The first of our terms, mediators, describes a set of physical agencies that work together to enable communication. Mediators are physical structures that enable, intervene, and control the flow of messages between communicators. These mediators include conventional channels of communication such as the light we see by, the air our face to face verbal interaction reverberates through, the senses we touch and taste with, along with paper, electrical signals, radio waves, light impulses and other transmission channels. These mediators also include the storage media that allow messages to persist, the switching equipment that allows messages to reach the correct destination, mechanisms for translating signals between channels, sources of "noise", and the people who sort, edit, print, deliver, proofread and otherwise mediate various forms of communication.
The relationship of mediators to medium is not precise. A medium cannot exist without a set of mediators, but no set of mediators defines a medium. Indeed, even in cases where a mediator all but defines perceptions of a medium (usually because it was there first, or because it is physically accessible to communicators), mediators will frequently be interchangeable with other mediators.
Much of what constitutes a medium is invisible to its users. We cannot see the vibration of air molecules in face to face communication. We cannot see the movement of light as it strikes surfaces and is reflected (in part) into our visual field. We are not generally aware of the nervous system that makes physical contact meaningful through the agencies of touch or taste.
One could go on endlessly on this point. The switches that control the movement of telephone conversations and letters are usually tens or hundreds of miles away. The electronics that convert radio waves into television images are buried inside the case of a television set, and are not readily apparent even when the television is disassembled. Most of the mediators that comprise a medium of communication are invisible to the agents that make use of it. The transparency of the mediators does not, however, make them unimportant to the definition of a medium.
We will use five terms to represent general classes of mediator: Carrier, Memory, Switch, Filter, and Interface. A carrier is something that moves a message from one place to another. A memory is something that maintains a message over time; that moves information from one point in time to another. A switch is something that directs the movement of a message to a particular place. A filter is something that alters a message as it moves from one place to another. An interface is something that captures an agent's message in a form in which it can be moved, maintained, switched and or changed by the medium; something that presents a message to an agent after it has been processed by a medium.
These terms, like those we have used to describe medium, exist in dialectical relationship. One can readily generalize any of these terms to encompass any of the others. One can just as easily narrow these terms, as we have done here, to capture a narrow part of what it is to be a medium. One can treat memory as carrier by transporting it, much as paper becomes a carrier when the postal system transports it from a mailbox to a mailing address. One can as readily treat carrier as storage by constantly retransmitting the message to the sender by a route that ensures that the message takes at least as long to move from sender to sender as the message is long.
The physical agencies which these terms represent are not, moreover, always distinct. A person can act as carrier, storage, switch, filter, and even interface. The magic of a computer, as a mediator of human communication, is its unique ability to provide agency, as a person can, through all five classes of mediation. Most mediators are much more restricted in scope. Paper can act as memory and interface, but as a component of communication media, it routinely depends on other mediators to provide carrier, switching and filtering services.
Indeed, most mediators can be associated with only a single function. Telephone lines can act as carrier, but routinely depend on other mediators to provide interface, switching, filtering, and (if necessary) memory services. The telephone PBX is similarly restricted, acting only as a switch for directing telephone calls. The telephone, although also providing a filtering function (transforming audio signals to electrical waveforms or digital signals) acts primarily as an interface. It functions as an agent's entry point to the medium, allowing the agent to select the destination with which messages will be exchanged and to both transmit and receive messages through the media. A tape recorder, if used in conjunction with the telephone or other media, can act as memory.
A carrier is a mediator that moves a message from one place to another. Examples include light, sound, touch, radio waves, microwaves, electricity and other agencies of movement that can be controlled in a way that carries information. The key to being a carrier is the ability to move information from one place to another. Hence any form of transportation can be regarded as a carrier and potentially used as a component mediator in a medium of communication.
There is a tendency, within the field of communication, to treat carriers (using the word "channel") as a key defining attribute of media. Indeed, one implication of this tendency, the interchangeable use of channel and medium, has already been the subject of extended discussion. This tendency may be rooted in our use of the Shannon and Weaver model as a metaphor for the study of communication. Shannon's (1948) mathematical theory of communication is, in a real sense, a theory of carriers and their information-carrying potential.
Carriers are, however, only one form of mediator. Indeed, they are one of the least important mediators to the definition of a medium of communication. With the exception of the most general carriers, the light, sound and touch that carry messages in face to face communication, carriers can frequently be interchanged quite freely without changing the nature of a medium. In the movement of mail, people can be replaced by horses, horses by trains, trains by trucks, and trucks by planes, all without changing the essential character of personal mail as a medium of communication. In the movement of telephone messages, electricity can be replaced by radio waves, radio waves by microwaves, microwaves by satellite communications, and satellite communications by light waves, again without changing the nature of the medium.
A medium's associated carriers remain an important element in determining a medium's characteristics, however. It is one's choice of carrier which, more than any other mediator, will determine the speed with which messages will move from sender to receiver.
A memory is a mediator that maintains a message over time. Examples of memories include rock, canvas, paper, plastic, static RAM, computer disks (floppy, fixed, magnetic, and optical), human memory, and other physical substances which are capable of maintaining information in a format which allows its ready retrieval. There is a tendency, within the field of communication, to treat memory as if it were a carrier (we speak of paper, for instance, as a channel of communication). The function of carriers and memories is ultimately quite different, however. A memory can be carried from one place to another, but its purpose is not the movement of information from one place to another, but the movement of information from one point in time to another.
A medium's associated memories are important in determining such media characteristics as message persistence and asynchrony.
A switch is a mediator that directs the movement of a message from one place to another. It decides, in effect, who has a chance to receive a message. Examples of switches include telephone switches (crossbars, PBX, PABX, and telephone operators), post offices and regional centers (where mail is sorted in route to its destination, warehouses, computers and people. The idea of a switch is not as clear-cut as that of carrier or memory. A switch is less a structure than an action taken by a structure, and that action can be taken in a variety of ways by a variety of structures.
One can recognize a switch by one of two principle forms. A switch can be recognized in a structure that receives large numbers of messages from a large variety of sources and distributes them to a large number of individually selected destinations. A switch can also be recognized in a structure that allows individuals to decide whether to expose themselves to a message or not.
These two varieties of switch are interesting in that they reveal an imperfect, but somewhat useful, difference between interpersonal and mass media. The switches of interpersonal media are centralized switches that receive messages from a wide variety of individual sources and distribute them to a similarly wide variety of individual destinations. The switches of mass media are the switches that allow individuals to decide whether or not to expose themselves to a message.
The differentiation is clearly imperfect:
These imperfections do not deny the relationship. Interpersonal media have centralized switching in addition to selective switching. Mass media, even when they broadcast through a centralized switch, either target their messages indiscriminately to groups of people (as is the case with junk mail) or to individuals that have asked to receive the message (as is the case with newspapers).
The operation of switches is nearly always contingent on the structure of messages. Centralized switches depend on addressing information being attached to the message in some predictable fashion. The postal system switch looks for addressing information to be centered on the front of an envelope, with various elements of the address listed from most specific (the name of the individual) to most general (state or country). Telephone switches look for addressing information at the beginning of a message. The address, in the form of a telephone number, again has an expected format, with various elements of the address listed from most general (area code) to most specific (exchange and phone number).
Similar addressing conventions can be identified in all centrally switched messages. Electronic mail, for instance, maintains addressing information in a header. Address information often takes the form of "userid" (a specific person or agency) "at" (sometimes expressed as "@" or "/") "node" (a specific computer whose location is known to other computers on a computer network).
User switches are often contingent on the structure of messages as well. Junk mail, for instance, will frequently be recognizable as such without being opened, with the result that a great deal is not. Selection of television and radio programs will often be based on very limited exposure to a range of selections on different stations. A "snapshot" of a small portion of the selection becomes a major decision criteria. Individuals at a party will generally be able to select among a variety of conversations. Which conversation is selected will frequently depend on the content of that conversation.
A filter is a mediator that alters a message as it moves from one place to another. Its purpose may be to add something to a message, much as a coffee filter serves to add flavor to the hot water that passes through it. Its purpose may be to remove content, much as a water filter removes particulates from drinking water. Its impact, whether it ultimately adds or subtracts information from a message, is to change the message.
Filters can be recognized in a variety of common forms, including:
Labels like editor and censor tell only a part of the story of resistors, however. Any time a person acts as an intermediary between a sender and receiver there is the possibility, whether due to forgetfulness or intent, that the intermediary will act as a resistor.
Noise may be the least important filter in a communication system, in part because of the relative ease with which most noise sources can be overcome. Routine chaotic noise can frequently be removed from the system with resisters that remove the noise signal. Both random and chaotic noise can potentially be removed through the use of redundant information that allows translators to recover the original message. When encountered in large amounts, however, noise can have the effect of garbling or even obscuring a message. When encountered persistently, as can be the case for Citizen's Band in heavily populated areas, it can have the effect of rendering the medium useless.
The varied capabilities of filters to modify and reproduce messages can be expected to result in varied impacts on the characteristics of a medium. An amplifier will have the effect of increasing the number of people that can be exposed to a message. A resistor or capacitor can, for different reasons, have the effect of increasing the amount of time it takes a message to travel from sender to receiver.
An interface is a mediator that gives agents access to a medium. Interfaces serve three essential functions in a medium:
The successful operation of a medium's interfaces depends on a correct mapping of that interface to the capabilities of the agents that use it. If an agent is capable of hearing sounds in the range of 50 to 25000 hertz (roughly the boundaries of human hearing), a medium that seeks to exploit this range of capabilities must present sounds in this range. If an agent is capable of seeing a range of light frequencies, a medium that seeks to exploit this range of capabilities must present light frequencies in this range.
It is a happy coincidence of human communication that the human message capture interfaces (our senses) are well coordinated with human message production interfaces (our voices, body motions, and touch). We make sounds we can hear. We motion in ways we can see. We touch in ways that we can feel. If our human message capture and production interfaces were not well coordinated, we might never have invented/learned ways to communicate with each other.
The interfaces of the media we use must be similarly well coordinated with human message capture and production interfaces. It is not, then, entirely surprising that the microphone and camera we use to capture sound and sight are highly similar to the ear and eye. The microphone, like the ear, vibrates in response to vibrations in the air, and turns those vibrations into another form. The camera, like the eye, measures light at a large number of points discrete points in a two dimensional array and converts what it senses into a different form.
This kind of cybernetic modeling (however unintentional it may have been) is not shared by all interfaces:
The structuring of interfaces in some media sometimes make it possible to recognize the medium from the structure of interface. Where this is possible, it is not the interfaces so much as their use that allows for recognition. The list of interfaces commonly used in human communication media is short. Message capture generally entails variations of stylus, keyboard, microphone, and camera. Presentation generally entails a combination of page, speaker, and screen. The controls built into media are designed around switches (on/off and channel selection; a keyboard is actually a large collection of switches) and sliders (analog volume control, for instance). Both switches and sliders are readily replicated in a computer environment with on-screen depictions that are manipulated with a pointer or keyboard. Each of these interface elements is used in a variety of elements in a variety of ways.
Consider, for instance the telephone, tape recorder (as used in a medium like tape correspondence), citizen's band radio, and broadcast radio:
The interfaces used in these media are the same. The structure of the interfaces are clearly different. Only one, the telephone, gives communicators at both ends of the connection simultaneous and parallel access to both microphone and speaker. In another, the tape recorder, controls provide selective use of speaker and microphone, without the possibility of live connection with other communicators. Use of a microphone in the live multi-communicator environment of citizen's band radio is similarly selective, but the selectivity is more immediate. In broadcast radio, by contrast with all of these, the microphone is used to create a live but one-way connection to a large number of speakers.
Similar differences can be found in control interfaces. The volume control of the telephone is generally obscurely placed and rarely used, a striking contrast to the other media. The tape recorder has, as is appropriate for a medium with no direct connection between the media interface and a tape message's target, no controls for specifying addressing. The addressing available to citizen's band and broadcast radio are limited, and the switches (typical of citizen's band radio) and sliders (typical of broadcast radio) of these media reflect these limited choices as a linear array. The addressing used in telephones is highly complex, and the multiswitch addressing control interface of telephones simplifies this complexity by sequencing a series of switch selections.
The differences in these media are not in the selection of interfaces. The differences are in the way these interfaces are used; in who has control of what interface elements when and in the way the medium can be controlled through its interfaces. Interfaces become uniquely associated with media more for the processes they entail than for the elements selected. These interface processes, much more than the interfaces themselves, are inventions selected from an infinite variety of choices. The process of learning to use a medium is, in no small part, a process of learning these processes.
The choice of interfaces and interface processes in a medium affects the characteristics of media in at least two ways. First, they affect the ease with which a medium can be used. The success of a medium will depend, in no small part, on its selection of interfaces that can be learned without undue effort and used without excess expenditure of attention. Second, they affect a medium's interactiveness. A medium that provides symmetrical access to mechanisms of both capture and presentation (as is the case for letters, the telephone, and citizen's band radio) will most likely be used in an interpersonal context. A medium that provides message capture access to a small group while distributing the means of message presentation to a large audience will most likely be used in a mass media context.
Control interfaces accentuate the relationship between controls and mediators outlined in the "Grammar as Model" discussion. Control interfaces exist to allow control of a medium. Some of those control interfaces will act to create and/or enforce rules (the frequency one can broadcast on, for instance). It is in the selection and use of switches, control interfaces, filters, and even memories that the rules and processes we might normally associate with controls can be transformed into mediators. Mediators can evolve on the basis of controls, sometimes subtlely changing the medium in the process.
The working of this relationship is clear in the evolution of media. Increasing expectation of professional presentation in publications undoubtedly started as an informal expectation. These expectations were eventually formalized and then routinized in the introduction of filters such as proofreaders, editors, typesetters, layout artists and other publishing professionals. Similar systematic complications can be observed in the evolution of the movie and television production process.
The increasing use of the telephone and growing complexity of telephone networks was accompanied by a growing expectation of speedy and reliable connections. Growth eventually swamped the capabilities of human operators. Solutions to this conflict between expectation and reality came first in the creation of compact addressing schemes (telephone numbers). These addressing schemes were then systematized in switches that routed calls to these addresses automatically and then control interfaces that allowed users to route calls without operator intervention. Similar problems with limits of human operators in telegraph and telex operations were solved first by rules that governed who transmitted messages when. These rules were eventually routinized in interfaces that implicitly enforced the rules and later through the use of memory in the form of paper tape.
This kind of transformation of practice to mediator is simplified in the computer environment, where highly flexible software memories, switches, controls, and filters allow communication to be routinely shaped in a wide variety of ways. Hence one sees, in computer media, a continuation of a general trend in media towards an increasing amount of mediation between message capture and presentation. It also initiates a trend toward the integration of message capture and presentation interfaces (commands, menus, icons, etc.) into media control interfaces. Computer conferencing is but one example of software routinely shaping human communication in specific ways.