This conventional 4 channel circuit with the 4017 and 555 was found in the November 1991 Radio Electronics magazine. The potentiometer can be 47k and it controls the speed of the effect.
Found in a 1967 Electronic Design, this transistorized circuit compresses and then expands a pulse....
This squelch is activated by its own noise, acting automatically. It is used in communications...
This is the battery-powered version of the previous circuit. The circuit is from the same Radio...