TI , ▁but ▁many ▁systems ▁can ▁be ▁mode led ▁as ▁L TI ▁for ▁simplicity ▁in ▁determ ining ▁what ▁their ▁output ▁will ▁be . ▁ ▁All ▁systems ▁have ▁some ▁dependence ▁on ▁things ▁like ▁temperature , ▁signal ▁level ▁or ▁other ▁factors ▁that ▁cause ▁them ▁to ▁be ▁non - linear ▁or ▁non - time - in variant , ▁but ▁most ▁are ▁stable ▁enough ▁to ▁model ▁as ▁L TI . ▁ ▁Linear ity ▁and ▁time - in var iance ▁are ▁important ▁because ▁they ▁are ▁the ▁only ▁types ▁of ▁systems ▁that ▁can ▁be ▁easily ▁solved ▁using ▁conventional ▁analog ▁signal ▁processing ▁methods . ▁ ▁Once ▁a ▁system ▁becomes ▁non - linear ▁or ▁non - time - in variant , ▁it ▁becomes ▁a ▁non - linear ▁differential ▁equations ▁problem , ▁and ▁there ▁are ▁very ▁few ▁of ▁those ▁that ▁can ▁actually ▁be ▁solved . ▁ ▁( H ay kin ▁& ▁Van ▁Ve en ▁ 2 0 0 3 ) ▁ ▁See ▁also ▁ ▁Sign al ▁processing ▁Digital ▁signal ▁processing ▁Sign al ▁Anal og ▁electron ics ▁Anal og ▁recording ▁vs . ▁digital ▁recording ▁Electron ics ▁Elect rical ▁engineering ▁Cap ac itor ▁Ind u ctor ▁Res istor ▁Trans istor ▁ ▁circ uits ▁ ▁RC ▁circuit ▁L C ▁circuit ▁R LC ▁circuit ▁Series ▁and ▁parallel ▁circ uits ▁ ▁filters ▁ ▁Band - pass ▁filter ▁Band - stop ▁filter ▁High - pass ▁filter ▁Low - pass ▁filter ▁ ▁References ▁▁ ▁Hay kin , ▁Simon , ▁and ▁Barry ▁Van ▁Ve en . ▁ ▁Sign als ▁and ▁Systems . ▁▁ 2 nd ▁ed . ▁ ▁Hob oken