Ear-candy is what my friend calls the upper frequency range and its resulting nuances in music. Often, when a stereo source is mixed to mono for use in a PA system, some ear-candy is lost from phase cancellation. The resulting sound can be dull and lifeless. One could simply boost treble to compensate -- but can we do better? We could instead measure the treble content that's thrown away in the mono mix and then dynamically boost a corresponding amount.
Here, a crossover splits the mono mix into low and high frequency bands. Separately, the HF difference sound (lost in the mono mix) is measured and a ratio taken with the mono HF. This adjusts the treble boost.
A divide block is needed to figure the ratio, but here the Sigma 100 (ADAU1701, etc.) divide block isn't up to the task -- it cannot handle divisors smaller than 0.0625 (see https://ez.analog.com/message/141869#141869). Instead, feedback around a multiplier provides both the divide function and a low-pass filter. Any lost ear-candy adds to the base gain of 1.0, causing a boost in the HF channel.
It's fascinating to view the Real-Time Display showing the boost -- and thus, just how much HF we have thrown away in the mono mix. With a mono podcast input, the display flat-lines at 1.0 -- nothing lost, no boost. Below are examples of stereo music inputs -- the left trace with Chuck Mangione's Feels So Good, the right trace with a dramatic movie soundtrack. In both cases, the aural effect is both subtle and welcome.