# (6) Demonstrate aliasing by sampling a chirpΒΆ

We demonstrate aliasing by sampling a linear chirp signal and then try to reconstruct the original signal using a cubic spline interpolator through the samples. Ideally, we should do this via the Shannon-Whittaker sinc function but alas not in GMT yet. As the frequency of the chirp increases we find it harder and harder to reconstruct a reasonable representation of the original signal from the samples. The morale is you need to sample data as often as you are able to. Here, we added a title slide visible for 6 seconds, then fade out to the animation. The scripts are a bit longer due to lots of little details. The finished movie is available in our YouTube channel as well: https://youtu.be/3vB53hoLsls The movie took ~3 minutes to render on a 24-core MacPro 2013.

```
#!/usr/bin/env bash
#
# We demonstrate aliasing by sampling a linear chirp signal and then try to reconstruct
# the original signal using a cubic spline interpolator through the samples. Ideally, we
# should do this via the Shannon-Whittaker sinc function but alas not in GMT yet. As the
# frequency of the chirp increases we find it harder and harder to reconstruct a reasonable
# representation of the original signal from the samples. The morale is you need to sample
# data as often as you are able to. Here, we added a title slide visible for 6 seconds, then
# fade out to the animation. The scripts are a bit longer due to lots of little details.
# The finished movie is available in our YouTube channel as well:
# https://youtu.be/3vB53hoLsls
# The movie took ~3 minutes to render on a 24-core MacPro 2013.
rate=24 # Frames per seconds
frames=$(gmt math -Q 60 $rate MUL =)
# 0. Initial parameters
cat << EOF > init.sh
R=-R-7.5/2.5/-1.5/2 # Fixed plot domain window
J=-JX22c/11.5c # Frame size after removing margin space
f=2 # Frequency of chirp in Hz at end time
rate=$rate
frames=$frames
EOF
# 1. Make a title slide explaining things
cat << 'EOF' > title.sh
gmt begin
echo "12 11.5 Demonstration of aliasing by sampling a linear chirp" | gmt text -R0/24/0/13.5 -Jx1c -F+f26p,Helvetica-Bold+jCB -X0 -Y0
echo "12 10.5 y(t) = cos (2@~p@~t@+2@+/60)" | gmt text -F+f36p,Times-Italic+jTC
gmt text -M -F+f14p <<- END
> 12 6.5 16p 20c j
We will simulate sampling the continuous phenomenon described by @%6%y(t)@%% every 0.5 seconds,
meaning our Nyquist frequency is 1 Hz. The samples are then interpolated with a cubic spline to
reconstruct the original signal@+*@+. At first, this works great, but as the chirp increases
in frequency we find the interpolation starts to deviate from the actual phenomenon, and eventually
the reconstruction is clearly dominated by aliased frequencies much longer than those in the phenomenon.
END
gmt legend -Dx12c/3.5c+w20c+jTC+l1.2 -C0.3c -F+p+gazure1 <<- END
N 3
V 0 1p
S 1.5c - 2.5c - 1p,red 3.2c Chirp (pheonomenon)
S 1.5c c 0.3c blue - 2c Sampled values
S 0.5c - 2.5c - 2.5p,blue 2.2c Spline interpolator
END
echo "@+*@+Sorry, we do not have a Shannon-Whittaker @%6%sinc@%% interpolator in GMT (yet)" | gmt text -F+f10p+cBR -Dj0.5c
gmt end show
EOF
# 2. Create background plot and build data files needed in the main script
cat << 'EOF' > pre.sh
gmt begin
gmt plot $R $J -X1c -Y1c -B+gcornsilk -S1c -Ggreen <<- END
0 -1.5 t
0 +2.0 i
END
gmt basemap -Bxa2.5g10 -By0g10 --FORMAT_FLOAT_MAP=%+5.1f
# Display Nyquist frequency in lower left corner
echo "Nyquist frequency = 1 Hz" | gmt text -F+f18p,Helvetica-Bold+jBL+cBL -Dj0.3c
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
EOF
# 3. 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 $J chirp_shifted.txt -W1p,red -X1c -Y1c
# Compute index of most recent sample number
last_sample=$(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 ${MOVIE_COL1} SUB = samples.txt
gmt plot -Sc0.3c -Gblue samples.txt
fi
# Take a new sample every 12 frames = 0.5 seconds
take_sample=$(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
fi
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
fi
# Add time counter in upper left corner
printf "t = %6.3f s\n" ${MOVIE_COL1} | gmt text -F+f18p,Helvetica-Bold+jTL+cTL -Dj0.3c
# Add cycles counter in upper right corner
fnow=$(gmt math -Q ${MOVIE_COL1} 60 DIV $f MUL =)
printf "f = %6.4f Hz\n" $fnow | gmt text -F+f16p,Helvetica-Bold+jTR+cTR -Dj0.3c
# Add frame counter in lower right corner
printf "%04d\n" ${MOVIE_FRAME} | gmt text -F+f14p,Helvetica-Bold+jBR+cBR -Dj0.3c
gmt end
EOF
# 4. Run the movie
gmt movie main.sh -Sbpre.sh -CHD -Iinit.sh -Tframe_times.txt -D$rate -Etitle.sh+d6s+fo1s+gwhite -Nanim06 -H8 -Zs -Fmp4 -W -V
```