Sometimes circuits need to be isolated from each other for safety reasons, noise reduction or even to simplify a circuit function. An early classic example is the photoresistor that Gibson and Fender used to add a tremolo effect in early guitar amplifiers. This photoresistor was an early form of an optical coupler (optocoupler) that varied the resistance of a cadmium sulphide (CdS) cell using a light source to modulate the amplifier bias to create the tremolo effect.