• Choose a category
• All
• Classic-Fiction

By Root 1090 0

of the sound that are not audible. This is called lossy compression, because information is lost during the compression process, but when properly implemented there can be a major reduction of data size with little noticeable loss in audio quality. This is the approach that is used with MPEG-1 level 3 audio (MP3), which can achieve compression levels of 10:1 over the original digital audio. Another lossy compression algorithm that achieves similar results is Ogg Vorbis, which is popular with many Linux users because it avoids patent issues with MP3 encoding. Other compression algorithms are optimized for human speech, such as the GSM encoding used by some digital telephone systems. The algorithms used for encoding and decoding audio are sometimes referred to as codecs . Some codecs are based on open standards, such as Ogg and MP3, which can be implemented according to a published specification. Other codes are proprietary, with the format a trade secret held by the developer and people who license the technology. Examples of proprietary codecs are Real Networks' RealAudio, Microsoft's WMA, and Apple's QuickTime.

We've focused mainly on audio up to now. Briefly turning to video, the storing of image data has much in common with sound files. In the case of images, the samples are pixels (picture elements), which represent color using samples of a specific bit depth. Large bit depths can more accurately represent the shades of color at the expense of more storage requirement. Common image bit depths are 8, 16, 24, and 32 bits. A bitmap file simply stores the image pixels in some predefined format. As with audio, there are raw image formats and self-describing formats that contain additional information that allows the file format to be determined.

Compression of image files uses various techniques. Standard compression schemes such as zip and gzip can be used. Run-length encoding, which describes sequences of pixels having the same color, is a good choice for images that contain areas having the same color, such as line drawings. As with audio, there are lossy compression schemes, such as JPEG compression, which is optimized for photographic-type images and designed to provide high compression with little noticeable effect on the image.

To extend still images to video, one can imagine simply stringing together many images arranged in time sequence. Clearly, this quickly generates extremely large files. Compression schemes such as that used for DVD movies use sophisticated algorithms that store some complete images, as well as a mathematical representation of the differences between adjacent frames that allows the images to be re-created. These are lossy encoding algorithms. In addition to the video, a movie also contains one or more sound tracks and other information, such as captioning.

We mentioned Compact Disc Digital Audio, which stores about 600 MB of sound samples on a disc. The ubiquitous CD-ROM uses the same physical format to store computer data, using a filesystem known as the ISO 9660 format. This is a simple directory structure, similar to MS-DOS. The Rock Ridge extensions to ISO 9660 were developed to allow storing of longer filenames and more attributes, making the format suitable for Unix-compatible systems. Microsoft's Joliet filesystem performs a similar function and is used on various flavors of Windows. A CD-ROM can be formatted with both the Rock Ridge and Joliet extensions, making it readable on both Unix-compatible and Windows-compatible systems.

CD-ROMs are produced in a manufacturing facility using expensive equipment. CD-R (compact disc recordable) allows recording of data on a disc using an inexpensive drive, which can be read on a standard CD-ROM drive. CD-RW (compact disc rewritable) extends this with a disc that can be blanked (erased) many times and rewritten with new data.

DVD-ROM drives allow storing of about 4.7 GB of data on the same physical format used for DVD movies. With suitable decoding hardware or software, a PC with a DVD-ROM drive can also view DVD movies. Recently, dual-layer