306 THE LANGUAGE AND CONCEPTS OF CONTROL 



above some critical minimum strength,*** otherwise the nerve will not fire. 

 That the threshold is not really as critical as is often claimed, and that the 

 spike, or "wave of negativity," modifies its shape under certain circum- 

 stances, are useful facts to know and are thought by some physiologists to 

 be more important that the spike itself. The main point for the moment, 

 however, is that the passage of a stimulus is a binary process, to a first ap- 

 proximation always the same. Only the pulse-repetition frequency (pulses 

 per second) can change; this is frequency modulation. 



For example, in the case of transmission of a signal from the pressure- 

 sensing device which reports blood pressure, the nerve encodes the informa- 

 tion as a frequency: the higher the pressure the greater the number of pulses 

 per second (e.g., 125 pulses per sec for high pressure, 70 for low). There is 

 an inherent accuracy in the counting, or digital, method of transmitting in- 

 formation — more so than in the decimal-expansion method. The accuracy 

 comes from repetition, or redundancy. 



The Digital Computer 



Information can be fed into a machine in either of two ways: intermit- 

 tently or continuously. If done intermittently, it takes the form of pulses of 

 energy. The number of pulses then becomes the important thing, for in the 

 number is contained the information in question. Thus five pulses means 

 one thing, three another, and so on. (The Morse code was an early example 

 of this idea.) Since number is important, counting and recording of number 

 are necessary. Therefore, the performing of operations on the information 

 becomes simply a matter of arithmetic, nothing more. Since it is numbers, 

 or digits with which the arithmetic is done, a machine which processes in- 

 formation in the form of numbers is known as a digital computer. An adding 

 machine is a primitive example; IBM's "650" has intermediate complexity; 

 and IBM's 7090 (see Figure 11-5) is a 20,000-component, complicated ex- 

 ample. It has 32,000 words of high-speed memory and can add two 10-digit 

 decimal numbers in 4.5 microseconds — facts to be compared with 2000 

 words for the 650 and an addition time of 800 microseconds. 



In computation with digits we normally use the decimal system, with 

 units often. This system was chosen quite arbitrarily by our ancestors dur- 

 ing a process of arithmetical evolution in which they counted in twos 

 (hands), tens (fingers), twenties (fingers and toes), etc. Other systems 

 could have been chosen equally well. For instance the binary system (units 

 of two), it is now realized, more closely represents many naturally occurring 

 phenomena than does the decimal system. Thus only two digits are needed 



***That is, a minimum energy must pass through the nerve membrane — most simply stated: 

 a current, at some voltage, for some length of time (amps x volts x sec = joules). 



