 Zener diodes can also be used in place of a voltage source and potentiometer, hence setting the offset at the Zener voltage.Clamping circuits were common in analog television receivers.These sets have a DC restorer circuit, which returns the voltage of the video signal during the 'back porch' of the line blanking (retrace) period to 0 V. Low frequency interference, especially power line hum, induced onto the signal spoils the rendering of the image, and in extreme cases causes the set to lose synchronization.