Ua ntej tshaj plaws, cov khoom siv PCM thiab cov khoom siv PDH yog cov khoom sib txawv kiag li.PCM yog cov cuab yeej siv kev pabcuam kev sib koom ua ke, thiab cov khoom siv PDH yog cov khoom siv kho qhov muag.

Cov teeb liab digital yog tsim los ntawm kev ua piv txwv, quantizing thiab encoding qhov tsis tu ncua hloov analog teeb liab, uas yog hu ua PCM (pulse code modulation), uas yog, mem tes code modulation.Qhov hom hluav taws xob digital teeb liab hu ua digital baseband teeb liab, uas yog generated. los ntawm PCM hluav taws xob davhlau ya nyob twg.Cov tshuab xa xov digital tam sim no txhua tus siv pulse-code modulation (Pulse-code modulation) system.PCM tsis yog siv los xa cov ntaub ntawv hauv computer, tab sis kom muaj lub cev kab ntawm cov keyboards es tsis txhob xa xov tooj teeb liab xwb.

PDH optical kis khoom siv, nyob rau hauv cov kev sib txuas lus digital, cov teeb liab kis tau yog tag nrho cov digitized pulse sequences.Thaum cov kwj ntawm cov teeb liab digital kis tau los ntawm cov khoom siv hloov pauv digital, lawv cov nqi yuav tsum ua kom zoo ib yam kom ntseeg tau qhov tseeb ntawm cov ntaub ntawv xa mus.Qhov no hu ua "synchronization."Hauv kev sib kis digital, muaj ob qhov kev sib kis digital, ib qho yog hu ua "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), abbreviated li PDH;lwm qhov yog hu ua "Synchronous Digital Hierarchy" (Synchronous Digital Hierarchy), abbreviated as SDH.

Nrog rau txoj kev loj hlob sai ntawm kev sib txuas lus digital, muaj tsawg dua thiab tsawg dua kev xa mus ncaj qha, thiab feem ntau cov kev sib txuas lus digital yuav tsum tau hloov.Yog li, PDH series tsis tuaj yeem ua tau raws li cov kev xav tau ntawm kev lag luam kev sib txuas lus niaj hnub no thiab cov kev xav tau ntawm kev tswj hwm kev sib txuas lus niaj hnub..SDH yog ib qho kev sib kis uas tau tshwm sim los ua kom tau raws li qhov xav tau tshiab no.