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The digital evolution in the HDTV industry is causing legacy analog audio technology to migrate to increasingly powerful digital signal processors (DSPs). This is leading to the development of more sophisticated audio subsystems that feature increased I/O multiplexing capabilities and a plethora of audio algorithms.
Consequently, designers face the challenge of cost effectively managing the audio subsystem's growing complexity while ensuring that the subsystem is free of noise artifacts, such as pops and clicks. This often-overlooked aspect of audio deserves attention, as unwanted noise artifacts considerably diminish the HDTV experience for the end-user.
Ensuring a pop and click free audio signal chain requires careful design on behalf of both the hardware engineer and software engineer. This article discusses how hardware system and software functionality issues cause pops and clicks and how engineers can prevent them.
Typical hardware system issues include managing the power-up and power-down of the audio subsystem, and muting quickly and smoothly on a system error. Typical software functionality issues include switching audio sources, enabling/disabling audio effects, muting, and adjusting volume/balance controls.
Understanding pops and clicks
Pops and clicks are a direct consequence of undesirable currents flowing through a loudspeaker. The main property of these undesirable currents is that they have a discontinuity - or sharp change in current - with respect to time. Understanding why a discontinuity causes a pop or click requires a brief examination of the electromechanical structure of a loudspeaker.
The most common type of loudspeaker consists of a fixed permanent magnet and an electromagnet attached to a diaphragm that is free to move in a supporting structure. The electromagnet moves when a current flows through it, forcing the attached diaphragm to move, resulting in sound. In an ideal loudspeaker, the displacement of the diaphragm is proportional to the current flowing through it.
It is worth noting that a time varying current results in a moving diaphragm, while a constant current results in a diaphragm that is displaced, but not moving. A discontinuity in the current causes a sharp, sudden movement of the diaphragm, giving rise to a pop or click. Generally, clicks are low amplitude events with high frequency content, while pops are higher amplitude events with a wider frequency spectrum. Preventing pops and clicks from appearing in the audio subsystem essentially boils down to ensuring that no discontinuity arises in the current flowing through the loudspeaker.
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