Category Archives: cd

Att välja DVD-skiva för långtidslagring av bilder, data och film

Jag hittade en intressant blog om att välja DVD-R eller DVD+R skiva – men för de flesta vanliga människor kanske lite väl djup artikel om att välja brännbara DVD. Författaren snöar kanske in lite väl på visa bitar men som helhet har har säkert rätt. Men för dom av oss som har stora bildarkiv på diverse media är detta intressant läsning. När man har lagt ner massor med jobb på att fotografera en bild vill man ju att den finns kvar för framtiden.

Man kan i princip sammanfatta hans artikel med

  • Använd inte återskrivbara skivor för långtidslagring (CD-RW, DVD-RW, DVD+RW)
  • Använd DVD+R istället för DVD-R
  • Välj skivor av bra kvalitet – han rekommenderar skivor från Taiyo Yuden
  • Förvara dem mörkt i syrafria förpackningar

Det går normalt inte att hitta skivor sålda under Taiyo Yuden i Sverige. Men de tillverkar skivor åt flera andra företag som säljer dem under sitt eget varumärke (som med alla annan elektronik). Vad jag vet så är i princip alla skivor från Verbatim tilverkade av Taiyo Yuden, men även lite dyrare skivor från Maxell. Det finns mer information om vilka skivor som är producerade av Taiyo Yuden på The Taiyo Yuden FAQ.

De har skrivits en hel del om brännbara skivor i olika tidskrifter, Datormagazin nr 10-2006, någon av de senaste Ljud och Bild (aprilnumret?). Jag har även en kort artikel på DigitalFotografen.se – Är dina skivor säkra.

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CD and DVD production

The purpose of this report is to give some guiding and to be technological starting point for distribution of CD or DVD disc. I don’t cover the distribution and packaging, just mastering and replication.

There are several books and articles written on CD and DVD production. Very few are on the topic of discs with mixed content (video and software), and most are targeted towards small volume projects. I have interviewed some people in the business and made some conclusions from this.

This text was originally produced as internal report on CD and DVD production for IKEA. It was written in august 2004 so some fact may have changed during those years.

* Images has been deleted due to copyright issues.

CD and DVD technology in brief
The DVD is in many ways an evolution from the CD. From a user perspective it can be seen as a high capacity CD. From a technical and manufacture perspective they belong to the same family, but even family members can be very different. The technology can be divided in a hard and a soft part. The hard part describes how data is physically stored on the disc. The soft part, how data is encoded and organized.

The hard side
CD and DVD store data in spiral tracks of groves on a reflective layer. A laser pickup recognizes the transition between land and grove, and this is decoded as data bits (0 or 1). A DVD stores more data on the same area compared to the CD. This is possible through two enhancements over the CD. The first is to use smaller groves and a “thinner tracks”. The DVD has about half the track spacing and about half the grove size. The second difference is that a DVD can have several layers, and that layers of CD and DVD are at different depth of the disc.

* Illustration from p 76 of “DVD Demystified” by Jim Taylor

A CD contains one reflective layer that is located at the top (just under the label) of the disk. A DVD has up to four reflective layers in the middle of the disc. Two layers can be read from the same side. The outer layer is then semi-reflective. The inner layer is read through the outer by changing the focus of the lens system. Just like you can see through a fence. A double-sided disc must be manually turned by the user, this makes them awkward. Normally a video DVD has two layers that are read from the same side (DVD-9). Shorter videos or data DVD (DVD-ROM) are normally single side single layer (DVD-5)

* Illustration from p 78 of “DVD Demystified” by Jim Taylor

All DVD players can read CD:s, while a standard CD-player can’t read a DVD.

Data capacity

The driving force in the DVD development was to squeeze in a full length movie without the need to turn the disc. Some space is needed for error correction, file structure, headers etc. How much depends on the content type, but 5-20% is a common. It’s possible to reduce the track spacing and gain more capacity. This is how 800 MB CD is implemented. 1 hour of average video takes about 2 gigabyte on a DVD.

Type Sides Layers Data Capacity GB
CD 1 1 0.68
DVD-5 1 1 4.70
DVD-9 1 2 8.54
DVD-10 2 1 9.40
DVD-18 2 2 17.08

Recordable discs
There are recordable CD and DVD discs. The disc contain a dye layer in front of the reflecting layer. A strong laser modifies this dye layer to record data. On a rewriteable disc this dye layer can also be restored to original state, so the disc can be reused. Recordable discs are a convenient way to store, backup and transport large volumes of data. CD-writers are today standard, and many computers are today shipped with DVD-writer. Rewritable DVD is still uncommon, but DVD-RW is soon expected to be available.

Disc types
CD-R Recordable CD
CD-RW Rewritable CD. Need special writer, disc must be erased after rewrite.
DVD-RAM First type of rewritable DVD. Can’t be used in all DVD-players, and disc is normally kept inside a cartridge. Normally only used for backup and data transfer. Can be double sided.
DVD-R
The most common type of recordable DVD
DVD+R Competing format with DVD-R. Some drives accept both DVD-R and DVD+R
Disc-at-Once vs Track-at-Once — Most CD-recorders only support Track-at-Once. They make a short pause in between tracks leaving a few blank sectors. This may result in clicks on some CD-players. For professional use (specially for mastering) Disc-at-Once should be used, which means that the whole CD is burnt without those pauses.

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