diff options
| -rw-r--r-- | util/spkmodem_recv/spkmodem-recv.c | 71 |
1 files changed, 64 insertions, 7 deletions
diff --git a/util/spkmodem_recv/spkmodem-recv.c b/util/spkmodem_recv/spkmodem-recv.c index f2b5e78d..a1ea0fd6 100644 --- a/util/spkmodem_recv/spkmodem-recv.c +++ b/util/spkmodem_recv/spkmodem-recv.c @@ -37,17 +37,74 @@ #include <string.h> #include <unistd.h> +/* + * spkmodem is essentially used FSK (Frequency Shift Keying) + * with two frequencies: tone A (bit 0) and tone B (bit 1), + * detected via pulse density inside a given window. + * Very cheap on CPU cycles and avoids neeing something more + * complicated like FFT or Goertzel filters, and tolerates + * weak/noisy signals. + */ + +/* + * Frequency of audio in Hz + */ +#define SAMPLE_RATE 48000 + +/* + * A frame is 5ms, so samples + * per frame is N / 48000 = 0.005 (5ms) + * => N = 0.005 * 48000 = 240 + */ #define SAMPLES_PER_FRAME 240 -#define MAX_SAMPLES (2 * SAMPLES_PER_FRAME) -#define SAMPLE_OFFSET (MAX_SAMPLES * sizeof(short)) -#define FREQ_SEP_MIN 5 -#define FREQ_SEP_MAX 15 +/* = 48000 / 240 = 200 Hz resolution */ +#define FRAME_RATE ((SAMPLE_RATE) / (SAMPLES_PER_FRAME)) + +/* + * Two FIR windows are maintained; one for data done, + * and one for the separator tone. They are positioned + * one frame apart in the ring buffer. + */ +#define MAX_SAMPLES (2 * (SAMPLES_PER_FRAME)) -#define FREQ_DATA_MIN 15 -#define FREQ_DATA_THRESHOLD 25 -#define FREQ_DATA_MAX 60 +/* + * Approx byte offset for ring buffer span, just for + * easier debug output correlating to the audio stream. + */ +#define SAMPLE_OFFSET ((MAX_SAMPLES) * (sizeof(short))) +/* + * Expected tone ranges (approximate, derived from spkmodem). + * These values are intentionally wide because real-world setups + * often involve microphones, room acoustics, and cheap ADCs. + */ +#define SEP_TONE_MIN_HZ 1000 +#define SEP_TONE_MAX_HZ 3000 + +#define DATA_TONE_MIN_HZ 3000 +#define DATA_TONE_MAX_HZ 12000 + +/* Mid point used to distinguish the two data tones. */ +#define DATA_TONE_THRESHOLD_HZ 5000 + +/* + * Convert tone frequencies within the sliding window, into pulse counts + * pulse_count ≈ tone_frequency / FRAME_RATE + * where FRAME_RATE = SAMPLE_RATE / SAMPLES_PER_FRAME. + */ +#define FREQ_SEP_MIN ((SEP_TONE_MIN_HZ) / (FRAME_RATE)) +#define FREQ_SEP_MAX ((SEP_TONE_MAX_HZ) / (FRAME_RATE)) + +#define FREQ_DATA_MIN ((DATA_TONE_MIN_HZ) / (FRAME_RATE)) +#define FREQ_DATA_MAX ((DATA_TONE_MAX_HZ) / (FRAME_RATE)) + +#define FREQ_DATA_THRESHOLD ((DATA_TONE_THRESHOLD_HZ) / (FRAME_RATE)) + +/* + * Sample amplitude threshold used to convert the waveform + * into a pulse stream. Values near zero regarded as noise. + */ #define THRESHOLD 500 #define READ_BUF 4096 |