Hey Aladan!
There are a few ways to do a black/white level clipper circuit. This method, of using two sets of rectifier circuits in series was one we came up with early on.
First off, this type of rectifier circuit (wideband opamp with schottky diodes) is very accurate all the way down to 0V. The point of a precision rectifier circuit is to split the signal into two – negative portions and positive portions.
In the first opamp, the input signal is biased up by CLIPREF and inverted. What this does is set the input signal’s WHITE LEVEL (0.714V) to 0V. If white level were the top of its skull, it’s like the signal is now standing on its head!
Now with the headstand, if we only take the positive output from the first rectifier, we get the input signal clipped off at 0V (signals above 0V are currently below white). So this is our white level clipper, but the output is inverted and biased so that white is at 0V.
The second rectifier inverts the signal again and then biases up by CLIPVREF a second time. This places white back where it’s supposed to be (0.714V) and restores 0V = BLACK LEVEL. If we take the positive portion from the second rectifier, we now have a signal where all voltages below 0V (BLACK) have been clipped off (black level clipping.)
Schottky diodes and a high bandwidth opamp are important for this circuit or else you can get ugly distortions and poor recovery near the clipping thresholds.
More reading:
1N5711 RF schottky diode:
https://www.st.com/en/diodes-and-rectifiers/1n5711.html
LM6172 wideband opamp:
http://www.ti.com/lit/ds/symlink/lm6172.pdf
On voltage references (a buffered vref is used for the CLIPREF):