Apparatus for providing a constant density trace of ECG analog signals

Abstract

Apparatus for providing a constant density trace of an analog signal which utilizes a thermal stylus is disclosed. The minimum rise and fall times of the analog signal are of the same order of magnitude as the thermal time constant of the thermal stylus. The thermal stylus is typically controlled by a galvanometer type movement which is responsive to the variations in an analog signal. In a preferred approach, the analog signals are those obtained from electrodes connected to a human body. The thermal stylus contains a heat generating element which, in the usual situations, heats up in response to a fixed heating voltage. In the present situation, the heating element is responsive to the absolute value of the rate of change of amplitude of the analog signal. Because it is desired to provide a constant density trace in an environment where the signal rise and fall times are marginally longer than the thermal time constant of the thermal stylus, the high rate of change of signal must be anticipated to ensure that the stylus is already at the desired temperature when the analog signal from the stylus driver deflects the stylus in the usual manner. This is achieved by providing means for introducing a delay which is a direct function of the thermal time constant of the thermal stylus into the deflection circuit. In this manner, then, the stylus is pre-heated to a desired temperature and gaps in the trace or over dark portions of the trace are avoided.

Dependencies are inferred from the claim text.