Often audio amplifiers and equipment use a small-valued capacitor across the input(s) to present a virtual short-circuitto radio-frequency signals - this really helps prevent the unintended radio-receiver effect (the non-linear elements inthe amplifier circuit can inadvertantly act like an amplitude-modulation detector at radio frequencies).Even quite a small value capacitor like 100pF can severely attenuate radio frequencies (MHz and above) without doing muchat all at audio frequencies.Often the source of unwanted noise is from the cables (which here can be acting as radio aerials / antennas). Mainswiring often has sporadic noise spikes (clicks) when switches and thermostats are operated - main wiring is everywhere inbuildings and thus tends to inject such noise into any other wiring running near it.
Electronic noise is any fluctuation in voltages that you don't want.It could be from the operation of a chip, capacitors help smooth this.Here is one example of noise being generated and being suppressed.http://www.thebox.myzen.co.uk/Tutorial/De-coupling.htmlNoise can also be generated by thermal agitation of electrons, this is know as thermal noise. There are other mechanisms that generate noise but in digital electronics this is normally too small to be a problem.Noise can also be injected into a circuit through radiation when the circuit is acting unintentionally like a radio receiver.
but i failed to understand how the IC'S here are producing the varying current and here it is shown that the varying current is going to the ICA and thus the other current going to ICB and ICcare also changing due to changing current produced by ICA .