I am experimenting with a 500 Kc/s carrier system with audio modulation. For various reasons I found it convenient to modulate the frequency of the carrier by controlling the duration of each cycle.
After some consideration I realised that this cannot correctly be called frequency modulation because it is the reciprocal of the frequency which is proportional to the modulation waveform, not the frequency itself. At high carrier frequencies with low deviation this may not matter, because the error due between the deviation and its reciprocal is very small, but the deviation on this system is more than 20% of the carrier frequency.
I then considered that "pulse duration modulation" might be a more appropriate name; but this is misleading because the system is not using a pulsed carrier, the information is contained in the duration between zero-axis crossings of a sinewave.
The term "time modulation" is probably the least inaccurate, but it is vague and doesn't really explain what is going on (it also suggest the surreal possibility that time can somehow be modulated). Can anyone come up with a better suggestion?