A sub-ether transmission is a signal that is broadcast through time and space.

In 1991, Doctor Emmett Brown used a sub-ether transmission to broadcast video and audio onto a television set mounted on the dashboard of the 8-passenger DeLorean time machine. He also used this type of transmission to control the time machine via remote control.

In 2015, Doctor Emmett Brown used this type of transmission to rewire a hovercam so he could broadcast his message back to the time travel volunteers at the Institute of Future Technology in 1991.

Doc's remote controlled mail delivery time machine likely used this type of transmission as well, since it was able to send a transmission across several decades to locations thousands of miles apart.