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27 lines
1.1 KiB
27 lines
1.1 KiB
2 years ago
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Exercise 3.75: Unfortunately, Alyssa’s
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zero-crossing detector in Exercise 3.74 proves to be insufficient,
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because the noisy signal from the sensor leads to spurious zero crossings. Lem
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E. Tweakit, a hardware specialist, suggests that Alyssa smooth the signal to
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filter out the noise before extracting the zero crossings. Alyssa takes his
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advice and decides to extract the zero crossings from the signal constructed by
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averaging each value of the sense data with the previous value. She explains
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the problem to her assistant, Louis Reasoner, who attempts to implement the
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idea, altering Alyssa’s program as follows:
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(define (make-zero-crossings
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input-stream last-value)
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(let ((avpt
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(/ (+ (stream-car input-stream)
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last-value)
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2)))
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(cons-stream
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(sign-change-detector avpt last-value)
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(make-zero-crossings
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(stream-cdr input-stream) avpt))))
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This does not correctly implement Alyssa’s plan. Find the bug that Louis has
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installed and fix it without changing the structure of the program. (Hint: You
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will need to increase the number of arguments to make-zero-crossings.)
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