Encoding with Feedback Shift Registers

• The encoding of the block codes described above can be done with feedback shift register circuits.
• The circuit below works as follows:
  • Switch 1 is closed for the first 4 shifts to load the shift registers.
  • Switch 2 is in the down position for the first 4 shifts to transmit the 4 information bits.
  • After transmission of the 4-th bit, switch 1 is opened and switch 2 is moved to the up position.
  • The 3 parity check bits are now in the shift register and are read out.
  • A total of 7 cycles is required.

  

Figure: Feedback shift register circuit for encoding rate $\frac{7}{4}$ block code.