MAS.632 Conversational Computer Systems - MIT OpenCourseWare

106 VOICE COMMUNICATION WITH COMPUTERS
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outline a number of design considerations and discuss some interactive techniques
in this light.
Most of the limitations of voice output described earlier can be summarized by
the broader observation that speech takes place over a periodof time. The amount
of time required to listen to a voice recording, the necessity of reciting menu items
in sequence, and much of the difficulty associated with the bulky nature of voice
are all a direct consequence of the temporal nature of speech.
Speed is thus one of the primary factors distinguishing voice from other interface
media; for example, most graphics applications can be written with the
assumption that display operations such as drawing lines or text are performed
nearly instantaneously. Much of the time the application is idle while the user
reads text, picks a menu item, etc.; this is punctuated by sporadic user input
and display update. For telephone-based interfaces, the exact opposite is true;
presentation accounts for most of the total time the user interacts with the
application.
Voice response systems must therefore strive to minimize the amount of time
required to retrieve information: they must be brief, selective about content, and
interruptible. Applications must plan what to say so as to avoid utterances that
are irrelevant by the time they are spoken. A real-time speech output system may
have to formulate conflicting goals and choose among them if there is not suffident
time to say everything, which requires estimating how long it will take to
speak an utterance. The awareness that speech takes time (and acceptance that
time is a commodity of which we never have enough) should permeate the design
process for building any speech application. This argues in favor of shortening
spoken menus and precludes many applications that may be quite feasible in text
such as browsing through a newspaper. Time motivates strategies to filter an
application's output requiring careful judgment as to the relevance of a specific
system comment. The remainder of this section explores these design considerations
in more detail.
Because speech is so difficult to employ effectively, discretion is essential in determining
effective voice applications and in choosing the appropriate speech output
device. Poor quality speech, whether synthesized or recorded, quickly becomes
annoying unless it serves a clear purpose. For example, speech output has been
employed effectively as a warning medium in airplane cockpits. On the contrary,
few car drivers appreciate a tinny voice reminding them to close the doors oftheir
automobile. The cockpit is a busier place, and the consequences of an error may
be more severe. A more effective use of speech in the car may be traffic information
(drivers already tune to commuter traffic reports on their radios) or as part
of a navigation system (as described later in this chapter).
In retrospect, another example of a misguided speech interface is the recitation
of a customer's purchases at a cash register. As each item is scanned by the bar
code reader, its description and price are announced. The original motivation for