What it does
Stereo bass boost implemented using multirate recursive filters.
Another example of the use of multirate signal processing to achieve efficient filtering over wide frequency ranges. In this example, successive downsampling is used to reduce the sample rate by a factor of 16. Then, a low-pass IIR filter is used to shape the bass response before up-sampling again. This enables precise control of the bass response since the low-pass filter is running at 5kHz (instead of 44kHz) and the filter can be rather elaborate without excessive computational expense. Try experimenting with the parameters of the IIR low-pass block to appreciate the effect of different filter designs.
(Note: the 2-pole low-pass filters are used to further suppress aliasing/imaging before/after the down/up samplers)