Coupling Capacitors; Capacitors As Audio Filters - FIGnition FUZE Hardware Reference Manual

FIGnition uses this feature in the large capacitor C1 to provide a more constant power
supply to the board: a large capacitor provides some temporary electrical energy to cover
up any of the expected ebb and flow of power from the USB cable.
C.3 De-coupling Capacitors
FIGnition uses smaller capacitors in a similar way to smooth power to its high frequency
integrated circuits. The use of electrical power by a circuit's components will cause tran-
sient power surges and losses in other parts of the circuit. Small capacitors placed next to
individual chips (here we use 10nF capacitors) provide tiny charge reservoirs which
smooth out these transients, effectively de-coupling the chip's power source from the rest
of the circuit.
C.4 Capacitors As Audio Filters
The ability capacitors have for smoothing out transients is the same as filtering out the
amplitudes of high frequencies: they diminish as the signal becomes smoothed out.
Using different resistor values and capacitor values allows us to change the frequency at
which this happens. The frequency will be:
Audio circuits assume an inline resistance of about 75Ω so FIGnition uses a 100nF polyes-
ter capacitor to filter out frequencies above 20KHz since 1/(2π * 75 * 100*10 -9 =21.2KHz).
For audio output this is useful because the audio pin on a FIGnition outputs square waves,
which have nasty high-frequency components which audio equipment won't like.
On the audio input side it means that noise above 20KHz is filtered out which helps to
make the audio loader reliable.