to sound that goes into the mouth piece of the telephone handset. His acoustic coupler is known as the Weitbrecht Modem.

The Weitbrecht Modem inspired other engineers to develop other modems to work with 8-bit ASCII terminals at a faster rate. Such modems or couplers were developed around 1966 by John van Geen at the Stanford Research Institute (now SRI International), that mimicked handset operations. An early commercial model was built by Livermore Data