Modulate/demodulate audio file as FSK signal mixed with music signal

1. How FSK modulation and demodulation is done?
2. What is the most commonly used demodulation technique for FSK?
3. What can be used as FSK modulator?

How FSK modulation and demodulation is done?

The FSK signal input is given to the two mixers with local oscillator circuits. These two are connected to two band pass filters. These combinations act as demodulators and the decision circuit chooses which output is more likely and selects it from any one of the detectors.

What is the most commonly used demodulation technique for FSK?

The most used demodulation technique is the one using a PLL circuit (fig. 2). The FSK signal across the PLL input takes two frequency values. The error voltage supplied by the phase comparator follows such variations, and so, it constitutes the NRZ binary representation (high and low level) of the FSK input signal.

What can be used as FSK modulator?

The Circuit given here illustrates how FSK Modulated Wave can be generated. It is build using IC555. A Square Pulses are given as input to represent bit 1 and bit 0, and as an output IC555 generates FSK Modulated Wave. To generate Square Pulses one more IC555 is used.