Transfer functions. PTSD flashbacks of Laplace transforms ensue. Another interesting bit is you can do the majority of signal processing stuff digitally within

Transfer functions. PTSD flashbacks of Laplace transforms ensue. Another interesting bit is you can do the majority of signal processing stuff digitally within an ADC with DSPs or in software if you have enough processing power. There are quantized equivalents of LP/HP/BP/BS filters and other transfer functions that can operate on a stream of values post-ADC. The advantage of filtering digitally is FFTs can create filters with no phase shift and infinite rolloff like an ideal filter, properties unattainable in passive analog electronic filters.