new = 0;
  if( PHASE_A )
    new = 3;
  if( PHASE_B )
    new ^= 1;                   // convert gray to binary
  diff = last - new;                // difference last - new
  if( diff & 1 ){               // bit 0 = value (1)
    last = new;                 // store new as next last
    enc_delta += (diff & 2) - 1;        // bit 1 = direction (+/-)
  }