1](2 3)(4 5)(6 7)(8 9)(10 11)(12 CLK 13)(14 15)(16 17)(18 19)(20 21)(22 23)(24 25)(26 27)(28 29)(30 31)(32 33)(34 FS 35)(36 37)(38 DIN 39)(40 DOUT - PCM - Pulse-code Modulation - PCM (Pulse-code Modulation) is a digital representation of sampled analog. On the Raspberry Pi it's a form of digital audio output which can be understood by a DAC for high quality sound.