Transmission performance of telephone networks containing p.c.m. links

Public telecommunication networks are required to transmit various types of information, and any new method of transmission must provide a quality at least as good as the present standards. This paper describes the basis on which the fundamental characteristics were chosen for the new 24-channel short-distance p.c.m. system now being introduced by the British Post Office. Representative overall telephone connections were selected and assessments made of their speech-transmission performance when certain links in them are provided by p.c.m. systems having various sets of fundamental characteristics. Special attention was given to requirements concerning load capacity, quantising distortion (including performance at very low signal levels) and circuit noise. Acceptable values for each quantity were determined and certain compression characteristics examined in the light of these requirements The final choice of fundamental characteristics was based mainly on consideration of speech, and the new systems should provide a suitable standard for any conditions of use for telephony that can be envisaged at present. It is believed that other services that the systems may be required to transmit impose less onerous conditions than those for speech, and so it is expected that they will also satisfy requirements for these other services.