I need help in fixing this code as I keep getting errors

% Define filter specifications
fs = 1000;       % Sampling frequency in Hz
t = 0:1/fs:1;    % Time vector (1 second)
fc_low = 50;     % Cutoff frequency for low-pass filter in Hz
fc_high = 100;   % Cutoff frequency for high-pass filter in Hz
fc_bandpass_low = 30;  % Lower cutoff for band-pass filter in Hz
fc_bandpass_high = 150; % Upper cutoff for band-pass filter in Hz
order = 4;       % Filter order

% Create a mixed signal (voltage output) with multiple frequency components
signal = sin(2*pi*10*t) + sin(2*pi*200*t) + 0.5*randn(size(t)); % 10 Hz, 200 Hz signal with noise

% Apply Low-Pass Filter
Wn_low = fc_low / (fs / 2);  % Normalize the cutoff frequency
[b_low, a_low] = butter(order, Wn_low, 'low');
filtered_signal_low = filter(b_low, a_low, signal);

% Apply High-Pass Filter
Wn_high = fc_high / (fs / 2); % Normalize the cutoff frequency
[b_high, a_high] = butter(order, Wn_high, 'high');
filtered_signal_high = filter(b_high, a_high, signal);

% Apply Band-Pass Filter
Wn_bandpass = [fc_bandpass_low, fc_bandpass_high] / (fs / 2); % Normalize band-pass range
[b_bandpass, a_bandpass] = butter(order, Wn_bandpass, 'bandpass');
filtered_signal_bandpass = filter(b_bandpass, a_bandpass, signal);

% Plot the original signal and the filtered signals
figure;

% Original signal plot
subplot(4, 1, 1);
plot(t, signal);
title('Original Signal (Voltage Output)');
xlabel('Time [s]');
ylabel('Voltage [V]');

% Low-pass filtered signal plot
subplot(4, 1, 2);
plot(t, filtered_signal_low);
title('Low-Pass Filtered Signal (f < 50 Hz)');
xlabel('Time [s]');
ylabel('Voltage [V]');

% High-pass filtered signal plot
subplot(4, 1, 3);
plot(t, filtered_signal_high);
title('High-Pass Filtered Signal (f > 100 Hz)');
xlabel('Time [s]');
ylabel('Voltage [V]');

% Band-pass filtered signal plot
subplot(4, 1, 4);
plot(t, filtered_signal_bandpass);
title('Band-Pass Filtered Signal (30 Hz < f < 150 Hz)');
xlabel('Time [s]');
ylabel('Voltage [V]');

% Magnitude Spectrum
figure;

% Compute and plot the magnitude spectrum (FFT) of the filtered signals
N = length(signal)  % Number of points for FFT

N = 1001

f = fs * (0:(N/2)) / N % Frequency vector for plotting

f = 1×501

0 0.9990 1.9980 2.9970 3.9960 4.9950 5.9940 6.9930 7.9920 8.9910 9.9900 10.9890 11.9880 12.9870 13.9860 14.9850 15.9840 16.9830 17.9820 18.9810 19.9800 20.9790 21.9780 22.9770 23.9760 24.9750 25.9740 26.9730 27.9720 28.9710

<mw-icon class=""></mw-icon>

<mw-icon class=""></mw-icon>

% Original signal magnitude spectrum
subplot(4, 1, 1);
Y = fft(signal);
plot(f, abs(Y(1:N/2+1)));

Warning: Integer operands are required for colon operator when used as index.

title('Magnitude Spectrum of Original Signal');
xlabel('Frequency [Hz]');
ylabel('Magnitude');

% Low-pass filtered signal magnitude spectrum
subplot(4, 1, 2);
Y_low = fft(filtered_signal_low);
%plot(f, abs(Y_low(1:N/2+1)));
plot(f, abs(Y_low(1:floor(N/2)+1)))
title('Magnitude Spectrum of Low-Pass Filtered Signal');
xlabel('Frequency [Hz]');
ylabel('Magnitude');

% High-pass filtered signal magnitude spectrum
subplot(4, 1, 3);
Y_high = fft(filtered_signal_high);
plot(f, abs(Y_high(1:N/2+1)));

Warning: Integer operands are required for colon operator when used as index.

title('Magnitude Spectrum of High-Pass Filtered Signal');
xlabel('Frequency [Hz]');
ylabel('Magnitude');

% Band-pass filtered signal magnitude spectrum
subplot(4, 1, 4);
Y_bandpass = fft(filtered_signal_bandpass);
plot(f, abs(Y_bandpass(1:N/2+1)));

Warning: Integer operands are required for colon operator when used as index.

title('Magnitude Spectrum of Band-Pass Filtered Signal');
xlabel('Frequency [Hz]');
ylabel('Magnitude');

Answer 1

[Fangjun Jiang]

No error but warnings.

The cause is that N/2 is not an integer because in your case, N is 1001.

It is surprise that in the example below, y(index) does not give warning but y(1:N/2+1) does, where index is equal to 1:N/2+1

But anyway, the plots show up and the warning is not critical.

N=11;
index=1:N/2+1

index = 1×6

1 2 3 4 5 6

<mw-icon class=""></mw-icon>

<mw-icon class=""></mw-icon>

y=rand(1,20);
y(index)

ans = 1×6

0.0055 0.9275 0.7893 0.7231 0.6331 0.9171

<mw-icon class=""></mw-icon>

<mw-icon class=""></mw-icon>

figure
plot(y(index))

●

figure
plot(y(1:N/2+1))

Warning: Integer operands are required for colon operator when used as index.