(6) Demonstrate aliasing by sampling a chirpΒΆ

We demonstrate how aliasing works by sampling a chirp at a fixed interval of 0.5 seconds. We try to reconstruct the signal by using a cubic spline on the sampled values. This works well for at the start but once we approach the Nyquist frequency it breaks down badly.

#!/usr/bin/env bash
#               GMT ANIMATION 06
# Purpose:      Demonstrate aliasing by sampling a chirp
# GMT modules:  math, convert, basemap, plot, text, movie, sample1d
# Unix progs:   echo, cat
# Note:         Run with any argument to build movie; otherwise 300th frame is plotted only.

if [ $# -eq 0 ]; then   # Just make master PostScript frame 300
	opt="-M300,ps -Fnone"
else	# Make movie in MP4 format and a thumbnail animated GIF using every 10th frame
	opt="-Fmp4 -A+l+s5"
rate=6			# Frames per seconds
frames=`gmt math -Q 60 $rate MUL =`
# 0. Initial parameters
cat << EOF > init.sh
R=-R-7.5/2.5/-1.5/2	# Fixed plot domain window
J=-JX6.8i/4.2i		# Frame size after removing margin space
f=2			# Frequency of chirp in Hz at end time
# 1. Create background plot and data files needed in the loop
cat << EOF > pre.sh
gmt begin
	gmt basemap \$R \$J -X0.2i -Y0.3i -Bxa2.5g10 -By0g10 -B+gcornsilk --FORMAT_FLOAT_MAP=%+5.1f
	cat <<- END | gmt plot -S0.3i -Ggreen
	0	-1.5	t
	0	+2.0	i
	# Add Nyquist frequency in lower left corner
	echo "-7.5 -1.5 Nyquist frequency = 1 Hz" | gmt text -F+f18p,Helvetica-Bold+jBL -Dj0.1i/0.1i
gmt end
# Build chirp data sets for 1 minute (60 secs); one every 1 ms and one every 0.5 sec as samples
gmt math -T0/60/0.001 T 2 POW 2 DIV 60 DIV \$f MUL 2 MUL PI MUL COS = chirp.txt
gmt math -T0/60/0.5 -Ca T -C1 2 POW 2 DIV 60 DIV \$f MUL 2 MUL PI MUL COS = chirp_samples.txt
gmt math -T0/\$frames/1 T \$rate DIV = frame_times.txt
# 2. Set up the main frame script
cat << EOF > main.sh
gmt begin
	# Shift the chirp in time to simulate paper movement
	gmt math chirp.txt -C0 \${MOVIE_COL1} SUB = chirp_shifted.txt
	# Plot the shifted chirp
	gmt plot \$R \$J chirp_shifted.txt -W1p,red -X0.2i -Y0.3i
	# Compute index of most recent sample number
	last_sample=\`gmt math -Q \${MOVIE_FRAME} \$rate 2 DIV DIV FLOOR RINT =\`
	# Extract all the old samples before the present
	gmt convert chirp_samples.txt -Z:\$last_sample > tmp.txt
	if [ -s tmp.txt ]; then
		gmt math tmp.txt -C0 \${MOVIE_COL1} SUB = samples.txt
		gmt plot -Sc0.3c -Gblue samples.txt
	# Take a new sample every 12 frames = 0.5 seconds
	take_sample=\`gmt math -Q \${MOVIE_FRAME} \$rate 2 DIV MOD 0 EQ =\`
	if [ \${MOVIE_FRAME} -gt 12 ]; then	# Interpolating up to most recent sample
		gmt sample1d samples.txt -I0.001 > resampled.txt
		gmt plot -W2.5p,blue resampled.txt
	if [ \$take_sample -eq 1 ]; then	# Take and plot sample at zero time
		y=\`gmt math -Q \${MOVIE_COL1} 2 POW 2 DIV 60 DIV \$f MUL 2 MUL PI MUL COS =\`
		echo 0 \$y | gmt plot -Sc0.5c -Gred
	# Add time counter in upper left corner
	printf "%4.1f 2 t = %6.3f s\n" -7.5 \${MOVIE_COL1} | gmt text -F+f18p,Helvetica-Bold+jTL -Dj0.1i/0.1i
	# Add cycles counter in upper right corner
	fnow=\`gmt math -Q \${MOVIE_COL1} 60 DIV \$f MUL =\`
	printf "2.5 2 f = %6.4f Hz\n" \$fnow | gmt text -F+f16p,Helvetica-Bold+jTR -Dj0.1i/0.1i
	# Add frame counter in lower right corner
	printf "2.5 -1.5 %04d\n" \${MOVIE_FRAME} | gmt text -F+f14p,Helvetica-Bold+jBR -Dj0.1i/0.1i
gmt end
# 3. Run the movie
gmt movie main.sh -Sbpre.sh -C7.2ix4.8ix100 -Iinit.sh -Tframe_times.txt -D$rate -Nanim06 -Z $opt
rm -rf init.sh main.sh pre.sh

Demonstrate aliasing by sampling a chirp.