1 /*!
2 * Fixed-point tone generator.
3 *
4 * The code here implements a simple fixed-point tone generator that uses
5 * integer arithmetic to generate a sinusoid at a fixed sample rate of
6 * 16kHz.
7 *
8 * To set the initial state of the state machine, you specify a frequency
9 * and duration using tone_reset. The corresponding C file embeds a
10 * sinusoid look-up table. The total number of samples is computed for
11 * the given time and used to initialise 'remain', 'time' is initialised
12 * to 0, and 'step' gives the amount to increment 'time' by each iteration.
13 *
14 * The samples are retrieved by repeatedly calling tone_next. This
15 * advances 'time' and decrements 'remain'. The tone is complete when
16 * 'remain' is zero.
17 *
18 * Author Stuart Longland <me@vk4msl.id.au>
19 * Copyright (C) 2015 FreeDV project.
20 *
21 * This program is free software; you can redistribute it and/or modify it
22 * under the terms of the GNU Lesser General Public License version 2.1,
23 * as published by the Free Software Foundation. This program is
24 * distributed in the hope that it will be useful, but WITHOUT ANY
25 * WARRANTY; without even the implied warranty of MERCHANTABILITY or
26 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
27 * for more details.
28 *
29 * You should have received a copy of the GNU Lesser General Public
30 * License along with this program; if not, see
31 * <http://www.gnu.org/licenses/>.
32 */
33
34 #include "tone.h"
35
36 /*! Fixed-point shift factor */
37 #define TONE_SHIFT (12)
38
39 /*! Static compiled quarter-sinusoid. */
40 static const int16_t partial_sine[] = {
41 830, 2488, 4140, 5781, 7407, 9014, 10598, 12155,
42 13681, 15171, 16623, 18031, 19394, 20707, 21967, 23170,
43 24314, 25395, 26411, 27360, 28238, 29043, 29774, 30429,
44 31006, 31503, 31919, 32253, 32504, 32672, 32756
45 };
46
47 /*! Length of quarter-sinusoid in samples */
48 #define TONE_PART_SINE_LEN (sizeof(partial_sine)\
49 /sizeof(partial_sine[0]))
50
51 /*! Total length of sinusoid */
52 #define TONE_SINE_LEN ((TONE_PART_SINE_LEN*4)+4)
53
54 /*!
55 * Generate a sine from the quarter-waveform.
56 */
tone_sine(uint8_t sample)57 static int16_t tone_sine(uint8_t sample)
58 {
59 /* Key points */
60 if ((sample % (TONE_SINE_LEN/2)) == 0)
61 /* Zero crossings */
62 return 0;
63 if (sample == TONE_SINE_LEN/4)
64 /* Maximum */
65 return INT16_MAX;
66 if (sample == (3*TONE_SINE_LEN)/4)
67 /* Minimum */
68 return INT16_MIN;
69
70 if (sample < TONE_SINE_LEN/4)
71 /* First quarter of sine wave */
72 return partial_sine[sample-1];
73
74 if (sample < (TONE_SINE_LEN/2))
75 /* Second quarter */
76 return partial_sine[(TONE_SINE_LEN/2)-sample-1];
77 if (sample < ((3*TONE_SINE_LEN)/4))
78 /* Third quarter */
79 return -partial_sine[(sample-3) % TONE_PART_SINE_LEN];
80 if (sample < TONE_SINE_LEN)
81 /* Final quarter */
82 return -partial_sine[TONE_SINE_LEN-sample-1];
83 /* We should not get here */
84 return 0;
85 }
86
87 /*!
88 * Re-set the tone generator.
89 *
90 * @param tone_gen Tone generator to reset.
91 * @param freq Frequency in Hz, 0 = silence.
92 * @param duration Duration in milliseconds. 0 to stop.
93 */
tone_reset(struct tone_gen_t * const tone_gen,uint16_t freq,uint16_t duration)94 void tone_reset(
95 struct tone_gen_t* const tone_gen,
96 uint16_t freq, uint16_t duration)
97 {
98 if (freq)
99 /* Compute the time step */
100 tone_gen->step = (((2*freq*TONE_SINE_LEN) << TONE_SHIFT)
101 / ((2*TONE_FS) + 1) + 1);
102 else
103 /* DC tone == silence */
104 tone_gen->step = 0;
105
106 /* Compute remaining samples */
107 tone_gen->remain = (uint16_t)(
108 ((uint32_t)(TONE_FS * duration)) / 1000);
109
110 /* Initialise the sample counter */
111 tone_gen->sample = 0;
112 }
113
114 /*!
115 * Retrieve the next sample from the tone generator.
116 * @param tone_gen Tone generator to update.
117 */
tone_next(struct tone_gen_t * const tone_gen)118 int16_t tone_next(
119 struct tone_gen_t* const tone_gen)
120 {
121 if (!tone_gen)
122 return 0;
123 if (!tone_gen->remain)
124 return 0;
125 if (!tone_gen->step) {
126 /* Special case, emit silence */
127 tone_gen->remain--;
128 return 0;
129 }
130
131 /* Compute sample index */
132 uint16_t sample_int = ((tone_gen->sample) >> TONE_SHIFT)
133 % TONE_SINE_LEN;
134
135 /* Advance tone generator state */
136 tone_gen->sample += tone_gen->step;
137 tone_gen->remain--;
138
139 return tone_sine(sample_int);
140 }
141
142 /*!
143 * Retrieve the current time in milliseconds.
144 */
tone_msec(const struct tone_gen_t * const tone_gen)145 uint32_t tone_msec(const struct tone_gen_t* const tone_gen)
146 {
147 uint64_t ms = tone_gen->sample;
148 ms *= 1000;
149 ms /= TONE_FS;
150 return ms >> TONE_SHIFT;
151 }
152