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7

The Transmission of Information

the ear, converted into nervous impulses, and processed by the brain of the listener.

According to the nature of the message, muscles may then have been stimulated in

order to make the listener run outside and secure objects from being blown away,

or whatever. Perhaps, during the broadcast, some words may have been rendered

unintelligible by unwanted interference (noise). It should also be mentioned that

the whole process did not, of course, happen spontaneously, but the satellite and

attendant infrastructure had previously been launched by the meteorologist with the

specific purpose of providing images useful in weather forecasting. From this little

anecdote, we may gather that the transmission of information involves coding and

decoding (i.e., transducing) messages, that transmission channels are highly varied

physically, and that noise may degrade the information received.

Inside the living cell, it may be perceived that similar processes are operating.

Sensors on the cell surface register a new carbon source, more abundant than the

one on which the bacterium has been feeding, a conformational change in the sensor

protein activates its enzymatic (phosphorylation) capability, ¹ some proteins in its

vicinity are phosphorylated, in consequence change conformation, and then bind to

the promoter site for the gene of an enzyme able to metabolize the new food source.

Messenger RNA is synthesized, templating the synthesis of the enzyme, which may

be modified after translation. The protein folds to adopt a meaningful, enzymati-

cally active structure and begins to metabolize the new food perfusing into the cell.

Concomitant changes may result in the bacterium adopting a different shape—its

phenotype changes. ²

In very general terms, semiotics is the name given to the study of signals used

for communication. In the previous chapter, the issues of the accuracy of signal

transmission, the syntactical constraints reducing the variety of possible signals, the

meaning of the signals (semantics), and their ultimate effect were broached. In this

chapter, we shall be mainly concerned with the technical question of transmission

accuracy, although we shall see that syntactical constraints play an important rôle in

considering channel capacity. We noted at the beginning of Chap. 6 that information

theory has traditionally focused on the processes of transmission. In classical infor-

mation theory, as exemplified by the work of Hartley (1928) and especially Shannon

(1948), the main problem addressed is the capacity of a communication channel for

error-free transmission. This problem was highly relevant to telegraph and telephone

companies, but they were not in the least concerned with the nature of the messages

being sent over their networks.

Some features involved in communication are shown in Fig. 7.1. There will always

be a source (emitter), channel (transmission line), and sink (receiver), and encoding

is necessary even in the simplest cases: For example, a man may say a sentence to a

messenger who then runs off and repeats the message to the addressee, but of course

to be able to do that he had to remember the sentence, which involved encoding

the words as patterns of neural firing. Even if one simply speaks to an interlocutor

1 Section 18.7.

2 These processes are considered in more detail in Chap. 14.