A solid state switched attenuator
Remember the great journal Wireless World? That's where I published my first articles when I started to write audio stuff. In May 1980 they published my design for a Solid State Switched Attenuator. The design was for a 60dB range auydio attenuator where the attenuation was set with a 6-dit data word. The idea was to automate it but the initial design was with a left/right up/down 4-switch array, and it never developed beyond that.
But it documents a lot of issues reletaed to CMOS switches used in audio, and how to minimize their impact on noise and distortion. Those issues are still valid almost 40 years later, because basic solid state physics is still the same!