Fix signed_to_unsigned being off by 1

trackrip/pcm.py

@@ -2,15 +2,23 @@
 
 def signed_to_unsigned(data):
     """
-    Converts signed data to unsigned data
-    (that's not exactly what this does but I'm not sure what to call it)
-    TODO: Figure out exactly why this works.
+    Converts signed data to unsigned data.
     """
+    # Python reads bytes as unsigned chars (0 to 255).
+    # For example: Byte "FF" in a signed file is -1, but Python will read it
+    # as the unsigned value 255. -1 is halfway in the signed range (-127 to 127)
+    # at position 127 (counting the sign bit). However, 255 corresponds to the
+    # very end of the unsigned range. To match the original position as a signed
+    # number, we subtract 128, landing us at the original position at 127.
+    # While positive decimals will be the same signed or unsigned (under 127),
+    # their relative position will change again. 127 is position 255 while
+    # signed, and 128 while unsigned. We add 128, and arrive at 255, the correct
+    # position.
     converted = bytearray()
     for byte in data:
         if byte > 127:
-            signed = byte - 127
+            signed = byte - 128
         else:
-            signed = byte + 127
+            signed = byte + 128
         converted.append(signed)
     return converted