I get what PWM is and how it can be done using a 555 astable circuit. However I feel I don't completely get what PPM is how its circuit works. I understand the VCO circuit, but I don't see how it can be used as PPM modulator simply by varying UTP and LTP using the pin5. I don't see much difference between PWM and PPM in below circuits. Both seem to produce varying width pulses. Shouldn't the pulse width be fixed in PPM and just the position change ? Could somebody please help me understand how the VCO in second figure acts as a PPM modulator ?