1,061,602. Data transmission. INTERNATIONAL BUSINESS MACHINES CORPORATION. April 30, 1965 [May 5, 1964], No. 18241/65. Heading H4P. The outputs of the various circuits in a demodulator of, for example, a frequency shift signal subject to frequency or amplitude wander are checked to provide error signals when prescribed conditions are met. The received signal A fed to the demodulator, Fig. 4 (not shown), is transformed in a pulse forming circuit, waveform D. This is fed to a clock retrieval circuit whose output E coacts with D to give RZ signals F. Pulses E pass to a clock sync. circuit to appear as C, C and F being used to form the output demodulated data G. To provide an indication if the frequency of the received signals increases excessively, the pulses C, D, E and F are passed to individual circuits each comprising an AND gate to which the pulses are fed both direct and via an inverter series connected to a flip-flop. The unstable period of the flip-flop determines the maximum permitted frequency. As a further check, two AND gates are fed direct by E and F, respectively and, via a delay and a flip-flop, by C and D. The gate pulses if C and D, or E and F, are out of phase. In another circuit, waveform C is filtered, giving B. This is rectified and filtered, and fed to a threshold unit which indicates when the D.C. level is too high. The demodulated signals pass to a further checking circuit, in which they are remodulated and compared in phase and amplitude with the received modulated signals. Outputs are produced if the amplitudes differ excessively or if a noise pulse is present in the received signal.