MAS.632 Conversational Computer Systems - MIT OpenCourseWare

114 VOICE COMMUNICATION WITH COMPUTERS
when telephone exchanges were referred to by a name instead of number.6 This
alphabetic labeling tends to get carried over into other public keypad style
devices such as automated bank teller machines. But there are two problems
with this arrangement: each key maps into three letters (see Figure 6.3), and the
letters "Q" and "Z" do not appear on any key of the telephone.
For the two missing letters several options are in use. Both letters may be
mapped to the otherwise vacant 1 key; this seems the most common alternative.
They are also sometimes mapped to the 0key. For the implicit key-to-letter decoding,
described below, they can be assumed to occupy the keys on which they would
appear if they had been left in the alphabet. In this scheme, Q appears invisibly
on the 7 (PRS) key and similarly Z appears on the 9 key.
Alphabetic input techniques may decode the three-to-one letter-to-key mapping
either explicitly or implicitly. Explicit mapping requires the user to type two keys
for each letter; the first key chooses a group ofthree letters and the second selects
one from this set. The second key may be constrained to one ofthe set 1,2, or 3 or
better could be any key with the column position of the key selecting the first,
second, or third letter. For example, the letter H might be selected by entering 4
5; 4 selects the GHI set, and 5 from the middle column selects the middle letter
from the triplet.
The implicit letter mapping technique accepts the confusability of the user's
input and attempts to decode the input by matching it against the set of valid
inputs. For example, while selecting a name from a menu (see Figure 6.4), the
user is really picking one of a set of names, each of which may have a distinct
touch tone "spelling" uniquely specifying that each letter is not necessary. In fact,
in the illustrated example, four of the names would be uniquely specified after the
first touch tone. (One might, however, wish to gather all five digits to confirm that
the user was not trying to select a name not on the list.)
Explicit spelling requires twice as many keystrokes but is required for some
tasks, e.g., if a new subscriber to a service is asked to spell his or her last name to
create a new account. In the case of spelling to make a selection from a list,
implicit character mapping may be quite adequate, faster, and easier to explain
to the user; examples include looking up a last name in an electronic database,
spelling a street name, or logging in to a computer account.
A problem with implicit spelling is the degree of overlap between the items
being selected and their keypad equivalent. For example, the login names "Leo"
and "Ken" are both spelled "536."In such a case, the collision must be detected by
the system and an alternate selection mechanism invoked, e.g., "Ifyou mean Leo,
press one; if you mean Ken, press two." (Note that this is a menu and could be
either enumerated or temporal).
Clearly the degree of overlap between the items is a function of the particular
set of items, and therefore the confusability of implicit spelling will vary from
6 Where I grew up in Chicago we were in the EDgewater exchange, although this con­
vention is no longer followed.