You guessed right! It’s very much an optical circuit. The dynamics of mixed music and speech on TV is highly complex and extremely difficult to apply control to. My circuitry combines some active filtering (which other audio manufacturers try) together with some compression. The result, happily, is that neither is very noticeable, but the dialogue is clearer! One would think that digital control would be easier and better, but there’s more to it than meets the eye (or ear).