Narrow-bandwidth television

Narrow-bandwidth television (NBTV) is a type of television designed to fit into a low-bandwidth channel, in the extreme case using amateur radio voice frequency channels that only range up to a few kilohertz (though channels ranging into a few tens of kilohertz and beyond can also be used). This is in contrast to broadcast TV systems that use a channel about six to eight megahertz wide.

There are two ways to make this work: reduce the scan rate, or reduce the image size. When the scan rate is reduced, this is referred to as slow-scan TV. With the latter type, the number of lines in an image may be reduced to just a few dozen. The earliest mechanical television systems often used narrow channels for sending moving images. Often, the images were only a few dozen lines in size. However, most narrow-bandwidth TV nowadays uses computers and other electronic systems.

...
Wikipedia