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10 Most Recent Questions and Answers (out of 24)

1382. How does a phonograph work? — MS
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A phonograph record represents the air pressure fluctuations associated with sound as surface fluctuations in long, spiral groove. This groove is V-shaped, with two walls cut at right angles to one another—hence the "V". Silence, the absence of pressure fluctuations in the air, is represented by a smooth portion of the V groove, while moments of sound are represented by a V-groove with ripples on its two walls. The depths and spacings of the ripples determine the volume and pitch of the sounds and the two walls represent the two stereo channels on which sound is recorded and reproduced.

To sense the ripples in the V-groove, a phonograph places a hard stylus in the groove and spins the record. As the stylus rides along the walls of the moving groove, it vibrates back and forth with each ripple in a wall. Two transducers attached to this stylus sense its motions and produce electric currents that are related to those motions. The two most common transduction techniques are electromagnetic (a coil of wire and a magnet move relative to one another as the stylus moves and this causes current to flow through the coil) and piezoelectric (an asymmetric crystal is squeezed or unsqueezed as the stylus moves and this causes charge to be transferred between its surfaces). The transducer current is amplified and used to reproduce the recorded sound.


1296. In instructions for cleaning CDs, it always specifies cleaning the CD by wiping radially from the center out. Why does it matter? — AB, Toronto, Canada
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Whenever you wipe a CD to clean it, there is a chance that you will scratch its surface. If that scratch is wide enough, it may prevent the player's optical system from reading the data recorded beneath it and this loss of data may make the CD unplayable. It turns out that tangential scratches are much more serious than radial scratches. When the scratch is radial (extending outward from the center of the disc to its edge), the player should still be able to reproduce the sound without a problem. That's because sound information is recorded in a spiral around the disc and there is error-correcting information included in each arc shaped region of this spiral. Since a radial scratch only destroys a small part of each arc it intersects, the player can use the error correcting information to reproduce the sound perfectly.

But when the scratch is tangential (extending around the disc and along the spiral), it may prevent the player from reading a large portion of an arc. If the player is unable to read enough of the arc to perform its error correcting work, it can't reproduce the sound. That's why a tangential scratch can ruin a CD much more easily than a radial scratch can. That's why you should never wipe a CD tangentially. Always clean them by wiping from the center out.


1295. How does a CD player pause a CD if the CD continues to turn? — BC, Oxon, England
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A CD player reads ahead of the sound it is playing so that it always has sound information from at least one full turn of the disc in its memory. It has to read ahead as part of the error correcting process—the sound information associated with one moment in time is actually distributed around the spiral rather than squeezed into one tiny patch. This reading ahead is particularly important for a portable CD player, which usually saves several seconds of sound information in its memory so that it will have time to recover if its optical system is shaken out of alignment. When you pause the CD player, it reads ahead until its memory is full and then lets its optical system hover while the disc continues to turn. When you unpause the player, it uses the sound information it has saved in its memory to continue where it left off and its optical system resumes the reading ahead process.

1242. How can I clean a dirty CD which has a very difficult to remove stain? Which materials are best for cleaning? — AM, Mexico
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Most CD's are made from polycarbonate plastic (though other plastics with the same index of refraction are occasionally used). Polycarbonate is a pretty tough material, so it should survive most common stain or gum removing solvents. Try your favorite solvent on an unimportant CD first; such as one of the free discs that come occasionally in the mail. However, if the stain molecules have diffused into the plastic and have become trapped within the tangle of plastic molecules, you're probably out of luck. Removing such a stain will require wearing away some of the plastic. Since the disc's surface finish must remain smooth and the thickness of the disc shouldn't change much, serious resurfacing is likely to make the disc unplayable. Also, stay away from the printed side of the disc—it has only a thin layer of varnish protecting the delicate aluminum layer from injury. Solvents can wreck this side of the disc. Finally, if the stain is a white mark (or a scratch), you may be able to render the disc clear again by filling the tiny air gaps that make it white with another plastic. I'll bet that a clear furniture polish or liquid wax will soak into the white spot, replace the air, and render the disc clear and playable.

1158. Why do you hear different music coming from a compact disc when the laser of the CD player is just going around the same part of the CD over and over again? — KD, McMinnville, OR
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The CD player's laser doesn't really go over the same part of the CD over and over again. As the disc turns, the laser slowly moves outward from the middle of the disc toward its edge. The laser beam is focused to an extremely small spot inside the disc and it is carefully following a tight spiral ridge in the aluminum layer inside. This ridge runs continuously from the center of the disc to its edge. With each revolution of the disc, the laser works its way outward by one more turn of the spiral. The ridge has interruptions in it every so often and it is this pattern of interruptions that contains the information needed to reproduce sound.

1029. How do analog to digital converters change the analog input signal into a stream of numbers? — RME, Santa Monica, CA
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A typical analog-to-digital converter (ADC) uses a process called "successive approximation" to find a binary number that accurately represents the voltage on an input wire. It samples the voltage on the input wire at one moment in time and then gradually constructs a binary number representing that voltage. The ADC tries various binary numbers and uses a digital-to-analog converter to form a voltage from each number. It compares the two voltages, the original and its approximation, to determine how close its current guess is to the correct value. With each successive approximation, it adds a bit a precision to its measurement so that after 16 approximations, it has a 16 bit number that accurately represents the voltage on the input wire.

For applications requiring even faster measurements, there are flash ADCs. These devices synthesize the entire range of possible voltages and then compare the input voltage directly with the complete collection of possible voltages. Since 8 binary bits can represent 256 possible numbers, an 8 bit flash ADC synthesizes 255 different voltages and makes 255 voltage comparisons simultaneously. It instantly determines where among the various voltages the input voltage falls and it reports this value in billionths of a second.


737. Why is CD audio better than that of a cassette? — MK, Baltimore, MD
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CD audio is recorded in a digital form—as a series of numerical pressure measurements. This digital recording is a very accurate representation of the air pressure fluctuations associated with the original sounds that arrived at the microphones. During playback, these air pressure measurements are read from the CD and the original air pressure fluctuations are recreated by the speakers. While there are imperfections in the whole process of measuring air pressure fluctuations and recreating those fluctuations, the CD itself doesn't introduce any imperfections—the information read from the CD during playback is absolutely identical to the information that was recorded on the CD at the manufacturer's plant.

The same isn't true of analog recording on a cassette tape. Cassette audio is recorded in an analog form—as magnetizations of the tape surface that are proportional to the air pressure fluctuations associated with the original sounds. During playback, these magnetizations of the tape are analyzed and used to recreate the sounds. But the tape itself introduces imperfections in the reproduced sound. The information read from the tape during playback isn't quite the same as the information that was recorded on the tape at the manufacturer's plant. The tape isn't perfect and the sound that's reproduced by a tape player isn't quite the sound that was originally recorded.


639. How does a CD player work? — NL, Dearborn, MI
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A CD player uses a laser beam to determine the lengths of a series of ridges inside a compact disc. Infrared light from a solid-state laser is sent through several lenses, a polarizing beam splitter, and a special polarizing device called a quarter-wave plate. It's then focused through the clear plastic surface of the compact disc and onto the shiny aluminum layer inside the disc. Some of this light is reflected back through the player's optical system so that it passes through the quarter-wave plate a second time before encountering the polarizing beam splitter. The two trips through the quarter-wave plate switches the light's polarization from horizontal to vertical (or vice versa) so that instead of returning all the way to the laser, the light turns 90° at the polarizing beam splitter and is directed onto an array of photodiodes. These photodiodes measure the amount and spatial distribution of the reflected light. From this reflected light, the CD player can determine whether the laser beam is hitting a ridge or a valley on the disc's aluminum layer. It can also determine how well focused or aligned the laser beam is with the aluminum layer and its ridges. The player carefully adjusts the laser beam to follow the ridges as the disc turns and it measures how long each ridge is. The music is digitally encoded in the ridge lengths so that by measuring those lengths, the player obtains the information it needs to reproduce the music.

563. In a CD player, how is the digital optical signal transformed into an electrical signal? — IM, Oxford, UK
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The ridges and flat regions on a compact disc's aluminum layer determine how laser light is reflected from that layer. As the disc turns and the player's laser scans across ridges and flat regions, the intensity of the reflected light fluctuates up and down. This reflected light is directed onto an array of silicon photodiodes that provide both the signals needed to keep the laser focused tightly on the aluminum layer and the signal that the player uses to recreate sound. The sound is encoded in the lengths of the ridges. A computer monitors the amount of light returning from the disc to determine how long each ridge is and how much spacing there is between it and the next ridge. The computer uses this information to obtain a series of 16 bit binary numbers for each of the two sound channels that are represented by an audio CD. A digital-to-analog converter uses these 16 bit numbers to produce currents that are eventually amplified and used to produce sound.

435. Will light going in two directions in the same space create destructive interference?
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In general, the answer is no—there won't be large regions of space in which the two light waves cancel one another. That's because, while the electric fields from the two waves do add to one another at each moment, those fields go in and out of phase with one another very rapidly as the waves pass and the end result is that they do not interfere with one another over broad expanses. However, there can be points or surfaces in space at which the electric fields from the waves at least partially cancel for extended periods of time and at which there is destructive interference. These points and surfaces are often observed in experiments with single frequency laser beams.

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