Hardware: loudspeakers, CD’s, …. Loudspeakers Not that different today than the ones from 80...
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Transcript of Hardware: loudspeakers, CD’s, …. Loudspeakers Not that different today than the ones from 80...
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Hardware: Hardware: loudspeakers, CD’s, …loudspeakers, CD’s, …
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LoudspeakersLoudspeakersNot that different today than the ones from 80 years ago !
• based on magnets, solenoids
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Magnets have two poles, “north” and “south”.
• Equal poles repel
• Opposite poles attract
Without touching !
One can picture this action-at-distance as being mediated by a “force field”: the magnetic fieldmagnetic field
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Electric charges moving = electric Electric charges moving = electric currents also generate magnetic fieldscurrents also generate magnetic fields
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A loudspeaker is a straightforward application A loudspeaker is a straightforward application of this principleof this principle
http://electronics.howstuffworks.com/speaker5.htm
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Speaker response curve
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Response depends on the angle
http://images.google.com/imgres?imgurl=http://www.rjbaudio.com/Alpheus/Alpheus%2520gated%2520response.jpg&imgrefurl=http://www.rjbaudio.com/Alpheus/alpheus.html&h=326&w=500&sz=40&hl=en&start=57&um=1&tbnid=CsAniYgXuAQcWM:&tbnh=85&tbnw=130&prev=/images%3Fq%3D%2522speaker%2Bresponse%2B%2522%26start%3D40%26ndsp%3D20%26um%3D1%26hl%3Den%26safe%3Doff%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-US:official%26sa%3DN
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Hardware: cd’s, mp3 Hardware: cd’s, mp3 and digital recording …and digital recording …
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Discretization (digitalization)Discretization (digitalization)
time
pressure level continuous signal
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sampling time
sampling sampling precisionprecision
from analog to digital …
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From that digital information we can recover the original signal … with some loss
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Larger sampling rate and sampling precision Larger sampling rate and sampling precision improves fidelityimproves fidelity
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Discretization (digitalization)Discretization (digitalization)
Pressure level at one instant represented by 1’s and 0’s
Two levels: 0 or 1 1 bit
Four levels: 00, 01, 10 or 11 2 bits
Eight levels: 000, 001, 010, 100, 011, 101, 110 or 111 3 bits
…
65536 levels: 0000000000000000, 000000000000001, …
16 bits = 8 bytes
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What are the sampling rates and sampling precision What are the sampling rates and sampling precision we need for high fidelity ?we need for high fidelity ?
A high frequency signal disappears with this sampling rate
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What are the sampling rates we need for high fidelity ?What are the sampling rates we need for high fidelity ?
A sampling rate equal to the twice the maximum frequency
20.000 Hz 40.000 samples per second
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What are the sampling precision we need for high What are the sampling precision we need for high fidelity ?fidelity ?
216 = 65536 levels are enough for the error to be imperceptible
Dropping one bit reduces file sizes by a factor of 2 !
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Total requirements for one minute of musicTotal requirements for one minute of music
44.100 x 2 x 2 x 60 x 1 = 10584 kbytes
samplings per second
two bytes per second
two channels
seconds per minute
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Download one song (3 minutes) with a 56 kbit per second modem
10584 x 8 x 3/56.000 = 4536 seconds = 76 minutes
bytes bytes per per
minuteminute
bit per byte
minutes per song
bits bits downloaded downloaded per secondper second
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MP3 is better: compression
The string 100100100100100 can be abbreviated by 100101
pattern“5”
The Lempel-Ziv-Welch adaptive dictionary based The Lempel-Ziv-Welch adaptive dictionary based algorithm is based on this idea.algorithm is based on this idea.
This is an example of This is an example of losslesslossless compression compression
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Strategies for lossy compressionStrategies for lossy compression
• maskingmasking
• more precision in sounds we hear bettermore precision in sounds we hear better
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CD playersCD players
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read from the read from the inside outinside out
Tracks (a total of 3.5 Tracks (a total of 3.5 miles in each cd)miles in each cd)
larger
smaller
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How can the tiny indentations be read How can the tiny indentations be read (without touching them !!!) ?(without touching them !!!) ?
This is a cartoon, real systems involve This is a cartoon, real systems involve several mirrors, etc, …several mirrors, etc, …
constructive constructive interferenceinterference
destructive destructive interferenceinterference
depth = ¼ wavelengthdepth = ¼ wavelength
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in reality …in reality …
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• error correctionerror correction
• no bumps for a while lost track, 1’s are no bumps for a while lost track, 1’s are interspersed (8-14 bit modulation)interspersed (8-14 bit modulation)
• data spread over a full turn (interleaving) to data spread over a full turn (interleaving) to avoid burst erroravoid burst error
• results in signal/noise ratio > 90 db !results in signal/noise ratio > 90 db !
More can be found at http://electronics.howstuffworks.com/cd.htm