randomizer_impl.h

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/* -*- c++ -*- */
/*
 * Copyright 2001 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * GNU Radio is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3, or (at your option)
 * any later version.
 *
 * GNU Radio 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 GNU Radio; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

#ifndef _ATSC_RANDOMIZER_H_
#define _ATSC_RANDOMIZER_H_

#include <gnuradio/atsc/api.h>
#include <gnuradio/atsc/types.h>

/*!
 * \brief ATSC data "whitener"
 *
 * The data randomizer described in ATSC standard A/53B.
 * See figure D4 on page 54.
 */

class ATSC_API atsci_randomizer
{
    friend class qa_atsci_randomizer;

public:
    atsci_randomizer();

    /*! \brief reset randomizer LFSR
     *
     * must be called during the Data Segment Sync interval prior to the
     * first data segment.  I.e., the LFSR is reset prior to the first
     * field of each VSB data frame.
     */
    void reset();

    //! randomize (whiten) mpeg packet and remove leading MPEG-2 sync byte
    void randomize(atsc_mpeg_packet_no_sync& out, const atsc_mpeg_packet& in);

    //! derandomize (de-whiten) mpeg packet and add leading MPEG-2 sync byte
    void derandomize(atsc_mpeg_packet& out, const atsc_mpeg_packet_no_sync& in);

    unsigned int state() const { return d_state; }

private:
    static void initialize_output_map();
    static unsigned char slow_output_map(int st);

    static unsigned char fast_output_map(int st)
    {
        return s_output_map[(st & 0xb23c) >>
                            2]; // Magic const with 8 bits set improves cache
                                // utilization.  The bits correspond to the taps
                                // used in output calculation.  Others may be
                                // safely ignored.
    }

    //! return current output value
    unsigned char output() { return fast_output_map(d_state); }

    //! clock LFSR; advance to next state.
    void clk()
    {
        if (d_state & 0x1)
            d_state = ((d_state ^ MASK) >> 1) | 0x8000;
        else
            d_state = d_state >> 1;
    }

    //! return current output value and advance to next state
    unsigned char output_and_clk()
    {
        unsigned char r = output();
        clk();
        return r;
    }

    unsigned int d_state;

    static const unsigned int PRELOAD_VALUE = 0x018f; /* 0xf180 bit reversed */
    static const unsigned int MASK = 0xa638;
    static unsigned char s_output_map[1 << 14];
    static bool s_output_map_initialized_p;
};

#endif /* _ATSC_RANDOMIZER_H_ */