Hard discs are the main form of permanent data storage used in computers and, increasingly these days, portable music players, video recorders and even car music players. In a PC, a hard disc is invariably used to store the operating system (e.g. Linux, MacOS), the application programs that you use (e.g. Firefox, an office suite) and the data that you create, download or receive (documents, MP3 files, email).
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A hard disc drive consists of several main components. The first is one or more thin discs that spin at anything from 3650 to 15000 RPM. The slower speeds - 3650 up to 5400 or occasionally 7200 - are used on laptops, mainly to minimise power consumption and noise. Speeds of 5400 and 7200 and occasionally 10000 RPM (10k) are used on desktop computers where power consumption is not so much of an issue and noise can be more easily suppressed. Drives running at 10k and 15k are generally used only in server computers. The 15k drives especially are very noisy and generate quite a bit of heat. It goes without saying that the higher the rotational speed of the disc, all else being equal, the better the performance. Although there is no physical manifestation of them, the surface of a disc is treated as having a number of tracks - concentric circles (unlike a vinyl disc which has a single spiral). If there is more than one recording surface (usually there are at least two), then the tracks in corresponding positions on each surface are said to form a cylinder (in fact, you can have a cylinder with one track in it but this is now virtually unseen). The disc assembly is rotated at a constant speed by an electronically controlled motor. any slight variations in speed are fed back to the electronics by sensors and this enables them to keep track of exactly where the disc is in it's rotation and to correct the speed.
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The second major component is the head assembly. This consists of record/playback heads mounted on very low mass but very rigid arms which can pivot so that the heads can be placed over any of the tracks. The pivot that the heads assembly swivels on is a genuine marvel of modern technological engineering, particularly as it is mass produced at such a low price. The head assembly is moved by what is known as a "voice coil". The term comes from loudspeaker manufacturing where the coil which moves the speaker's cone has been called a "voice coil", with some justification!, for around 100 years. The coil is mounted on the opposite side of the pivot from the arms carrying the heads. It is placed between two very powerful permanent magnets. Passing an electric current through the coil creates an electromagnetic force which moves the whole assembly on its pivot. Feedback from sensors allows the drive electronics to know exactly where the assembly is and how fast it is moving. This information is used to control the current in the coil to make the heads arrive at the the required track in the shortest possible time. This process is called "seeking".
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The surfaces of the disc are coated with a very thin layer of magnetic material. This is then coated with one or more protective layers. When the disc is rotating, the heads (which are on tiny spring mounts on the end of the arms) "fly" above the surface of the disc at a distance of about 1/10th the diameter of a human hair. In fact, in some modern drives it is considerably less than this. The interaction between the disk surface, the tiny, specially shaped head and the air between them gives rise to aerodynamic forces which keep the head at exactly the required height for optimum reading & writing performance. These forces are considerably greater than the force of gravity so that a modern disc unit can run happily in virtually any orientation - vertical, horizontal etc.
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Because of the tiny distance between the head and disc, any tiny dust particle present could easily get between them and force them apart. This could result in anything from some data being skipped to the head bouncing and scratching the disc. The latter is referred to as a "Head crash" and invariably results in permanent loss of recorded data and usually the need to replace the drive.
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To write a single data bit to the disc, the drive electronics passes a tiny, precisely shaped pulse of current through the coil of the read/write head. This causes a small part of the disc coating to become magnetised in a particular orientation. In older drives, each bit was recorded with a N and a S pole along the direction of spin of the disc. Modern drives store data at a much greater density and in order to achieve this the bits are stored as tiny magnets which are perpendicular to the surface of the disc - one of the poles is beneath the other, within the magnetic layer. In practice, a single bit is never written and a complex waveform s generated by the drive in order to write a "block" of data.
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Each track is subdivided into a number of blocks. The exact number varies with the size of the drive and the block size selected when you format the drive (but that's another story). Data is always read and written as a whole number of blocks. So, a block occupies an arc along a track. The blocks are numbered and that number (along with the cylinder and track numbers) is recorded in a part of the block that does not hold data. Each block is actually divided into three "fields" - Count, Key and Data. This allows the drive to check, after a seek, that it has indeed found the location on the disc that it was seeking and that it has the right size. These extra bits of information (Count and Key) plus some other housekeeping information and the gaps between the blocks, account for the difference between the nominal size of a disc drive (e.g. 80 GB) and the formatted size (e.g. 76 GB).
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When the drive has to read data, it moves the head so that it passes over the area of the disc containing the required data. The combination of physical movement and they tiny magnetic fields on the disc cause a minuscule electric current to be generated in the coil of the read/write head. That current is first amplified by an integrated circuit which is as close to the head as can be arranged, in order to minimise signal loss due to long wires. The amplified signal is then passed to other circuitry which performs more amplification and conditioning of the signal before it is subject to Digital Signal Processing which recovers the recorded data. So, the controller circuit board on the drive is a fairly complex computer in its own right - it will often have two or three embedded processors: One for the interface to the host computer, another for the read/write processing and perhaps a third to control the spindle motor (the discs are mounted on the spindle - or axle).
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A further article will explain how the data - which arrives at & comes from the read/write head as a stream of bits - gets transferred to the host computer (e.g. your ) and ends up as the collection of bytes which makes up, e.g., your Word document. http://blog.lowlightimages.com/blog
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Article Source: http://EzineArticles.com/?expert=Dave_Harris
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