fsk.h

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00001 /*
00002  * SpanDSP - a series of DSP components for telephony
00003  *
00004  * fsk.h - FSK modem transmit and receive parts
00005  *
00006  * Written by Steve Underwood <steveu@coppice.org>
00007  *
00008  * Copyright (C) 2003 Steve Underwood
00009  *
00010  * All rights reserved.
00011  *
00012  * This program is free software; you can redistribute it and/or modify
00013  * it under the terms of the GNU General Public License version 2, as
00014  * published by the Free Software Foundation.
00015  *
00016  * This program is distributed in the hope that it will be useful,
00017  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00018  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
00019  * GNU General Public License for more details.
00020  *
00021  * You should have received a copy of the GNU General Public License
00022  * along with this program; if not, write to the Free Software
00023  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
00024  *
00025  * $Id: fsk.h,v 1.22 2007/04/10 16:12:20 steveu Exp $
00026  */
00027 
00028 /*! \file */
00029 
00030 /*! \page fsk_page FSK modems
00031 \section fsk_page_sec_1 What does it do?
00032 Most of the oldest telephony modems use incoherent FSK modulation. This module can
00033 be used to implement both the transmit and receive sides of a number of these
00034 modems. There are integrated definitions for: 
00035 
00036  - V.21
00037  - V.23
00038  - Bell 103
00039  - Bell 202
00040  - Weitbrecht (Used for TDD - Telecoms Device for the Deaf)
00041 
00042 The audio output or input is a stream of 16 bit samples, at 8000 samples/second.
00043 The transmit and receive sides can be used independantly. 
00044 
00045 \section fsk_page_sec_2 The transmitter
00046 
00047 The FSK transmitter uses a DDS generator to synthesise the waveform. This
00048 naturally produces phase coherent transitions, as the phase update rate is
00049 switched, producing a clean spectrum. The symbols are not generally an integer
00050 number of samples long. However, the symbol time for the fastest data rate
00051 generally used (1200bps) is more than 7 samples long. The jitter resulting from
00052 switching at the nearest sample is, therefore, acceptable. No interpolation is
00053 used. 
00054 
00055 \section fsk_page_sec_3 The receiver
00056 
00057 The FSK receiver uses a quadrature correlation technique to demodulate the
00058 signal. Two DDS quadrature oscillators are used. The incoming signal is
00059 correlated with the oscillator signals over a period of one symbol. The
00060 oscillator giving the highest net correlation from its I and Q outputs is the
00061 one that matches the frequency being transmitted during the correlation
00062 interval. Because the transmission is totally asynchronous, the demodulation
00063 process must run sample by sample to find the symbol transitions. The
00064 correlation is performed on a sliding window basis, so the computational load of
00065 demodulating sample by sample is not great. 
00066 
00067 Two modes of symbol synchronisation are provided:
00068 
00069     - In synchronous mode, symbol transitions are smoothed, to track their true
00070       position in the prescence of high timing jitter. This provides the most
00071       reliable symbol recovery in poor signal to noise conditions. However, it
00072       takes a little time to settle, so it not really suitable for data streams
00073       which must start up instantaneously (e.g. the TDD systems used by hearing
00074       impaired people).
00075 
00076     - In asynchronous mode each transition is taken at face value, with no temporal
00077       smoothing. There is no settling time for this mode, but when the signal to
00078       noise ratio is very poor it does not perform as well as the synchronous mode.
00079 */
00080 
00081 #if !defined(_SPANDSP_FSK_H_)
00082 #define _SPANDSP_FSK_H_
00083 
00084 /*!
00085     FSK modem specification. This defines the frequencies, signal levels and
00086     baud rate (== bit rate for simple FSK) for a single channel of an FSK modem.
00087 */
00088 typedef struct
00089 {
00090     const char *name;
00091     int freq_zero;
00092     int freq_one;
00093     int tx_level;
00094     int min_level;
00095     int baud_rate;
00096 } fsk_spec_t;
00097 
00098 /* Predefined FSK modem channels */
00099 enum
00100 {
00101     FSK_V21CH1 = 0,
00102     FSK_V21CH2,
00103     FSK_V23CH1,
00104     FSK_V23CH2,
00105     FSK_BELL103CH1,
00106     FSK_BELL103CH2,
00107     FSK_BELL202,
00108     FSK_WEITBRECHT,     /* Used for TDD (Telecom Device for the Deaf) */
00109 };
00110 
00111 extern fsk_spec_t preset_fsk_specs[];
00112 
00113 /*!
00114     FSK modem transmit descriptor. This defines the state of a single working
00115     instance of an FSK modem transmitter.
00116 */
00117 typedef struct
00118 {
00119     int baud_rate;
00120     get_bit_func_t get_bit;
00121     void *user_data;
00122 
00123     int32_t phase_rates[2];
00124     int scaling;
00125     int32_t current_phase_rate;
00126     uint32_t phase_acc;
00127     int baud_frac;
00128     int baud_inc;
00129     int shutdown;
00130 } fsk_tx_state_t;
00131 
00132 /* The longest window will probably be 106 for 75 baud */
00133 #define FSK_MAX_WINDOW_LEN 128
00134 
00135 /*!
00136     FSK modem receive descriptor. This defines the state of a single working
00137     instance of an FSK modem receiver.
00138 */
00139 typedef struct
00140 {
00141     int baud_rate;
00142     int sync_mode;
00143     put_bit_func_t put_bit;
00144     void *user_data;
00145 
00146     int min_power;
00147     power_meter_t power;
00148     /*! \brief The value of the last signal sample, using the a simple HPF for signal power estimation. */
00149     int16_t last_sample;
00150     /*! \brief >0 if a signal above the minimum is present. It may or may not be a V.29 signal. */
00151     int signal_present;
00152 
00153     int32_t phase_rate[2];
00154     uint32_t phase_acc[2];
00155 
00156     int correlation_span;
00157 
00158     int32_t window_i[2][FSK_MAX_WINDOW_LEN];
00159     int32_t window_q[2][FSK_MAX_WINDOW_LEN];
00160     int32_t dot_i[2];
00161     int32_t dot_q[2];
00162     int buf_ptr;
00163 
00164     int baud_inc;
00165     int baud_pll;
00166     int lastbit;
00167     int scaling_shift;
00168 } fsk_rx_state_t;
00169 
00170 #if defined(__cplusplus)
00171 extern "C"
00172 {
00173 #endif
00174 
00175 /*! Initialise an FSK modem transmit context.
00176     \brief Initialise an FSK modem transmit context.
00177     \param s The modem context.
00178     \param spec The specification of the modem tones and rate.
00179     \param get_bit The callback routine used to get the data to be transmitted.
00180     \param user_data An opaque pointer.
00181     \return A pointer to the modem context, or NULL if there was a problem. */
00182 fsk_tx_state_t *fsk_tx_init(fsk_tx_state_t *s,
00183                             fsk_spec_t *spec,
00184                             get_bit_func_t get_bit,
00185                             void *user_data);
00186 
00187 /*! Adjust an FSK modem transmit context's power output.
00188     \brief Adjust an FSK modem transmit context's power output.
00189     \param s The modem context.
00190     \param power The power level, in dBm0 */
00191 void fsk_tx_power(fsk_tx_state_t *s, float power);
00192 
00193 void fsk_tx_set_get_bit(fsk_tx_state_t *s, get_bit_func_t get_bit, void *user_data);
00194 
00195 /*! Generate a block of FSK modem audio samples.
00196     \brief Generate a block of FSK modem audio samples.
00197     \param s The modem context.
00198     \param amp The audio sample buffer.
00199     \param len The number of samples to be generated.
00200     \return The number of samples actually generated.
00201 */
00202 int fsk_tx(fsk_tx_state_t *s, int16_t *amp, int len);
00203 
00204 /*! Get the current received signal power.
00205     \param s The modem context.
00206     \return The signal power, in dBm0. */
00207 float fsk_rx_signal_power(fsk_rx_state_t *s);
00208 
00209 /*! Adjust an FSK modem receive context's carrier detect power threshold.
00210     \brief Adjust an FSK modem receive context's carrier detect power threshold.
00211     \param s The modem context.
00212     \param power The power level, in dBm0 */
00213 void fsk_rx_signal_cutoff(fsk_rx_state_t *s, float cutoff);
00214 
00215 /*! Initialise an FSK modem receive context.
00216     \brief Initialise an FSK modem receive context.
00217     \param s The modem context.
00218     \param spec The specification of the modem tones and rate.
00219     \param sync_mode TRUE for synchronous modem. FALSE for asynchronous mode.
00220     \param put_bit The callback routine used to put the received data.
00221     \param user_data An opaque pointer.
00222     \return A pointer to the modem context, or NULL if there was a problem. */
00223 fsk_rx_state_t *fsk_rx_init(fsk_rx_state_t *s,
00224                             fsk_spec_t *spec,
00225                             int sync_mode,
00226                             put_bit_func_t put_bit,
00227                             void *user_data);
00228 
00229 /*! Process a block of received FSK modem audio samples.
00230     \brief Process a block of received FSK modem audio samples.
00231     \param s The modem context.
00232     \param amp The audio sample buffer.
00233     \param len The number of samples in the buffer.
00234     \return The number of samples unprocessed.
00235 */
00236 int fsk_rx(fsk_rx_state_t *s, const int16_t *amp, int len);
00237 
00238 void fsk_rx_set_put_bit(fsk_rx_state_t *s, put_bit_func_t put_bit, void *user_data);
00239 
00240 #if defined(__cplusplus)
00241 }
00242 #endif
00243 
00244 #endif
00245 /*- End of file ------------------------------------------------------------*/

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