MIDI uses a current loop (mark = 0 mA, space = 5 mA). Two pins are needed to drive the required current through the receiver's IR LED. The three resistors (two in the sender, one in the receiver) are needed to limit the current to 5 mA; the resistors are distributed this way to make the connectors short-circuit tolerant.

The actual voltages at pins 4 and 5 are not really important. (However, some cheap interfaces omit the optoisolator and just measure the voltage at pin 5; in that case, pin 4 is not needed.)

what's use of pin 2 ? shield... security if midi cable is broken.

current loop goes to pin 5 to 4 ?

but on this link, are there are a mistake ? because diode stop the current to pin 4 :

https://cdn.sparkfun.com/assets/learn_tutorials/4/0/8/signaling-one.png

thanks

The shield (connected to pin 2 at the sender) blocks electrical noise.

The current goes from pin 4 to pin 5. To transmit a 0 bit, the sender pulls pin 5 low. To transmit a 1 bit, the sender pulls pin 5 high, or leaves it open.