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view spandsp-0.0.3/spandsp-0.0.3/src/spandsp/fsk.h @ 5:f762bf195c4b
import spandsp-0.0.3
author | Peter Meerwald <pmeerw@cosy.sbg.ac.at> |
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date | Fri, 25 Jun 2010 16:00:21 +0200 |
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/* * SpanDSP - a series of DSP components for telephony * * fsk.h - FSK modem transmit and receive parts * * Written by Steve Underwood <steveu@coppice.org> * * Copyright (C) 2003 Steve Underwood * * All rights reserved. * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2, as * published by the Free Software Foundation. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. * * $Id: fsk.h,v 1.17 2006/10/24 13:45:28 steveu Exp $ */ /*! \file */ /*! \page fsk_page FSK modems \section fsk_page_sec_1 What does it do? Most of the oldest telephony modems use incorent FSK modulation. This module can be used to implement both the trasmit and receive sides of a number of these modems. There are integrated definitions for: - V.21 - V.23 - Bell 103 - Bell 202 - Weitbrecht (Used for TDD - Telecoms Device for the Deaf) The audio output or input is a stream of 16 bit samples, at 8000 samples/second. The transmit and receive sides can be used independantly. \section fsk_page_sec_2 The transmitter The FSK transmitter uses a DDS generator to synthesise the waveform. This naturally produces phase coherent transitions, as the phase update rate is switched, producing a clean spectrum. The symbols are not generally an integer number of samples long. However, the symbol time for the fastest data rate generally used (1200bps) is more than 7 samples long. The jitter resulting from switching at the nearest sample is, therefore, acceptable. No interpolation is used. \section fsk_page_sec_3 The receiver The FSK receiver uses a quadrature correlation technique to demodulate the signal. Two DDS quadrature oscillators are used. The incoming signal is correlated with the oscillator signals over a period of one symbol. The oscillator giving the highest net correlation from its I and Q outputs is the one that matches the frequency being transmitted during the correlation interval. Because the transmission is totally asynchronous, the demodulation process must run sample by sample to find the symbol transitions. The correlation is performed on a sliding window basis, so the computational load of demodulating sample by sample is not great. Two modes of symbol synchronisation are provided: - In synchronous mode, symbol transitions are smoothed, to track their true position in the prescence of high timing jitter. This provides the most reliable symbol recovery in poor signal to noise conditions. However, it takes a little time to settle, so it not really suitable for data streams which must start up instantaneously (e.g. the TDD systems used by hearing impaired people). - In asynchronous mode each transition is taken at face value, with no temporal smoothing. There is no settling time for this mode, but when the signal to noise ratio is very poor it does not perform as well as the synchronous mode. */ #if !defined(_FSK_H_) #define _FSK_H_ /*! FSK modem specification. This defines the frequencies, signal levels and baud rate (== bit rate for simple FSK) for a single channel of an FSK modem. */ typedef struct { const char *name; int freq_zero; int freq_one; int tx_level; int min_level; int baud_rate; } fsk_spec_t; /* Predefined FSK modem channels */ enum { FSK_V21CH1 = 0, FSK_V21CH2, FSK_V23CH1, FSK_V23CH2, FSK_BELL103CH1, FSK_BELL103CH2, FSK_BELL202, FSK_WEITBRECHT, /* Used for TDD (Telecom Device for the Deaf) */ }; extern fsk_spec_t preset_fsk_specs[]; /*! FSK modem transmit descriptor. This defines the state of a single working instance of an FSK modem transmitter. */ typedef struct { int baud_rate; get_bit_func_t get_bit; void *user_data; int32_t phase_rates[2]; int scaling; int32_t current_phase_rate; uint32_t phase_acc; int baud_frac; int baud_inc; int shutdown; } fsk_tx_state_t; /* The longest window will probably be 106 for 75 baud */ #define FSK_MAX_WINDOW_LEN 128 /*! FSK modem receive descriptor. This defines the state of a single working instance of an FSK modem receiver. */ typedef struct { int baud_rate; int sync_mode; put_bit_func_t put_bit; void *user_data; int min_power; power_meter_t power; int carrier_present; int16_t last_sample; int32_t phase_rate[2]; uint32_t phase_acc[2]; int correlation_span; int32_t window_i[2][FSK_MAX_WINDOW_LEN]; int32_t window_q[2][FSK_MAX_WINDOW_LEN]; int32_t dot_i[2]; int32_t dot_q[2]; int buf_ptr; int baud_inc; int baud_pll; int lastbit; int scaling_shift; } fsk_rx_state_t; #ifdef __cplusplus extern "C" { #endif /*! Initialise an FSK modem transmit context. \brief Initialise an FSK modem transmit context. \param s The modem context. \param spec The specification of the modem tones and rate. \param get_bit The callback routine used to get the data to be transmitted. \param user_data An opaque pointer. \return A pointer to the modem context, or NULL if there was a problem. */ fsk_tx_state_t *fsk_tx_init(fsk_tx_state_t *s, fsk_spec_t *spec, get_bit_func_t get_bit, void *user_data); /*! Adjust an FSK modem transmit context's power output. \brief Adjust an FSK modem transmit context's power output. \param s The modem context. \param power The power level, in dBm0 */ void fsk_tx_power(fsk_tx_state_t *s, float power); void fsk_tx_set_get_bit(fsk_tx_state_t *s, get_bit_func_t get_bit, void *user_data); /*! Generate a block of FSK modem audio samples. \brief Generate a block of FSK modem audio samples. \param s The modem context. \param amp The audio sample buffer. \param len The number of samples to be generated. \return The number of samples actually generated. */ int fsk_tx(fsk_tx_state_t *s, int16_t *amp, int len); /*! Get the current received signal power. \param s The modem context. \return The signal power, in dBm0. */ float fsk_rx_signal_power(fsk_rx_state_t *s); /*! Adjust an FSK modem receive context's carrier detect power threshold. \brief Adjust an FSK modem receive context's carrier detect power threshold. \param s The modem context. \param power The power level, in dBm0 */ void fsk_rx_signal_cutoff(fsk_rx_state_t *s, float cutoff); /*! Initialise an FSK modem receive context. \brief Initialise an FSK modem receive context. \param s The modem context. \param spec The specification of the modem tones and rate. \param sync_mode TRUE for synchronous modem. FALSE for asynchronous mode. \param put_bit The callback routine used to put the received data. \param user_data An opaque pointer. \return A pointer to the modem context, or NULL if there was a problem. */ fsk_rx_state_t *fsk_rx_init(fsk_rx_state_t *s, fsk_spec_t *spec, int sync_mode, put_bit_func_t put_bit, void *user_data); /*! Process a block of received FSK modem audio samples. \brief Process a block of received FSK modem audio samples. \param s The modem context. \param amp The audio sample buffer. \param len The number of samples in the buffer. \return The number of samples unprocessed. */ int fsk_rx(fsk_rx_state_t *s, const int16_t *amp, int len); void fsk_rx_set_put_bit(fsk_rx_state_t *s, put_bit_func_t put_bit, void *user_data); #ifdef __cplusplus } #endif #endif /*- End of file ------------------------------------------------------------*/