve this problem empty discs also carry grooves. The recording electronics tracks these grooves and steers the laser head towards the center ( the valley ) of the groove, where the data needs to be stored ( or retrieved when the disc is being read ).

The data is stored as round or oblong pits inside this groove.

These grooves do not describe a perfect circle, but they have a slight wobble. The tracking mechanism follows this wobble. The wobble is done on purpose and holds information.

It is first of all used to maintain the correct rotational speed. The frequency of this wobble is measured and motor speed adjusted until the disk spins at a preset wobble frequency.

But it goes way beyond that. The wobble itself is also a carrier of information. And this is where the first differences are between DVD-R and DVD+r.

DVD-R stores information between grooves. Big pits are blasted on the edge of the groove. As explained before the groove wobble is being tracked during read and write to maintain the correct speed.

The big pits near the edge of the groove show up as spikes on this tracking signal. A detection circuit detects these and decodes them.

Their relative position to each other can be used to store 1's and 0's. A drawback of this technology is that if a small damage occurs this information might become corrupted.

The detection electronics also has to be able to detect these small bumps. With recording speeds increasing ( and this disk speeds ) this can get tricky. This turns out to be an inherent weakness of the DVD-R technology.

DVD+R uses a technique called Phase modulation. Here the phase of the groove suddenly changes. The wobble is still present and thus correct speed is maintained. However if the signal suddenly changes 180 degree out of what would be expected this is marked as a phase jump.

The delay between such jumps is used to store information. A 0 could be represented by 16 periods of opposite phase, while a 1 could be 32 periods of opposite phase. The advantage is that small damages to the groove are not detected falsely. This methodology is much more robust and protected against read errors. Also since now information is stored over a very long time ( multiple periods of the wobble instead of only a fraction of a period as in DVD-R ) the electronics has less problems reading correctly. Even at higher recording speeds this poses no problem.

This also explains the compatibility problems between DVD-R and DVD+R. The principle of recording remains the same. Blast holes of different shape inside a groove. But decoding the information present on the groove wobble is different. For playback this information is not needed. But with DVD+R the phase shift in the wobble causes older readers to loose the groove ( like an old record where the needle skips a track ) Older playback drive can easily handle the DVD-R system as here the disturbance is very small.

Newer drives ( playback ) have no problems with any of these systems. Actually some systems now even decode this information to retrieve information that can be useful during playback. There are more technicalities to both systems and explaining them goes way beyond the scope of this article. But the above is a simplified overview on the real differences between the 2 systems.

We are more interested in the information contained in these hidden information.

We already mentioned track index information but there is information on recording speed and power settings as well. Some blocks are used to store the manufacturer’s information. And this is very interesting since it allows us to find out who really made the disc.

Almost any manufacturer that sells drives also has media with his name on it. It was already a well known fact that most of the drives out there are based on either Pioneer or Ricoh chassis. The same can be said for empty media. There are only a few select companies that actually make media. Most of these companies never sell under their name but act as suppliers to other vendors.