tone: Exploit sine-wave symmetry to save space.

The sine wave basically can be divided into four quadrants, 0→0.5pi,
0.5pi→pi, pi→1.5pi and 1.5pi→2pi.  If we call these Q1, Q2, Q3 and Q4,
we can see:

- Q2 is basically Q1 in reverse
- Q3 is Q1 negated
- Q4 is Q2 negated

We can therefore exploit this to store ¼ of the waveform.  We do some if
statement trickery to stitch the partial waveforms together.  End result
is we've now quadrupled the definition of our sine wave at minimal
code-size cost.

CPU wise is a wee bit more expensive, but still a lot better than
actually calculating sine with a math library!

git-svn-id: https://svn.code.sf.net/p/freetel/code@2381 01035d8c-6547-0410-b346-abe4f91aad63

diff --git a/codec2-dev/stm32/src/tone.c b/codec2-dev/stm32/src/tone.c
index 7f02fa549ce9f0ec3449ba0ab66b9860446f2192..b834dc3be35974fa4b2299a0e7231f3430c4ad69 100644 (file)
--- a/codec2-dev/stm32/src/tone.c
+++ b/codec2-dev/stm32/src/tone.c
@@ -36,16 +36,53 @@
 /*! Fixed-point shift factor */
 #define TONE_SHIFT     (12)
 
-/*! Static compiled sinusoid.  Nicked from original FreeDV code. */
-static const int16_t tone_sine[] = {
-     -16,    6384,   12528,   18192,   23200,   27232,   30256,   32128,
-   32752,   32128,   30256,   27232,   23152,   18192,   12528,    6384,
-     -16,   -6416,  -12560,  -18224,  -23184,  -27264,  -30288,  -32160,
-  -32768,  -32160,  -30288,  -27264,  -23184,  -18224,  -12560,   -6416
+/*! Static compiled quarter-sinusoid. */
+static const int16_t partial_sine[] = {
+   830,   2488,   4140,   5781,   7407,   9014,  10598,  12155,
+ 13681,  15171,  16623,  18031,  19394,  20707,  21967,  23170,
+ 24314,  25395,  26411,  27360,  28238,  29043,  29774,  30429,
+ 31006,  31503,  31919,  32253,  32504,  32672,  32756
 };
 
-/*! Length of sinusoid in samples */
-#define TONE_SINE_LEN  (sizeof(tone_sine)/sizeof(tone_sine[0]))
+/*! Length of quarter-sinusoid in samples */
+#define TONE_PART_SINE_LEN     (sizeof(partial_sine)\
+                       /sizeof(partial_sine[0]))
+
+/*! Total length of sinusoid */
+#define TONE_SINE_LEN  ((TONE_PART_SINE_LEN*4)+4)
+
+/*!
+ * Generate a sine from the quarter-waveform.
+ */
+static int16_t tone_sine(uint8_t sample)
+{
+       /* Key points */
+       if ((sample % (TONE_SINE_LEN/2)) == 0)
+               /* Zero crossings */
+               return 0;
+       if (sample == TONE_SINE_LEN/4)
+               /* Maximum */
+               return INT16_MAX;
+       if (sample == (3*TONE_SINE_LEN)/4)
+               /* Minimum */
+               return INT16_MIN;
+
+       if (sample < TONE_SINE_LEN/4)
+               /* First quarter of sine wave */
+               return partial_sine[sample-1];
+
+       if (sample < (TONE_SINE_LEN/2))
+               /* Second quarter */
+               return partial_sine[(TONE_SINE_LEN/2)-sample-1];
+       if (sample < ((3*TONE_SINE_LEN)/4))
+               /* Third quarter */
+               return -partial_sine[(sample-3) % TONE_PART_SINE_LEN];
+       if (sample < TONE_SINE_LEN)
+               /* Final quarter */
+               return -partial_sine[TONE_SINE_LEN-sample-1];
+       /* We should not get here */
+       return 0;
+}
 
 /*!
  * Re-set the tone generator.
@@ -99,7 +136,7 @@ int16_t tone_next(
        tone_gen->sample += tone_gen->step;
        tone_gen->remain--;
 
-       return tone_sine[sample_int];
+       return tone_sine(sample_int);
 }
 
 /*!