Insight Through Computing 25. Working with Sound Files Cont’d Frequency Computations Touchtone Phones.

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Presentation transcript:

Insight Through Computing 25. Working with Sound Files Cont’d Frequency Computations Touchtone Phones

We looked at the time domain Sample number Time

Insight Through Computing What about the frequency domain? >> phone

Insight Through Computing A “pure-tone” sound is a sinusoidal function = the frequency Higher frequency means that y(t) changes more rapidly with time.

Insight Through Computing Still looking at the time domain

Insight Through Computing Digitize for Graphics % Sample “Rate” n = 200 % Sample times tFinal = 1; t = 0:(1/n):tFinal % Digitized Plot… omega = 8; y= sin(2*pi*omega*t) plot(t,y) Digitize for Sound % Sample Rate Fs = % Sample times tFinal = 1; t = 0:(1/Fs):tFinal % Digitized sound… omega = 800; y= sin(2*pi*omega*t); sound(y,Fs)

Insight Through Computing Equal-Tempered Tuning 0 A A# B C C# D D# E F F# G G# A Entries are frequencies. Each column is an octave. Magic factor = 2^(1/12). C3 = , A4 =

Insight Through Computing Middle C: A above middle C: + = “Adding” Sinusoids

Insight Through Computing “Adding” Sinusoids  averaging the sine values Fs = 32768; tFinal = 1; t = 0:(1/Fs):tFinal; C3 = ; yC3 = sin(2*pi*C3*t); A4 = ; yA4 = sin(2*pi*A4*t); y = (yC3 + yA4)/2; sound(y,Fs)

Insight Through Computing Application: touchtone telephones Make a signal by combining two sinusoids

A frequency is associated with each row & column So two frequencies are associated with each button. The “5”-Button corresponds to (770,1336) Each button has its own 2-frequency “fingerprint”!

Insight Through Computing Signal for button 5: Fs = 32768; tFinal =.25; t = 0:(1/Fs):tFinal; yR = sin(2*pi*770*t); yC = sin(2*pi*1336*t) y = (yR + yC)/2; sound(y,Fs) MakeShowPlay.m

Insight Through Computing This is the “fingerprint” of button ‘5’

Insight Through Computing The sinusoids for buttons 1,2,3, and 4

Insight Through Computing To Minimize Ambiguity… No frequency is a multiple of another The difference between any two frequencies does not equal any of the frequencies The sum of any two frequencies does not equal any of the frequencies Why is this important? I dial a number (send signal). The receiver of the signals get a “noisy” version of the real signal. How will the noisy data be interpreted? SendNoisy.m

Insight Through Computing How to compare two signals (vectors)? Given two vectors x and y of the same length, the cosine of the angle between the two vectors is a measure of the correlation between vectors x and y: cos_xy.m ShowCosines.m Small cosine  low correlation High cosine  highly correlated

Insight Through Computing Sending and deciphering noisy signals Randomly choose a button Choose random row and column numbers Construct the real signal ( MakeShowPlay ) Add noise to the signal ( SendNoisy ) Compute cosines to decipher the signals ( ShowCosines ) See Eg13_2

Insight Through Computing What does the signal look like for a multi-digit call? Buttons pushed at equal time intervals Each band matches one of the twelve “fingerprints” “Perfect” signal

Insight Through Computing “Noisy” signal Buttons pushed at unequal time intervals Each band approximately matches one of the twelve “fingerprints.” There is noise between the button pushes. One of the most difficult problems is how to segment the multi-button signal!