Some mono‑maker plug‑ins, such as Tone Projects’ Basslane Pro pictured here, allow you to specify the steepness of the crossover filtering.
I have two plug‑ins with a mono‑making function that makes all frequencies below an adjustable threshold mono. I’d surmise that how a plug‑in actually makes the transition from stereo to mono below a selected threshold matters, but neither of these plug‑ins describe in their manual how the transition is made. In other words, if it’s set to 100Hz would a 99Hz signal be mono and 101Hz stereo, or is there a range of transition that sees stereo signals gradually taper into mono around the threshold? If there’s a transition range, how wide would it typically be? And does anyone know of a mono‑making plug‑in that offers some control over the shape of the transition?
SOS Forum post
SOS Technical Editor Hugh Robjohns replies: There is a transition range, and the mono‑making can be achieved in a couple of different ways. First, you can convert the L‑R signal to Mid‑Sides, then high‑pass filter the Sides at 100Hz and convert back to left‑right. Second, you could introduce low‑pass filtered (at 100Hz) crosstalk, left‑to‑right and right‑to‑left.
...no filter, not even a digital one, could pass 100Hz at full level and 101Hz fully attenuated!
In both cases the filtering typically involves second‑order filters (12dB/octave slopes). So if the crosstalk‑style low‑pass system with second‑order filter is used, with the corner frequency at 100Hz, the signal will be ‑15dB at 200Hz, ‑27dB at 400Hz, and so on. I’ve seen some 6dB/octave mono‑making filters too, and some plug‑ins allow steeper filters to be used — but no filter, not even a digital one, could pass 100Hz at full level and 101Hz fully attenuated!