f we apply variable voltage at control input (pin 5) of IC555 in astable mode then the width of output pulse will change accordingly. When we apply low frequency audio signal to the control input, internally the control voltage will change the reference voltage of comparator so charging time of capacitor changes every time and so width of o/p pulse also changes. So here as shown in figure the audio signal is applied at the control input through positive clamper circuit. The values of R1, R2 and C1 are chosen to generate pulse train of around 64 KHz. The width of o/p pulse will change in accordance with the amplitude of audio signal as shown in figure in Circuit Diagram 1tab.