This problem can occur when using a laptop onboard sound card, or USB External Audio Adapter. Mobile DJ's may use a laptop computer to provide digital audio to their mixers and amplifiers. Often there is a buzz or hum that is related to activity on the laptop. The harmonic noise can change when the screen brightness is adjusted, when the laptop is switched from AC to DC power, or general activity on the laptop such as disk access. These harmonics are a symptom of the laptop's internal electronics.