83 lines
3.2 KiB
C++
83 lines
3.2 KiB
C++
/* -*- c++ -*- */
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/*
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* gr-satnogs: SatNOGS GNU Radio Out-Of-Tree Module
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*
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* Copyright (C) 2018, Libre Space Foundation <http://librespacefoundation.org/>
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#ifndef INCLUDED_SATNOGS_QUAD_DEMOD_FILTER_FF_H
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#define INCLUDED_SATNOGS_QUAD_DEMOD_FILTER_FF_H
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#include <satnogs/api.h>
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#include <gnuradio/sync_block.h>
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namespace gr {
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namespace satnogs {
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/*!
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* \brief The goal of this block is to act as a valve for a signal
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* commonly originating from a clock recovered quadrature demodulated stream.
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*
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* A very common problem in such cases, is that after the quadrature
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* demodulation the signal is on the phase domain and the detection
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* of signal presence or absence cannot be performed with energy detection.
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* Why do we need to identify signal presence? Most of the amateur satellites
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* uses quite poor and short training sequences, especially in the AX.25
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* AFSK1200 mode. Thus, the decoders have too many false alarms and due to
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* this, they miss frames.
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*
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* Some may argue that you can perform energy detection before the quadrature
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* demodulation, but this implies a 'magic' threshold should be set.
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* In the SatNOGS case, this is not possible. Every ground station is unique
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* in terms of performance and RF characteristics. On the other hand the
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* phase domain has the nice property that the signal level is known. This
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* level can be +-pi. This block tries to find an SNR-like metric based
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* on the variance and the mean of the signal.
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* \ingroup satnogs
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*
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*/
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class SATNOGS_API quad_demod_filter_ff : virtual public gr::sync_block
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{
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public:
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typedef boost::shared_ptr<quad_demod_filter_ff> sptr;
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/**
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* Creates a Quadrature Demodulate filter. The block will output samples
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* only in signal presence acting as a "valve" to reduce false alarms of
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* the FSK decoders. In case of no signal, zero samples are produced.
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*
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* @param gain this MUST be the gain set on the quadrature demodulate
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* gain or an appropriate value if the amplitude of the quadrature
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* demodulated signal somehow is scaled. By default if the signal is
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* on the +-pi region, the gain should be one. If it is on the +-2pi, the
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* gain should be set to 2.0 etc.
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*
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* @param window the window size to calculate the moving average and
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* variance.
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*
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* @return shared pointer of the block
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*/
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static sptr make(float gain, int window = 80);
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};
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} // namespace satnogs
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} // namespace gr
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#endif /* INCLUDED_SATNOGS_QUAD_DEMOD_FILTER_FF_H */
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