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When storing analog information in digital form, the trick is to produce a representation that is very close to the original If the numbers are exact enough (such as a thermometer reading of 714329 degrees) and repeated often enough, they closely represent the original analog information3 Digital video is a sheet of dots, called pixels, each holding a color value This is similar to drawing a picture by coloring in a grid, where each square of the grid can only be filled in with a single color If the squares are small enough and there is sufficient range of colors, the drawing becomes a reasonable facsimile of reality For Blu-ray, each grid of 1920 squares across by 1080 squares down represents a still image, called a frame Twenty-four to sixty frames are shown each second to convey motion Digital audio is a series of numbers representing the intensity, or amplitude, of a sound wave at a given point For BD, these numbers are sampled over 48,000 times a second (sampling may be performed as frequently as 192,000 times a second for super-high fidelity audio), providing a much more accurate recording than is possible with the rough analogues (pun intended) of vinyl records or magnetic tape When a digital audio recording is played back, the stream of numerical values is converted into a series of voltage levels, creating an undulating electrical signal that drives a speaker
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Pits and Marks and Error Correction
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Data is stored on optical discs in the form of microscopic pits (see Figure 21) The space between two pits is called a land On writable discs, pits and lands are often referred to as marks and spaces Read-only discs are stamped in a molding machine from a liquid plastic such as polycarbonate or acrylic and then coated with a reflective metallic layer Writable discs are made of material designed to be physically changed by the heat of a laser, creating marks As the disc spins, the pits (or marks) pass under a reading laser beam and are detected according to the change they cause in the intensity of the beam These changes happen very fast (over 980,000 times per second on Blu-ray Discs) and create a stream of transitions spaced at varying intervals: an encoded digital signal Figure 21 Optical Disc Pits
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3Ironically, digital data is stored on analog media The pits and lands on a DVDor a BD are not of a uniform depth and length, and they do not directly represent ones and zeros They produce a waveform of reflected laser light that represents coded runs of zeros and transition points Digital tape recordings use the same magnetic recording medium as analog tapes Digital connections between AV components (digital audio cables, HDMI cables, etc) encode data as square waves at analog voltage levels However, in all cases, the digital signal threshold is kept far above the noise level of the analog medium so that variations do not cause errors when the data is retrieved
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To help put the burning process in perspective, consider that at 16x DVD recording speeds a disc makes 10,800 rotations per minute, corresponding to a linear velocity of 56 meters per second (over 200 km/h), while marks are burned with a precision of less than 005 micrometer!
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Many people assume that the digital ones and zeros that comprise the data stored on the disc are encoded directly as pits and lands, but the reality is actually much more complicated Pits and lands both represent strings of zeros of varying lengths, and each transition between them represents a one, but neither is a direct representation of the contents of the disc Almost half of the information has been used to pad and rearrange (modulate) the data in sequences and patterns designed to be accurately readable as a string of pulses Modulation makes sure strings of zeros aren t too long (no more than 10) or too short (no fewer than two) and that there is only a single one between them (since ones are represented by edges and it s not physically possible to have two edges together) About 13 percent of the digital signal before modulation is extra information for correcting errors Errors can occur for many reasons, such as imperfections on the disc, dust, scratches, a dirty lens, and so on A human hair is about as wide as 150 pits, so even a speck of dust or a minute air bubble can cover a large number of pits However, the laser beam focuses past the surface of the disc so the spot size at the surface is much larger and is hardly affected by anything smaller than a few millimeters This is similar to the way dust on a camera lens is not visible in the photographs because the dust is out of focus As the data is read from the disc, the error correction information is separated and checked against the remaining information If it doesn t match, the error correction codes are used to try to correct the error The error correction process is like a number square, where you add up columns and rows of numbers (see Figure 22) You could play a game with these squares where a friend randomly changes a number and challenges you to find and correct it If the friend gives you the sums along with the numbers, you can add up the rows and columns and compare your totals against the originals If they don t match, then you know that something is wrong either a number has been changed or the sum has been changed4 If a number has been changed, Figure 22 Number Squares
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4It is possible for more than one number to be changed in such a way that the sum still comes out correct, but the BD encoding format makes this an extremely rare occurrence
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