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59 lines
1.5 KiB
C
59 lines
1.5 KiB
C
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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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// 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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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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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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u8 byte_in=*pin, byte_out=0;
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for ( int b=8; b--; byte_in<<=1 ) {
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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
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#endif // LEANSDR_IESS_H
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