Abstract: In the particular embodiment of the invention described in the specification, a frame slip detector for a detecting frame slip in a 24 channel PCM transmission system using an 8 KHz frame rate supplies a 1 KHz test tone to one channel of the system. A phase shift detector detects phase shifts in the same channel and supplies them to a frame slip indicator which responds to phase shift signals within the range from 40 degrees to 50 degrees which recur at periodic intervals of at least one second and supplies corresponding indications of frame slip for each phase shift detected.

Abstract: The adaptive and predictive capabilities of Adaptive Differential Pulse Code Modulation (ADPCM) equipment enable a telecommunication system to maintain acceptable signal/noise levels in voice transmission while utilizing a significantly lower encoding bit rate than that of conventional Pulse Code Modulation (PCM). ADPCM, however, has a deleterious effect on high-speed voiceband data transmission, yet due to its adaptive capabilities cannot readily be identified or evaluated by means of conventional ANSI/IEEE standard test signals and methods. The procedure of the present invention enables such identification and evaluation by imposing upon an ADPCM system a multiple-tone test signal which spans the voiceband and has amplitude characteristics similar to white noise. This signal thereby effectively overloads the adaptive and predictive capabilities of the system and causes the generation of a notably high level of quantizing noise.