Voice frequency tone signalling
C.L. Roualt · IRE Transactions on Vehicular Communications · 1952
In the Radio Communication field, the need for selective signalling devices has been recognized for the past several years, and many devices have been developed to satisfy the need. The devices which are described in this paper are the result of market analysis, experiment, and extensive field testing to make a system of voice-frequency, tone-signalling apparatus which fills substantially the need.