1. Field of the Invention
This invention relates to a method of and apparatus for producing hard copy displays of data signals, for example, analog physiological signals, and, in particular, plots of multi-valued functions. One example of a multi-valued function is a circle, the mathematical equation for which is X.sup.2 + Y.sup.2 = r.sup.2 where r is the radius of the circle. This relationship may also be stated as X = .+-. .sqroot.r.sup.2 - Y.sup.2. From this equation it is apparent that for each value of X there exists two values Y and vice versa. There are many, many other multi-valued functions. Curves of multi-valued functions, particularly when plotted on hard copy, are useful in the medical field; for example, such curves can indicate a relationship between heart beat signals derived from several different parts of the body. Such signals from three different planes in the body (referred to as the frontal, horizontal, and sagittal) are useful in the diagnosis of abnormal heart conditions.
2. Description of the Prior Art
Apparatus for providing single-channel or multi-channel hard copy displays of curves and/or records of the amplitude variations of periodically sampled analog data signals are known in the prior art. In one such known form of apparatus, the values of a plurality of data signals are plotted on a moving photosensitive record sheet by means of a cathode ray tube (CRT) operaating essentially in a single line scan mode. U.S. Pat. No. 3,605,109, assigned to the assignee of the present invention, shows a CRT system of this type. In that system, graphic displays are produced from periodically sampled analog data signals in a so-called connected sampling method the yields substantially continuous, and hence, readily discernible curves, notwithstanding the periodic nature of the sampled data from which the curves are produced. The traces are obtained by producing successive substantially parallel lines on a photosensitive record sheet that is arranged to move past the face of a normally blanked CRT operated in a line scan mode.
This prior art apparatus is generally satisfactory. However, it cannot generate a record of a multi-valued function from a series of stored sample data points.