Recorders, Graphic

Abstract

Abstract Graphic recorders are essentially measuring instruments that produce in real‐time graphic representations of (biomedical) signals, in the form of a permanent document intended for visual inspection. Recorded data are thus fixed on a two‐dimensional (2D) medium that can simply be called “paper”. Real‐time paper recordings have the benefit of direct visual access to signal information, allow immediate examination (and re‐examination) of trends (as long strips of paper can be used), present better graphic quality than most screens and can be used as a document for scientific evidence. Two primary aspects describing the performance of the graphic recorder are the properties of the instrument as a recorder of information (measuring accuracy and the ability to display additional information) and the quality of graphics, implying the clearness of the tracings and the overall graphic layout (including identification of the curves by color difference) in relation to ergonomics in visual examination. Secondary aspects are, in general, ease of control of the apparatus, maintenance, costs and service life, and, specifically, input amplification and signal conditioning facilities, waveform monitoring, digital processing, data storage and communication possibilities. In general, recorders use a transducer that converts electrical signal information into a position on the paper. This can be achieved by moving the writing element to the specific position (analog transducers) or by activating the correct point in a large array of stationary, equally spaced writing elements (digital transducers). In most applications the graphic transducer, not the electronic signal conditioning hardware, is the crucial stage in the measuring chain. In the technique of analog transducing, there are essentially two methods: The direct method and the servomethod. Positioning of the writing device can be accomplished by a rotation act (corrected to a translation) or a translation act. The time axis is generated by pulling a strip of paper at constant velocity or in small identical increments. Translational pen servorecorders are used for slow varying signals (bandwidth 0–5 Hz typically), galvanometric pen recorders can handle faster signals (bandwidth 0–100 Hz). Thermal array recorders are digital recorders, which can record higher frequencies in real time with the aid of memory facilities and, evidently, from memory as well with an appropriate paper speed. Nowadays, the borderline between digital graphic recorders, digital oscilloscopes, data‐acquisition systems and PC based virtual measuring instruments becomes less clear. Measurements where immediate visualization of the tracings on a long strip of paper is not required and data are to be digitally analyzed and stored, can be performed by PC with virtual instrument software and printer; Nowadays the majority of data collection is accomplished using digital PC approaches.