In our daily life, Radio Frequency (RF) signals from mobile phone, television, radio, and many other electrical appliances will also generate “images” that lead to further degradation in jitter performance in a digital system and Signal-Noise-Ratio in an analogue system. The diagram below shows the sampling images found in a typical audio system.
As we will find out in many audio systems, it really does not matter where the images are located, even if they are in the ultrasonic range and inaudible, crystal clear sound field will not be possible as long as there is unwanted frequency existed in the system. In order to get a near-perfect reconstruction of the original signal, it is necessary to annihilate any unwanted frequency and stop them from spreading out to the rest of the audio system.
It should be obvious the purpose of removing the unwanted frequency in a typical amplifier is to ensure that the analog circuitry will not become non-linear due to the inaudible frequency from both D/A conversion (From stereo system) and RF sources (External interference).