METHODS OF RECORDING 



signal frequency may be made as high as required. The disadvantage of 

 the system lies in the finite number of styli and hence number of possible 

 'quantized' values the record may have. 



The bulk of electrobiological recording above 100 cycles, and much 

 that occurs at lower frequencies, is performed by photographing the face of 



Signal 

 in 



► — 



Varying frequency 

 Oscillator 



Filterl 



Filter 2 



Filter 3 



Filter U 



Filters 



— etc. 

 Figure 40.1 



Battery of 

 styli 



a cathode ray tube. The method is accurate, the record can be read with 

 precision, and the frequency range is practically unlimited. A disadvantage 

 is that the record is not at once available for inspection, though a machine 

 has been described which presents a developed photographic record only 

 one second after exposure^-^. 35 mm film or paper is usual : the former is 

 generally perforated for sprocket engagement, which restricts somewhat 

 the useful recording area but ensures a positive drive, while paper is often 

 unperforated and relies for pulling through the camera on friction rollers, 

 which sometimes slip. Two ways of photographing the trace are possible, 

 which might be termed the electrical time-base, and the mechanical time- 

 base, methods. 



The electrical time-base method — This is suitable for recording phenomena 

 which are completed within, say, 500 msec of a definite stimulus. The 

 stimulus is delivered regularly and triggers an electronic time-base generator 

 such that the spot is swept across the cathode ray tube face, the response 

 being applied in a direction at right angles in the usual manner. Film can 

 be drawn through the camera at a constant speed in a direction also at 

 right angles to the time-base, so that the finished record might have the 

 appearance of Figure 40.2. 



A difficulty arises when the sweep time becomes comparable with the 

 interval between stimuli, or when a 'continuous-running' time-base is used, 

 for the record then assumes the somewhat unsatisfactory appearance of 

 Figure 40.3. This is because, on raising the stimulus rate, the film speed 

 has to be increased to prevent crowding of successive sweeps on the record. 

 When this is done the film speed becomes comparable with the cathode ray 

 tube spot sweep speed, producing the type of distortion shown. It is possible 



639 



