Laboratory 1

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Laboratory 1

• A simple Pitch tuner

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Objectives

• To lean how to interface analogue signals to
  digital systems
• To learn how to process audio signals in embedded
  systems.
• Aim
• To develop a pitch tuner using an embedded system.

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Signal processing steps

• Capture signal by microphone
• Amplify the signal
• Feed to ADC of AMR7 micro.
• Convert to digital , find autocorrelation coeff.
• Find pitch

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Hardware system

RS232
Mic amp
ADC input ARM7
PC for display result
Mic
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Amplifier

• What is the best bias voltage at pin3(non-invert.
  input) ?
• Condenser MIC output impedance is 75 Ohms, why
  this circuit is not a good design. How to correct
  it?
• Hint Input1KHZ, find impedance of C1 and input
  of the amp.
• Solution C1gt1uF, R21K, V3 100K(VR), use ,
  one VR to replace VR1,2. Discuss why?

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Software Algorithm to find period1/frequency
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Auto-correlation to find rj0, rj1, rj2,

• The signal is s0,s1,..,s511
• Example jtime lag
• Auto-correlation coefficients rj r0, r1,..,
  rn8, are

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Matlab example 1

• a1 3 9 3 2 3 8 3 2 ,
• r(xcorr(a)) ,round (r)
• Rr(jtime lag)
• r 2 9 35 60 96 72 86 123
  190 123 86 72 96 60 35 9 17 2
• Exercise
• Verify by hand the first 3 (r j ) elements
  after the signal overlapped with itself.

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Matlab Example1a1 3 9 3 2 3 8 3 2
,r(xcorr(a)) figure(1),clf,subplot(2,1,1)plot(
a),subplot(2,1,2)plot(r)
Data (a)
Period4,
Pick 2 peaks and measure period
Auto Corr. Xcorr (r)
The middle is the peak
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Since correlation is mirrored at the lag time
(j0) when the signal overlapped with itself

• Only positive time lag is considered
• Fundamental Periodj2

Auto correlation R(j)
Rthe_max (j1) Rsecond_max (j2)
Lag Time j in samples
j2
j10
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Matlab Example2ysin wave of 440 Hz

• dt0.0001
• time0dt2
• freq440
• nu2.0pifreq
• ysin(nutime) angular freq. (sin wave of 440
  Hz)
• r(xcorr(y))
• figure(2) , clf, subplot(2,1,1), plot(time,y)
• axis (0 0.01 -1 1)
• subplot(2,1,2), plot(time(1100),r(1100))

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Matlab Example2In fact any two peaks will give
you the answersine wave of 440
Hz,period0.0091-0.0068 (by inspection between 2
peaks)freq1/period434.7Hz
input Data (y)
Auto Corr. (r)
0.0068 0.0091
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Conclusion

• Studied a method of measuring the pitch of an
  audio sound.