Seems to work

src/lfo_freq_sine.m

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+function output = lfo_freq_sine(amplitude, frequency, phase, fs, duration, input)
+%LFO_FREQ_SINE Modulates the frequency of an input with a sine LFO
+
+% CONTRIBUTORS:
+% Neelay Joglekar: Original author
+
+% SOURCES:
+% Code based off of Benjamin Liou's functions (lfo_sine)
+% https://dsp.stackexchange.com/questions/2349/help-with-algorithm-for-modulating-oscillator-pitch-using-lfo
+
+% DOCUMENTATION:
+% Frequency is modulated by 20 semitones (20 above, 20 below)
+
+    % initialize local variables from input arguments
+    n = fs * duration; % number of samples (length of matrix)
+    dt = 1 / fs; % sampling period: time between two sample points
+
+    % create output array
+    output = zeros([1, n]);
+    
+    x = 0.0; % theoretical input index, assuming input not discrete
+    % populate output
+    for i = 1:n
+        t = i * dt; % time at the i'th sample
+        omega_ratio = 2 ^ (20/12 * sin(2 * pi * frequency * t - phase));
+        x = mod(x + omega_ratio, n);
+        x_0 = floor(x) + 1;
+        x_1 = mod(x_0 + 1, n) + 1;
+        output(i) = (input(x_1) - input(x_0)) * (x - x_0) + input(x_0);
+    end
+end
+