Comparison between compact disc (CD) and digital versetile disc (DVD).

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M. Tanseer Ali

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Comparison Between

Compact Disc (CD) and Digital Versetile Disc (DVD)

Compact Disc (CD):

Optical disc technology

CD just happens to be one of the first in a long line of optical media which are read using a laser beam which is reflected in varying intensity from the surface of a rapidly rotating metallic disc embedded in a protective plastic substrate. Other optical disc formats include the Philips Video Laserdisc (LD) for recording of analogue video signals, and more recently the minaturized  for digital audio and the much denser  which has myriad applications, including high-definition digital video.

CD is a very adaptable format which was initially only intended to store digital audio data. Today however, CDs are also used to store computer files, multimedia content and even digital video. Additionally, while CD was initially evisaged as a read only medium which was pressed in a die, cast forever with one digital imprint, it wasn't long before ways were found to exploit heat-sensitive dyes as well as the magneto-optical properties of some other compounds to allow CDs to be recorded and rerecorded by powerful lasers and magnetic heads.

The key technology needed to realise optical discs, apart from the laser, is error correction. Without it, optical discs would be far more sensitive and prone to errors than they are known to be today. Error correction, specifically the Cross-Interleaved Reed-Solomon Code (CIRC,) allows small errors in the read signal to be seemlessly corrected by applying mathematical transformations.

Prerecorded audio CDs

Audio CDs contain audio data which has been encoded into a fixed-rate digital bitstream. The audio is sampled at a rate of exactly 44.1 kHz to a resolution of 16 bits, and two channels of such data are stored to give stereo output. This yields a bit rate of approximately 1.4 Mb/s, and such measures are often invoked to compare sound quality of digital audio media and encoding schemes. Generally, a higher bitrate means a nicer sound, although there are schemes to achieve higher quality for a given bitrate (such as Digital Theatre Systems Coherent Acoustics (DTS) encoding, which delivers maximal sound quality over 6 channels at CD bitrate.)

The sampling rate of CDs is no accident. It is believed that the highest frequency a young human can detect is around 22kHz. By setting a sampling rate just above this, the standard supposedly allows perfect reproduction of all audible frequencies (which is not necessarily the same as "all audible waveforms.") The 16 bit linear PCM samples are signed numbers lying between -32767 and +32768, which represent the divergence of the audio waveform from the time axis on a linear (as opposed to logarithmic) scale. The magic number "16" comes from the power of two (and even better, multiple of 8) which allowed an hour of recording time at 44.1 kHz. It allows a theoretical dynamic range of greater than 100 dB. This scheme has become the benchmark for high quality audio in recent times, and all modern audio media and encoding schemes compare themselves to CD quality using phrases like "near CD quality" or "better than CD quality."

The most common type of audio CD is the prerecorded Red Book CD. Initially, the only way to produce optical disc media was through mastering and pressing. First comes the manufacture a glass master, which in turn gives rise to metal stampers which is then be used to press aluminium sheets with the required pits. These sheets are subequently embedded in a plastic substrate to keep them rigid and to afford protection to the aluminium surface. The result is a prerecorded CD.

The name "Red Book" refers to the original CD specification originated by Philips, which specified how audio data should be encoded on CD. The "Red Book" defines what is understood by discs bearing the words "Compact Disc Digital Audio" (often abbreviated to CD-DA in technical documents) and the corresponding logo. "Red Book" is the widely adopted name of the specification for the simple reason that the cover of this specification was red. Since then, in keeping with tradition, the various emerging CD standards have also been commonly referred to by the differing colours of the covers of the books in which their specifications are published.

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Data and hybrid CDs

The "Yellow Book" specification document (in combination with an associated document known as te "High Sierra Application Format") discusses an extension of the CD format, initially only intended to carry digital audio data, to accomodate more rigorously error-protected computer data in a hierarchical filing system. This is the format which forms the basis of all CD-ROM (Compact Disc Read Only Memory) discs in use today. It is more technically referred to as Compact Disc Digital Data (abbreviated to CD-DD.) The "Yellow Book" and associated "High Sierra" document will now simply be referred to collectively as "Yellow Book" in ...

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