Aren’t we all familiar with the good old “CD” albums? Well, I wouldn’t be too quick to say “yes” to that question. We’ve all seen CDs at some point in our lives, but there is probably much more to them that we do not know. If you might be serious about starting your own audio productions some day, then ask this – What is a Compact Disc?
I feel that today’s subject is a really important one, as compact discs are pretty much the physical representation of the final product of any audio or video project. That said, in today’s article, we will talk more about the purpose of the compact disc, as well as its various characteristics that makes it suitable for the job. Let us begin the learning!
Compact Disc – Its Purpose
Two companies, Philips and Sony, co-developed a digital optical disc data storage format, called the compact disc (CD). The original purpose for the development of CDs, was to store and play only audio recordings but was later redesigned to also allow for storage of data (CD-ROM).
There are also several other formats that were created, including write-once audio and data storage (CD-R), rewritable media (CD-RW), Video Compact Disc (VCD), Super Video Compact Disc (SVCD), CD-i, and Enhanced Music CD. Since October 1982, audio CD players have already been commercially available, when the first ever CD player was released in Japan.
Standard CDs have a measurement of 120 millimetres in diameter (4.7 inches) and can store up to about 80 minutes of uncompressed audio (equivalent to 700 MiB of data). On the other hand, the Mini CD comes in various sizes ranging from 60 to 80 millimetres in diameter (2.4 to 3.1 inches) and is able to store up to 24 minutes of audio. It is often used for installing device drivers.
Let us now take a look at the topics of discussion:
- Audio CD formats
- Recordable CD
- ReWritable CD
- CD copy protection
Audio CD Formats
In 1980, Sony and Philips had created a document known as the “Red Book” (due to the colour of the book cover). It describes the logical format of an audio CD (Compact Disc Digital Audio or CD-DA), which is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate per channel. “Four-channel” sound was to be considered in the Red Book as well, but was never implemented.
On the other hand, there is no existing standard created for monaural audio (mono) on a Red Book CD. Hence, mono audio sources are typically formatted as two identical channels in a standard Red Book stereo track (also known as “mirrored mono”). However, do take note that an “MP3 CD”, can store audio file formats with mono sound.
“CD-Text” refers to the additional text information such as “album name”, “song name” and “artist” on a standards-compliant audio CD. “CD-Text” is basically a 1996 extension of the Red Book specification. The information is stored either in the lead-in area of the CD (which has about 5 kilobytes of space available), or in the subcode channels R to W on the disc (which stores up to 31 megabytes).
This CD format contains both audio and graphics data. It can be played by a typical audio CD player, but will only output graphics signal when played on a special CD+G player (which typically connects to a television or a computer monitor). CD+G format is often used to display lyrics on a screen for karaoke purposes. This format takes advantage of the channels R through W.
CD + Extended Graphics (CD+EG)
Also known as “CD+XG”, this format is basically an improved version of the Compact Disc + Graphics (CD+G) format (explained above). Similar to “CD+G”, “CD+EG” utilizes basic CD-ROM features to display text and video information in addition to the music being played. This extra data is stored in subcode channels R-W. CD+EG discs are very rarely published in the market.
Known as CD-Rs, these discs are injection-molded with a “blank” data spiral. A photosensitive dye is then applied before the discs are metalized and lacquer-coated. A CD recorder’s write laser changes the colour of the dye to allow a standard CD player’s laser to “see” the data, just like with a standard stamped disc. Most audio CD players and CD-ROM drives are able to read CD-Rs.
Any recordings done on a CD-R will be permanent and cannot be re-written. But over time, there will be read errors and data loss, due to a change in the dye’s physical characteristics. Reading devices might also not be able to recover the data with error correction methods. CD-R’s lifespan is from 20 to 100 years, depending on the quality of the disc and writing drive, and also storage conditions.
However, do take note that testing has shown some discs degrading in quality by just 18 months, even though they are stored under normal storage conditions. This phenomenon is widely known as “disc rot”, which is largely caused by environmental circumstances, apart from other factors.
Known as “CD-RW”, this disc is re-recordable and uses a metallic alloy instead instead of a dye. In this case, the write laser heats and alters the alloy’s properties, thus changing its reflectivity. There isn’t a huge difference in reflectivity between a CD-RW and a CD-R, and many earlier CD audio players can’t read CD-RW discs, although later on, most CD audio players are able to.
Originally, the ReWritable CD cannot be written faster than 4x speed. But a “High Speed” ReWritable CD is designed to allow writing at speeds ranging from 4x to 12x. Also, early CD-RW drives are only able to write to original ReWritable CDs. But “High Speed” CD-RW drives are capable of writing to both original ReWritable CDs and High Speed ReWritable CDs.
Keep in mind that both CD-RW disc types can often be read in most CD drives. Today, there are also higher speed CD-RW discs available in the market, such as the Ultra Speed (16x to 24x write speed) and Ultra Speed+ (32x write speed).
The Red Book specification does not have any mechanism for copy protection (except for a simple “anti-copy” statement in the subcode). Ever since 2001, there have been attempts made by record companies to market “copy-protected” non-standard compact discs, which cannot be ripped, or copied, to hard drives or easily converted to other formats (like Flac, MP3 or Vorbis).
However, most of these copy-protected discs will not play on computer CD-ROM drives or any standalone CD players that use CD-ROM mechanisms. Philips has also stated that such discs cannot be labelled as Compact Disc Digital Audio, as they violate the Red Book specifications. Furthermore, numerous copy-protection systems have been negated by various free software.
We have come to the end of the article. Did you expect this much information about compact discs? Well, whatever the case, I hope you have learnt something new!
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