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Hardware: loudspeakers, CD’s, …. Loudspeakers Not that different today than the ones from 80 years ago ! based on magnets, solenoids. Magnets have two poles, “north” and “south”. Equal poles repel Opposite poles attract Without touching !
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Loudspeakers • Not that different today than the ones from 80 years ago ! • based on magnets, solenoids
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 field
Electric charges moving = electric currents also generate magnetic fields
A loudspeaker is a straightforward application of this principle http://electronics.howstuffworks.com/speaker5.htm
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
Discretization (digitalization) pressure level continuous signal time
from analog to digital … sampling precision sampling time
From that digital information we can recover the original signal … with some loss
Larger sampling rate and sampling precision improves fidelity
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
What are the sampling rates and sampling precision we need for high fidelity ? A high frequency signal disappears with this sampling rate
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
What are the sampling precision we need for high fidelity ? 216 = 65536 levels are enough for the error to be imperceptible Dropping one bit reduces file sizes by a factor of 2 !
Total requirements for one minute of music 44.100 x 2 x 2 x 60 x 1 = 10584 kbytes samplings per second two channels seconds per minute two bytes per second
Download one song (3 minutes) with a 56 kbit per second modem 10584 x 8 x 3/56.000 = 4536 seconds = 76 minutes bytes per minute bit per byte bits downloaded per second minutes per song
MP3 is better: compression The string 100100100100100 can be abbreviated by 100101 pattern “5” The Lempel-Ziv-Welch adaptive dictionary based algorithm is based on this idea. This is an example of lossless compression
Strategies for lossy compression • masking • more precision in sounds we hear better
larger smaller Tracks (a total of 3.5 miles in each cd) read from the inside out
How can the tiny indentations be read (without touching them !!!) ? depth = ¼ wavelength destructive interference constructive interference This is a cartoon, real systems involve several mirrors, etc, …
error correction • no bumps for a while lost track, 1’s are interspersed (8-14 bit modulation) • data spread over a full turn (interleaving) to avoid burst error • results in signal/noise ratio > 90 db ! More can be found at http://electronics.howstuffworks.com/cd.htm
