A setup to be used in a home theater or sound reproduction system for handling low frequency sound signals with a signal source providing several full bandwidth digital sound channels and at least one digital low frequency subwoofer channel includes a delay-and level control unit at least for each full bandwidth signal channel, which is equipped for a speaker not capable of reproducing low frequency sounds. A first summing stage creates the sum channel by summing the delay-and level controlled signal channels. A second summing stage adds a low frequency subwoofer channel to the sum channel in order to create a digital bass channel. A DA-conversion, amplifying, lowpass-filtering and volume control unit processes the digital bass channel in order to create an analog bass channel.

https://patents.google.com/patent/US6332026B1/en