UNIVERSITY OF GLASGOW Degrees of MEng, BEng, MSc and BSc in Engineering DIGITAL SIGNAL PROCESSING 1 MSC (ENG5027) Tuesday, 17/12/2019 09:30 – 11:30 Time 120 minutes. Total 100 marks Attempt ALL questions The numbers in square brackets in the right-hand margin indicate the marks allotted to the part of the question against which the mark is shown. These marks are for guidance only. Attempt all questions Page 1 of 5 /OVER Q1. Fourier transform Figure Q1: (a) The plot Q1 shows the frequency spectrum of a short sentence spoken into a micro- phone with flat frequency response. i. Do you expect just one fundamental frequency and its harmonics – or multiple fundamental frequencies and their harmonics? Provide an explanation and state at least two possible fundamental frequencies. [6] ii. Speech has a-periodic hissing sounds such as “s”, “z” or “f”. Provide an ap- proximate frequency range in the plot and an explanation why these sounds generate no distinct peaks in plot Q1. [6] (b) The sound recording of the speech is 2 secs long at a sampling rate of 50 KHz. Your task is to improve the quality of the speech by boosting the amplitudes of the high order harmonics in the frequency domain by manipulating the Fourier trans- formed frequency coefficients X [k] and then transforming them back into the time domain. Calculate the index range of k1 . . .k2 of the frequency samples based on the frequency range of the harmonics F1, . . .F2 and provide the formula(s) or code to boost the corresponding frequency amplitudes by a factor of two so that the inverse becomes real valued again. [8] (c) After having performed the inverse Fourier Transform you try to save the time series as a WAV file but the function crashes because the time series is complex. Provide one reason why it is the case and what can be done to fix it. [5] Page 2 of 5 /OVER Q2. FIR filtering (a) An electrocardiogram (ECG) has an unwanted DC drift. Which filter should be applied and what is the cut-off frequency considering the heart rate of a healthy person. [5] (b) An ECG is sampled at a rate of 250 Hz and filtering needs to be done with an FIR filter. State the theoretical number of taps and the actual number required for DC removal as implemented in the lab. [10] (c) State at least one strategy how FIR filters can be made more computationally effi- cient. [5] (d) The discrete convolution is defined as: y(n) = h(n)∗ x(n) = ∞ ∑ n=−∞ h(n)x(m−n) A Google search often claims that an FIR filter performs this operation. Is that statement 100% true if we don’t know anything about h(n) or x(n)? Provide an explanation. [5] Page 3 of 5 /OVER Q3. A digital filter has the transfer function: H(z) = (1− r0e jω0z−1)(1− r0e − jω0z−1) (1− r1e jω1z−1)(1− r1e− jω1z−1) (a) Draw a dataflow diagram of the filter. [8] (b) Which variable in the formula above determines the stability of the filter and which values guarantee a stable operation? [2] Figure Q3: (c) Butterworth filter coefficients of even filter order are evenly distributed in the left half s plane forming a half circle as shown in Fig. Q3. For example a 6th order filter has 3 pairs of poles as shown in Fig. Q3 where the imaginary values flip sign. How would you distribute the poles over a chain of the above filters so that processing is guaranteed real valued within every building block and that higher order filters can be created with minimal additional algebra? [10] (d) Assume the analogue filter is a lowpass filter, how would you transform the analogue poles of Fig. Q3 to the digital coefficients of the above transfer function? State the transform and provide a reason why it is appropriate. [5] Page 4 of 5 /OVER Q4. FIR filter design in the frequency domain (a) Sketch the frequency response of an ideal digital bandstop filter with cut off fre- quencies ω1 and ω2. [5] (b) Perform an inverse Fourier transform of the ideal bandstop filter to obtain the im- pulse response of the filter. [10] (c) Your task is to design a narrow bandstop filter. Which window function would you choose that maximises the bandstop rejection? [5] (d) You sample a signal at 250Hz and the delay of the bandstop filter needs to be less than 80ms. What is the maximum number of taps permitted? [5] Page 5 of 5 /END
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