Technical Information

The Next Step in DVD Evolution

The DVD Forum is a body, made up of representatives of all the major developers of disc technology and hardware, which decides the future direction of the optical disc industry. At the recent meeting of its steering committee in New York, the DVD Forum was confronted with two different strategies to implement the next-generation HD disc standard, and there is no obvious roadmap for success.

Up for vote in New York was the specification for a new format HD-DVD, specifying a blue-laser diode technology, and based on Advanced Optical Disc (AOD) technology, co-developed by Toshiba and NEC as a successor to the current DVD specification. Evolutionary, and designed to maintain full backward compatibility with current DVD discs, AOD adopts the same bonded-disc structure as the red-laser DVD systems currently used. However, the data capacity is increased three to six times to store a high-definition movie, according to HD-DVD proponents.

Meanwhile, the opposing Blu-ray group, developed by Philips and backed by ten companies including Sony, Samsung and Matsushita, had been gearing up for the completion of its revolutionary, non-backward compatible BD-ROM version 0.9 specification, but this project has now been put on hold.

In contrast to the HD-DVD proposal, BD-ROM rejects the use of a new video compression format and sticks with the same audio/video codecs such as MPEG-2 and Dolby Digital, which are specified by the US digital HDTV system. Observers believe that Blu-ray may have been withdrawn, for now, by Philips Research, because it may be too expensive for the average user, and it is unlikely many studios will master to it because of the cost.

The HD-DVD format was finally approved by the DVD Forum’s steering committee, and will be available in both read only and re-writable formats, but only the ROM format in its 0.9 version was approved by the committee, by a vote of 8-6.

HD-DVD is a blue laser-based optical disc system with a capacity of 15-20 GB per side, and uses the same disc structure as current DVD discs.

Masters & Archiving – How Long Will A CD Master Last?

Unlike paper or parchment, optical discs have not been around that long, so there is no actual experience of just how long the discs, and more importantly, the data, will last. Some say 10 years, some say 30, others say 300. Who’s right? Well, we’ll probably have to wait for 300 years to be absolutely sure, but until then, there are various accelerated lifetime tests that can provide some basic insight into disc longevity.

If you’re storing audio or CD-ROM masters, or archiving information on CDs, this helpful article will hopefully give more answers than it raises questions.

The most vulnerable part of a CD is the reflective layer. If this is damaged or degraded, then the disc can quickly become unplayable. If air gets to the metallised layer, either at the edge of the disc or by damaging the printed side, then life expectancy will fall off very quickly. Given enough time, moisture, oxygen and aggressive airborne pollutants such as sulphur, can also find their way through the polycarbonate substrate or protective lacquer. If these materials manage to reach the metallised layer, then a chemical reaction will take place, resulting in a loss of reflectivity. In the best-case scenario, the disc will be totally unplayable.

Another factor that can potentially affect disc life is writing on the disc. Any sharp or pointed writing implements should be avoided, as these might easily dig into the protective lacquer. And it would be best to avoid using inks with solvents. In some cases, these could have a detrimental reaction with the protective lacquer.

Another thing to avoid is the paper label. The paper itself may not be a problem, but the label adhesive could well be. But what if you have already stuck a label on an important disc? Leave it. Trying to pull the label off might well dislodge the protective lacquer, particularly if it is very thin or poorly applied, and this would spell disaster for the long-term survival of the disc.

Although the polycarbonate in a CD will not usually degrade or discolour if exposed to sunlight, care still has to be taken with sunlight as it is a heat source, and this can create serious problems, as those who have left their discs on a car dashboard have no doubt discovered. Humidity can also be a problem, but short-term exposure shouldn’t represent a serious problem. Polycarbonate naturally absorbs moisture, so if you have had problems playing your CDs in a steamy bathroom this could be the reason. Fortunately, if you do nothing, just let the disc dry off naturally, everything should return to normal.

So the best advice, is if you want your discs to really last, store them individually in a cool, dry, dark, stable place. Never play them and never touch them. And if you did that, how long would they last?

It depends on the type of CD we are talking about and some research suggests it also depends on which part of the disc you are looking at. Apparently, the centre section of the program area is the most robust part of the disc and this would make sense when you take into account the affect of edge damage, both on the outside of the disc and around the centre hole. So if the data is ultra important, don’t put it at the start or the end of the disc.

Also it may surprise some readers that rewritable discs (CD-RW, DVD-RW, DVD-RAM and DVD+RW) are not generally regarded as archival formats. This is because the materials used in the recording stacks can be adversely affected by heat and UV light. Some researchers also believe that the repeated re-recording process can accelerate ageing in the materials and this, too, reduces the archiving potential of the media.

Although a light and heat sensitive dye-based product like CD-R may seem more vulnerable, and therefore less suitable for archiving than a rewritable disc, the opposite is actually true. All the current recording dyes will break down given enough time. UV light (ie, sunlight) can accelerate this process, but once the disc has been recorded, this is no longer an issue. If longevity is your primary concern, then phthalocyanine probably has a slight edge over cyanine because it is less sensitive to light. If you keep your archive CD-Rs in the dark, then this obviously isn’t going to be a critical issue.

Today, there is a wide range of standards laid down for storing optical media. These include ISO18925, plus recommendations laid down by organisations such as the National Library of Canada and the National Archives of Australia. Typically, all these institutions provide recommended storage temperatures and relative humidity levels plus acceptable rates of change. A gradual change of temperature or humidity (no more than 5% or 10% over a 24-hour period) is far more preferable than a sudden blast of hot or cold air or rapid change in humidity.

It is interesting to note, that whilst a lot of people appear to be interested in long-term disc life, little is written about the initial shelf life of a recordable disc. Five to ten years seems a reasonable estimate according to some research, but actually finding a sell-by date on a box of CD-Rs or DVDRs is a lot harder to come by.

What is Postgap and why is it needed?

Postgap is the 2-second gap after every data track, occupying 150 sectors on a CD ROM master.