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80 lines
2.6 KiB
C++
80 lines
2.6 KiB
C++
// This file is part of LeanSDR Copyright (C) 2016-2018 <pabr@pabr.org>.
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// See the toplevel README for more information.
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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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#ifndef LEANSDR_IESS_H
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#define LEANSDR_IESS_H
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#include "leansdr/framework.h"
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namespace leansdr
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{
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// SELF-SYNCHRONIZING DESCRAMBLER
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// Per ETSI TR 192 figure 8 (except Q20/ not connected to CLOCK).
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// This implementation operates on packed bits, MSB first.
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struct etr192_descrambler : runnable
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{
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etr192_descrambler(scheduler *sch,
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pipebuf<u8> &_in, // Packed scrambled bits
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pipebuf<u8> &_out) // Packed bits
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: runnable(sch, "etr192_dec"),
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in(_in), out(_out),
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shiftreg(0), counter(0)
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{
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}
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void run()
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{
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int count = min(in.readable(), out.writable());
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for (u8 *pin = in.rd(), *pend = pin + count, *pout = out.wr();
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pin < pend; ++pin, ++pout)
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{
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u8 byte_in = *pin, byte_out = 0;
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for (int b = 8; b--; byte_in <<= 1)
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{
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// Levels before clock transition
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int bit_in = (byte_in & 128) ? 1 : 0;
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int reset_counter = (shiftreg ^ (shiftreg >> 8)) & 1;
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int counter_overflow = (counter == 31) ? 1 : 0;
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int taps = (shiftreg >> 2) ^ (shiftreg >> 19);
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int bit_out = (taps ^ counter_overflow ^ bit_in ^ 1) & 1;
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// Execute clock transition
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#if 1 // Descramble
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shiftreg = (shiftreg << 1) | bit_in;
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#else // Scramble
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shiftreg = (shiftreg << 1) | bit_out;
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#endif
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counter = reset_counter ? 0 : (counter + 1) & 31;
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byte_out = (byte_out << 1) | bit_out;
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}
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*pout = byte_out;
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}
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in.read(count);
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out.written(count);
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}
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private:
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pipereader<u8> in;
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pipewriter<u8> out;
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u32 shiftreg; // 20 bits
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u8 counter; // 5 bits
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}; // etr192_descrambler
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} // namespace leansdr
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#endif // LEANSDR_IESS_H
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