Circuit to limit negative (falling) slew rate only

Question

I am in search for a way to only limit the negative slew rate while leaving rising parts of the signal untouched. The signal will be anything from Subaudio till 20kHz, with potentially arbitrary waveforms and the amount of slew should be changable (via potentiometer, control voltage or current).

I found multiple ways of slewing both rising and falling edges, but nothing to slew only falling edges. I thought about some way to do it with positive and negative peak detectors and so on, but this gets quickly too complicated and to big for my intended applications (mostly a ±12V Audio environment).

The resulting slew can be linear or logarithmic, it doesn't really matter, for me it is more about getting unstuck with this topic.

The added slew is displayed in red (with different fall rates), the original waveform is the square wave.

Answer 1

Something like this, perhaps? The diodes effectively give you a different R value for different input edges in this RC lowpass filter.

^{simulate this circuit – Schematic created using CircuitLab}