Optical disc is the generic term when referring to a CD, DVD, blu-ray or other storage medium which required light to read or write data to. Optical discs come with a plethora of acronyms and abbreviations associated with each type of disc. We look at what these all mean, and decipher the jargon to offer you a straightforward guide. By the end of this guide, you’ll be able to identify any type of optical disc, and know its main uses.
Fig 1: CD-ROM’s can be identified from the silver data side of a disc, compared to a greenish/blue tint on CD-R’s.
CD stands for Compact Disc, and ROM stands for Read Only Memory. This type of disc, is supplied pre-pressed with content, this can be in the form of music, data or video. Once the disc is created it cannot be re-written to. Most CD-ROMs do not have any identifying logos, (unless it is included in the design of the CD printing), however, a CD-ROM can be distinguish from other types of CD formats, as the bottom of the disc is fully silver and it is quite difficult to make out a difference in shades between the written data section of the disc, and the section that remain data free.
Fig 2: A recordable CD-R data side has a tint compared to a silver CD-ROM.
A CD-R is a writable disc, (the R stands for Recordable). This type of disc can have data burnt to it once only. If the disc is not finalised or closed with burning, then additional content can be added to the disc, until it reaches its full capacity. A CD-R does not allow for existing content to be deleted off the disc.
You may find the CD-Rs in your collection to contain one of the above numbers after the CD-R acronym. These numbers refer to the length of uncompressed audio that can be written to the disc in minutes. 74 minutes is the industry standard which can contain about 630mb of data. An 80 minutes CD-R can hold about 703mb of data, a 90 minute about 790mb, and 870mb for a 99 minute CD-R. When burning a 90 or 99 minute CD-R most writing software needs to be told to ‘overburn’ the disc, in order to take advantage of the additional capacity. A disc that contains more than 74 minutes of data cannot be guaranteed to play in all CD players, although most new players should recognise the CD without an issue.
The RW stands for Re-Writeable. This type of disc can have data added to it, and once the data is no longer required, this can be deleted and new data added to the disc. In this way, the disc can be used multiple times. A CD-RW can be written to up to 1000 times. A CD-RW can be compared to a flash drive where the content can be wiped and then re-written with new data.
DVD stands for Digital Versatile Disc, with ROM being an abbreviation for Read Only Memory. A DVD-ROMs is supplied pre-pressed usually in DVD-video, although other formats are available. A DVD-ROM has a silver data side, whereas DVD-R’s have a purple die data side. A DVD-ROM has 4.7gb of capacity or almost 7 times that of a standard CD.
Fig 3: A DVD-5 disc with purple data side.
The R in DVD-R stands for writable. Early DVD writers were able to write only one or the other format as manufacturers were split on which format to back. One group of manufacturers is known as the DVD forum, made up of Mitsubishi, Hitachi, Time Warner and Sony support DVD-R, whilst the +R format is supported by the DVD+RW Alliance, who consist of Sony, Yamaha, Philips and Dell.
These days all modern writers support both types of DVD format. There are small technical differences in that the +R format allows for features such as a faster disc eject compared to -R or additional menu functions, but these differences are negligible in the wider scope of things. Both types of disc offer 4.7gb of storage.
For submitting a master for DVD replication there isn’t a single preferred format, both of the above formats can be provided as a master source for conversion to a DVD glass-master and stamper. The easiest way to tell the difference between a DVD-ROM and DVD R is a DVD-R or +R will have a purple data side die, where as a replicated DVD-ROM is silver on the data side.
Whilst comparing different DVD types, it is worth mentioning the quite unique M-Disc DVD. Made out of a rock like data layer, the disc is designed to last for 1000 years. It has even been military tested to withstand the very toughest conditions. The M-Disc has been design to be read in standard DVD drives. An M-Disc cannot be written in a standard DVD burner, but requires a dedicated drive called the M-DISC READY™ writing drive.
The RW stands for Re-Writable, and it works in the same way as a CD-RW. The user can store data to the disc one time or over a period of time, and can delete data and replace it with new content as required. A DVD-RW has a thicker coating so it can withstand more wear and tear, and is rated for up to 1000 write operations. The RW format has a storage capacity of 4.7gb.
A DVD-RAM (Random Access Memory), has the same functionality as a +RW or -RW, however, it has a lifetime rating of 30 years or more, and a possible 10000 erase or delete operations to the data on the disc. A DVD-RAM offers a more secure storage of data, with fewer chances of the data becoming corrupt over time.
Fig 4: DVD-9 disc with purple data side. It’s slightly darker and bluer than a DVD-5 disc
The DL stands for dual layer. This type of DVD of disc has almost double the storage capacity of a DVD-5 format, with a total space availability of 8.5gb. Physically a DVD-9 disc has the same dimensions as a CD or DVD-5, but will have a thicker protective lacquer coating to protect the data side.
All the above talked about formats of DVD-5 are available as a DVD-9 disc, all except the M-DISC format.
These are a rarer format of DVD-ROM offering larger data storage capacities. DVD-10, 14, and 18 are double sided discs, where if we are watching a film, then the disc needs to be flipped in the player to access the other half of the content. Using these types of DVDs offers a good piracy deterrent, and often used with padded out data to offer protection from the content being copied in a direct DVD-to-DVD copy procedure, as re-writeable versions of these discs are not available.
The HD stands for High Definition, and was launched as a direct competitor to Blu-ray disc. This format functions just as a DVD disc, but allows up to 30gb of data on a dual layer, and 15gb of data on a single layer. In February 2008 the HD-DVD format was abandoned leaving only Blu-ray as the sole optical disc format for HD.
BD is short for Blu-ray Disc. BD discs are a Read only format, and offer storage capacity of 25gb on a single layer and 50gb on to a dual layer disc. BD-XL format allows for a huge 128gb of data storage on a single disc. A Blu-ray disc is the ideal format for HD video on to optical media, and this includes 3D Blu-ray. An experimental format of Blu-ray is in development which allows for up to 200gb of data, this will be ideal for storage of the 4K HD format, it is worth nothing current Blu-ray readers and writers do not support the 200gb format as the current drives are not programmed to read the additional data layers found on a BD 200gb disc.
Fig 5: A blank Blu-ray disc data side, with 25gb capacity.
This stands for Blu-ray Disc Rewriteable. Same functionality as a DVD+RW or -RW but with the above mentioned storage capacity of 25gb or 50gb. The data can be written and erased as required.
When a disc is replicated, then the finished form is identified with the ROM extension, which stands for Read Only Memory. A replicated CD, DVD, or Blu-ray is created in an injection moulding process rather than data being burnt to the disc with a laser.
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For duplication any of the other mentioned format apart from ROM can be used for DVD, Blu-ray or CD duplication. However, the majority of duplicated discs are Recordable (R’s), and are created and finalised in a Blu-ray, DVD or CD duplicator towers.
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The image below depicts the data layer of popular disc formats, and on head to head comparision, it is quite clear and easy to identify different types of optical discs from the colour of the discs data surface.
Fig 6: Above we can see 5 types of disc data layers, CD-R (greenish tint), CD-ROM (silver), DVD-R (DVD-5) (purple), DVD-R (DVD-9) (blueish / purple) and Blu-ray BD-E 25gb (dark gold / brown).
As with most things, a variety of quality is available, however, for reliability it is advised to use grade A discs to minimise chances of any data corruption.
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