#lang racket/base

  Exercise 3.75

  Unfortunately ,  's zero - crossing detector in
  * Note Exercise 3 - 74 : : proves to be insufficient , because the noisy
  signal from the sensor leads to spurious zero crossings .    
  Tweakit , a hardware specialist , suggests that  smooth the
  signal to filter out the noise before extracting the zero
  crossings .    takes his advice and decides to extract the
  zero crossings from the signal constructed by averaging each value
;; of the sense data with the previous value.  She explains the
  problem to her assistant ,   , who attempts to
  implement the idea , altering  's program as follows :
;;
       ( define ( make - zero - crossings input - stream last - value )
         ( let ( ( avpt ( / ( + ( stream - car input - stream ) last - value ) 2 ) ) )
;;          (stream-cons (sign-change-detector avpt last-value)
                        ( make - zero - crossings ( stream - cdr input - stream )
;;                                            avpt))))
;;
  This does not correctly implement  's plan .   Find the bug
  that  has installed and fix it without changing the structure
;; of the program.  (Hint: You will need to increase the number of
  arguments to ` make - zero - crossings ' . )

 of the sense data with the previous value.  She explains the

          (stream-cons (sign-change-detector avpt last-value)
                                            avpt))))

 of the program.  (Hint: You will need to increase the number of
#lang racket/base

  Exercise 3.75

  Unfortunately ,  's zero - crossing detector in
  * Note Exercise 3 - 74 : : proves to be insufficient , because the noisy
  signal from the sensor leads to spurious zero crossings .    
  Tweakit , a hardware specialist , suggests that  smooth the
  signal to filter out the noise before extracting the zero
  crossings .    takes his advice and decides to extract the
  zero crossings from the signal constructed by averaging each value
  problem to her assistant ,   , who attempts to
  implement the idea , altering  's program as follows :
       ( define ( make - zero - crossings input - stream last - value )
         ( let ( ( avpt ( / ( + ( stream - car input - stream ) last - value ) 2 ) ) )
                        ( make - zero - crossings ( stream - cdr input - stream )
  This does not correctly implement  's plan .   Find the bug
  that  has installed and fix it without changing the structure
  arguments to ` make - zero - crossings ' . )
