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----------------------------- 2218 - Physics - in CD’s and DVD’s
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- You can date yourself by what you remember. When I was growing up in my small town my Dad ran the juke box business. He was self employed in his own one man business called Universal Games. He had juke boxes in all the restaurants and clubs in town. We would go around at each stop and put in new 45 RPM records , count the money, and split the total with the proprietor. We wrapped the nickels in paper tubes to take them to the bank. Our hands would be gray and near black from counting nickels.
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- 45 RPM was the right size recording to work with the mechanical marvels in juke boxes. Whenever one was not working we would load it up in our truck and take to his garage to fix it. I learned al lot of electrical and mechanical engineering from the schematics used in their repair manuals.
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- Audio technology has come a long way from analog to digital recordings. Remember Ben Franklin’s wax cylinder, then the wire recorder, 78 RPM vinyl records, 45 RPM records, 33.3 RPM records, reel-to-reel tapes, cassette tapes, video cassettes, compact disks, … this Review is about Compact Disks, CD’s and will take you up to DVD’s and holographic recordings.
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- Compact disks brought us digital recordings rather than the earlier analog recordings on vinyl records. A recording on a CD is digital, nothing more than a very long string of 1’s and 0’s. Each 1 or 0 is one “bit”. 8 bits make one data “byte” . 2 bytes or 16 bits make a number.
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- On a CD they use 16 bits to record the intensity of each sound at successive times. That is where the music comes from.
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- A CD holds 44,100 of these 16 bit numbers. To get stereo recordings they create a complicated coding scheme for these numbers. With this scheme they can get 88,200 numbers for the sounds that you hear in stereo.
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- Great fidelity is created from 16 bit numbers. Each binary number can produce 2^16 different sound intensity levels. 2^16 = 65,536 possible numbers for sound levels.
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- This high fidelity is very robust because the recording only has to recognize a 1 or a 0. There are now shades of gray in between.
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- The CD is stamped out in a plastic disk. When the stamp hit’s the plastic it puts little pits where the 1 needs to be. The flat plastic in between pits are zeros. The pits are stamped from above the disk but the laser reads the pits from below the disk. To the laser the pits appear to be raised bumps.
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- To the laser each pit, or bump, is 1 to 3 *10^-16 meters in length and 0.5 *106-6 meters wide. 10^-6 meters is a millionth of a meter, 0.000,001meters. A human hair is 100 times wider than one pit. If you took the width of a human hair and cut it into 100 equal pieces, one piece would be the size of one pit. This is too small to see with the naked eye.
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- The pits of 0’s ad 1’s are spread out in a spiral line from the outside to the center of the disk. The spiral tracks are only 1.6 *10^-6 meters apart. That leaves room for 22,000 tracks on a 5.25 inch CD. The total length of the track is more than 3 miles. It takes 74 minutes of audio to travel the full 3 miles, traveling 2.5 miles per hour.
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- The average rotation speed is 300 RPM . But, the speed varies. The speed of the rotating disk needs to be slower on the outside of the disk and gradually speeds up as it approaches the center of the disk in order to maintain a constant reading speed
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- The laser reads the bumps because the light intensity changes when it reflects off a bump versus reflecting off a flat surface. If the laser light hit’s a pit it scatters and a lower light reflection intensity is read as a “1’. If the laser light reflects of a flat surface , maximum reflection intensity is read as a “0”.
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- The CD focuses the laser at the level of the bumps but at the point it enters the surface of the disk it is 500 times wider. The lens in front of the laser focuses the beam in the shape of a cone. This has the effect of the laser beam going around any dust that is on the surface and focuses to a point only on the bumps at the information level of the disk and not on the surface. The CD dust particles on the surface are not a problem.
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- CD’s use LED laser light that has a wavelength of 0.78 * 10^-16 meters. The height of the bumps are ¼ of this wavelength. So, a reflected wave does more than scatter the light. Reflecting at one-half wavelength the waves experience destructive interference and the waves actually cancel each other out reducing the light intensity to as low as possible. Therefore, it is definitely read as a zero.
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- Exactly one-half wavelength reflected, lines up a crest with a trough of the incoming wave. The two waves cancel each other out , “destructively”.
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- Turn the CD upside down and look at the light reflecting off the surface. You see a rainbow effect. The exact opposite type of reflecting wave interference is occurring off the adjacent tracks that are 1.6*10-6 meters apart. The reflecting waves experience “constructive” interference, increasing their intensity, brightening the reflecting beam.
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- You see a reflecting rainbow because different colors have different wavelengths and the constructive interference pattern occurs at different angles of reflection for different wavelengths.
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- The 74 minutes of audio equates to 640 megabytes of computer data. If a single photograph takes up to 1 megabyte of data than you can store up to 640 pictures on a CD. To get a motion picture video you will need lot more storing capability.
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- A DVD is a digital versatile disk the same as a CD except the pits are even smaller and the tracks are even closer together. A DVD has 30 times more capacity than a CD.
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- A ‘ recordable” CD-R disk uses a chemical dye that becomes opaque when a high powered laser beam hits the die. This opaque spot is burn spot that serves the function of a pit.
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- A Re-recordable CD-RM disk are similar except they use crystal structures that change when exposed to the high power laser beam. The crystal process is reversible and the same disk can be recorded many times.
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- In each case the high power laser beam makes the recording but the lower power reading laser does not change the recording.
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- There is a limit to how tiny the pits, spots, bumps can be recorded and a still be read by the laser beam. The wavelength of red light is 0.7 * 20^-6 meters. The wavelength of blue light is 0.4 * 10^-6 meters. Once the pits, or bumps, get small enough to approach the single wavelength of laser light the ability to read the data becomes too ambiguous.
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- Blue lasers should be able to read twice as much data as red lasers cause their wavelengths are half the width. But, blue lasers are harder to manufacturer than red lasers.
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- Technology marches on and the next generation of DVD’s promise to store three times as much data. Current pit sizes and red lasers limit disk capacity of a disk to 4.7 gigabytes per layer. This is 8 hours of video for a four layer disk. Blue lasers have a capacity for 25 gigabytes per layer. The HD-DVD has 15 gigabytes storage capacity. With multiple layers they can hold a high definition movie. Wow, that’s is a whole lot of 1‘s and 0’s
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- Ultraviolet lasers are the new challenge to get even more capacity on a DVD. Another technology challenge is to have as asymmetric pits that reflect differently depending on their orientation. The pit are lopsided craters. The engineering dream is to pack 1,000 gigabytes on to a 4 layer DVD. One disk would hold all the episodes of the Simpson’s.
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- The new technology is called multiplexed optical data storage. This technology requires reconfigured lens to focus the laser of four layers of cratered pits. You will be buying MODS disks when this becomes available.
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- Holographic storage is the storage technology of the future. Holographic drives could combine terabytes (1,000 gigabytes) of capacity with rapid data transfers. Don’t get excited too early. These new multiplexed optical data storage technologies present many technical challenges to overcome. Stay in school to be ready for what is needed to change the future. The future ain’t going to be what it used to be.
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- December 27, 2018 534
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-------------------------- Friday, December 28, 2018 --------------------------
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