moved fade_in() to src folder

src/fade_in.m

@@ -0,0 +1,29 @@
+function output = fade_in(input, time)
+    % Creates a fade-in sound effect that lasts a given
+    % time parameter of the input sound signal
+    % By Yalu Ouyang
+    
+
+    % input: a 1D array that represents the sound signal in the time domain
+    % time: how long the fade in effect should last
+    % Shouldn't be longer than the input signal (in which case the function
+    % treats it as the duration of the signal)
+    % Returns modified signal in the time domain (output).
+    
+    len = length(input);
+    
+    % if time parameter longer than signal, treat time as
+    % the duration of original signal
+    if time > len
+        time = len
+    end
+
+    % set multiplier as 1D array
+    % fade in effect: from no volume to full volume of signal
+    multiplier = (1 : time) / time;
+
+    % the resulting fade-in output  
+    output = input .* multiplier;
+end
+
+% This is useful for making videos, specifically the intro part