libdsp: update some comments

libs/libdsp/Kconfig

@@ -26,8 +26,9 @@ config LIBDSP_PRECISION
 	---help---
 		With this option we can select libdsp precision for
 		some of calculations. There are 3 available options:
-		0 - the fastest calculation but the lowest precision
-		1 - a little better precision than above, but slowest
+		0 - the fastest calculation but the lowest precision,
+		1 - increased precision than for option 0 at the expense
+		    of a longer execution time,
 		2 - the most accuracte but the slowest one, use standard math functions.
 
 config LIBDSP_FOC_VABC

libs/libdsp/lib_misc.c

@@ -169,8 +169,6 @@ void dq_saturate(FAR dq_frame_f32_t *dq, float max)
  * Description:
  *   Fast sin calculation
  *
- *   Reference: http://lab.polygonal.de/?p=205
- *
  * Input Parameters:
  *   angle - (in)
  *
@@ -228,12 +226,11 @@ float fast_cos(float angle)
  * Name: fast_sin2
  *
  * Description:
- *   Fast sin calculation with better accuracy
- *
- *   Reference: http://lab.polygonal.de/?p=205
+ *   Fast sin calculation with better accuracy (quadratic curve
+ *   approximation)
  *
  * Input Parameters:
- *   angle
+ *   angle - (in)
  *
  * Returned Value:
  *   Return estimated sine value
@@ -287,7 +284,8 @@ float fast_sin2(float angle)
  * Name:fast_cos2
  *
  * Description:
- *   Fast cos calculation with better accuracy
+ *   Fast cos calculation with better accuracy (quadratic curve
+ *   approximation)
  *
  * Input Parameters:
  *   angle - (in)

libs/libdsp/lib_misc_b16.c

@@ -196,8 +196,6 @@ void dq_saturate_b16(FAR dq_frame_b16_t *dq, b16_t max)
  * Description:
  *   Fast sin calculation
  *
- *   Reference: http://lab.polygonal.de/?p=205
- *
  * Input Parameters:
  *   angle - (in)
  *
@@ -261,12 +259,11 @@ b16_t fast_cos_b16(b16_t angle)
  * Name: fast_sin2_b16
  *
  * Description:
- *   Fast sin calculation with better accuracy
- *
- *   Reference: http://lab.polygonal.de/?p=205
+ *   Fast sin calculation with better accuracy (quadratic curve
+ *   approximation)
  *
  * Input Parameters:
- *   angle
+ *   angle - (in)
  *
  * Returned Value:
  *   Return estimated sine value
@@ -282,7 +279,8 @@ b16_t fast_sin2_b16(b16_t angle)
  * Name:fast_cos2_b16
  *
  * Description:
- *   Fast cos calculation with better accuracy
+ *   Fast cos calculation with better accuracy (quadratic curve
+ *   approximation)
  *
  * Input Parameters:
  *   angle - (in)