Pulse-Interval Modulation Applicable to Narrowband Transmission

Pulse-interval modulation (PIM) is reviewed considering its application to the voice band transmission of still picture signals in industrial TV systems. Although PIM seems to have scarcely been utilized in practice, some advantages as an analog pulse modulation can be expected in the case of narrowband transmission. As to the signal-to-noise ratio and the analog message bandwidth, comparisions with PPM and FM are made, which proves that PIM is suited for the TV signal transmission. The dominant defect of PIM is the matter concerning the errors introduced into the pulse spacing by the mutual interferences of bandlimited pulses. Assuming an ideal filter, the effects of bandlimiting are discussed. The differential gain characteristics, and the interference outputs owing to the unwanted lower sidebands of a pulse train are examined, which indicates the effectiveness of lowpass filtering of post-detection signals.