← Back to Concept Indexbaudot-code
A five-impulse telegraph code using two symbols (current pulse presence or absence) to represent letters and control functions.
1 chapter across 1 book
chapter III INFORMATION FLOW OVER DISCRETE CHANNELS HAVING devised a measuring scheme for handling the basic problem of communications engineering, namely, the transmission of informa- tion over any given channel, it is time to proceed to the problem itself This consists simply of evaluating the operating efficiency of a com- munications channel, that is, matching its actual performance against its optimum potential. A natural measure thereof is the ratio of the actual rate of flow of information to its ultimate capacity. But what is a channel over which information flows and what is its capacity ? A channel is any physical medium such as a wire, a cable, a radio or television link, or magnetic tape, whereby we may either transmit information or store it as in a memory device like a tape. The trans- mission and/or storage takes place by a code of symbols which may be pulses of current of varying duration as in telegraphy, light flashes as in navigation, or radio signals of different intensity, polarity, and so forth. Thus, in teletype, signals are transmitted by a code of two symbols made out of the presence or absence of a current pulse for a given duration which is the same for both. These two symbols enable a modern printing telegraph system to transmit any given English text by means of what is commonly called the Baudot code. In this system five impulses are transmitted for every letter, any one of which may be either a current pulse or a gap. That is, in each of the five impulses the circuit is either closed (current present) or open (current absent). With such a code it is possible to obtain 2 x 2 x 2x2x2 = 2^ = 32 different permutations, of which twenty-six are assigned to letters of the alphabet and five to other functions such as space, figure shift, or letter shift, leaving one spare. The five impulses making up the code are sent to the line successively by means of a rotating distributor or commutator and are distributed at the 22This chapter addresses the evaluation of communication channel efficiency by comparing actual information flow rates to the channel's theoretical capacity. It explains the concept of a communication channel as a physical medium transmitting coded symbols, illustrating this with examples like the Baudot code in teletype systems and Morse code in telegraphy. The chapter further explores the mathematical modeling of information transmission over discrete channels, including cases with symbols of equal and unequal durations, and introduces finite difference equations as a tool to handle complexities arising from unequal symbol durations.