C-Netz: SDR now uses zero-crossing to detect level changes.

This does not work with analog radio, but with SDR it works.
The quality should be better and the process is faster.

src/cnetz/fsk_demod.h

@@ -0,0 +1,68 @@
+
+#define BITS_PER_SUPERFRAME	12672.0	/* super frame (Oberrahmen) has duration of excactly 2.4 seconds */
+#define BITS_PER_BLOCK		198.0	/* block has duration of excactly 37.5 milli seconds */
+#define BITS_PER_SPK_BLOCK	66.0	/* spk block has a duration of exactly 12.5 milli seconds */
+
+/* fsk rx sync state */
+enum fsk_sync {
+	FSK_SYNC_NONE = 0,
+	FSK_SYNC_POSITIVE,
+	FSK_SYNC_NEGATIVE,
+};
+
+enum demod_type {
+	FSK_DEMOD_SLOPE, /* check for highest slope (good for sound cards) */
+	FSK_DEMOD_LEVEL, /* check for zero crossing (good for SDR) */
+};
+
+typedef struct cnetz cnetz_t;
+
+typedef struct fsk_fm_demod {
+	cnetz_t		*cnetz;			/* pointer back to cnetz instance */
+
+	/* clock */
+	double		bit_time;		/* current time in bits inside superframe */
+	double		bit_time_uncorrected;	/* same as above, but not corrected by sync */
+
+	/* bit detection */
+	enum demod_type	demod_type;		/* how to demodulate bits */
+	sample_t	*bit_buffer_spl;	/* samples ring buffer */
+	int		bit_buffer_len;		/* number of samples in ring buffer */
+	int		bit_buffer_half;	/* half of ring buffer */
+	int		bit_buffer_pos;		/* current position to write next sample */
+	double		level_threshold;	/* threshold for detection of next level change */
+	double		bits_per_sample;	/* duration of one sample in bits */
+	double		next_bit;		/* count time to detect bits */
+	int		bit_count;		/* counts bits, to match 4 bits at distributed signaling */
+	int		last_change_positive;	/* flags last level change direction */
+	enum fsk_sync	sync;			/* set, if we are in sync and what polarity we receive */
+	double		sync_level;		/* what was the level, when we received the sync */
+	double		sync_time;		/* when did we receive sync, relative to super frame */
+	double		sync_jitter;		/* what was the jitter of the sync */
+
+	/* speech */
+	sample_t	*speech_buffer;		/* holds one chunk of 12.5ms */
+	int		speech_size;
+	int		speech_count;
+
+	/* bit decoder */
+	uint64_t	rx_sync;		/* sync shift register */
+	char		rx_buffer[151];		/* 150 bits + termination */
+	int		rx_buffer_count;	/* counter when receiving bits */
+
+	/* statistics */
+	double		change_levels[256];	/* ring buffer to store levels */
+	double		change_when[256];	/* ring buffer to store time when level has changed */
+	uint8_t		change_pos;		/* index for next write */
+
+	/* debug */
+	FILE		*debug_fp;		/* file pointer for debugging output */
+} fsk_fm_demod_t;
+
+int fsk_fm_init(fsk_fm_demod_t *fsk, cnetz_t *cnetz, int samplerate, double bitrate, enum demod_type);
+void fsk_fm_exit(fsk_fm_demod_t *fsk);
+void fsk_fm_demod(fsk_fm_demod_t *fsk, sample_t *samples, int length);
+void fsk_correct_sync(fsk_fm_demod_t *fsk, double offset);
+void fsk_copy_sync(fsk_fm_demod_t *fsk_to, fsk_fm_demod_t *fsk_from);
+void fsk_demod_reset(fsk_fm_demod_t *fsk);
+