What is the most common filter, particularly audio-related ones, being used by majority of appliances currently? Is it RC Filters? Are Switched Capacitor Filters increasing in popularity and are they being commonly used by products currently?
A common audio filter is a tone control. It is simply a resistor and a capacitor for bass and another pair for treble. A single resistor and single capacitor produce a gradual slope.
I used a switched-capacitor lowpass filter IC that has a very steep slope to reduce the distortion of a stepped sine-wave (made from square-waves) to almost nothing.
I do not know of a product that uses one.
A good application for a switched capacitor filter is one that has to be tuned to work at various frequencies. By changing the switching time the response can be varied, and that is easier than trying to vary the actual capacitance in many applications.