Abbreviation for Concurrent Multi-tone Hell.

An amateur radio facsimile mode, which is a variant of Hellschreiber. Instead of the original 7 pixels per column, the text is transmitted in a grid 5 pixels high. Whereas the original Hellschreiber transmit all information on the same frequency, and sends each pixel sequentially from low to high, C-MT Hell sends each pixel with different frequencies, ranging from a low frequency for the lowest pixel, to a high frequency for the highest pixel. All pixels in one row are transmitted simultaneously.

Although C-MT Hell is sent and received with special software, it has the amusing property of being able to be read directly from a standard waterfall display.