AHICDM is a simple digital mode that transmits bits by varying the amount of silence between pulses. It is a narrowband mode that transmits slowly.

I only came across this modulation while looking at the Recently changed pages list of sigidwiki. It looked easy to implement, and didn’t have much written about it, so I decided to play around with the provided audio example.

I have never seen this modulation used in the wild, and the only mention I have seen of it is in the Signal Identification Wiki. This page is written by analyzing the audio recording provided on the wiki page.