G/H 161
224
107
F/K 87
65
43
220
false alarm
A/L 618
95
367
F/T no detection after 954 seconds
S/H 227
false alarm
90
75
83
K/G 38
421
20
143
S/W 257
229
no detection after 260 seconds
[Illustration: _Data from table have been arranged in this histogram
to show wide range of delay detection times that were recorded in
the experiment. Each block represents a single delay detection,
identified by initials of conversers._]
number of cases
6
S/H
5
S/H
4
S/H S/W
3
K/G A/L S/H
2
K/G F/K K/G F/K
1
F/K F/K G/H G/H S/W A/L K/G A/L
0
0 50 100 150 200 250 300 350 400 450 500 550 600 650
detection time (seconds)
Now, to answer our question about whether people can converse over our
circuit, we can say something like this: We have found some cases (of a
certain type of conversation) where people _can_ use a circuit with 1.2
seconds of echo-free delay for the amounts of time listed in our table.
But there are two important things to remember about our results:
—_The findings are a_ non-negative _answer to the original question,
“Can people converse over a four-wire, echo-free circuit that has
delay like that of a synchronous satellite?”_
—_The experiment applies_ only _to four-wire, echo-free circuits, and
does not help with the problem of improving echo suppressors or with
that of finding out how good synchronous satellite circuits with the
best possible echo suppressors would be if they were used to
interconnect the world’s telephones._
Since we didn’t get a “no” answer to our question, we have been
encouraged to do more experiments with other subjects and other types of
conversations. We also have begun to look for more than a yes-or-no
answer; we now want to find out how serious various amounts of pure
delay would be. Some of my colleagues have been working on this problem
by furnishing special four-wire telephones to a group of people, so that
echo-free delay can be inserted in the line when one member of this test
group calls another member. Their experiment has confirmed our finding
that conversation with a round-trip pure delay of 1.2 seconds is not
impossible, but it has also shown that the degradation of conversation
that results is not trivial.
Recently an international committee on commercial telephone standards
set the maximum permissible echo-free delay (round-trip) at 0.7 seconds.
However, the search for a more precise evaluation is still going on.
Peter D. Bricker _was born in Scranton, Pennsylvania, and received an
A.B. from Bucknell University in 1950 and an M.A. in 1952 and a Ph.D.
in 1954 from the Johns Hopkins University. He joined Bell Telephone
Laboratories in 1954, and has been engaged in psychological studies of
telephone color preferences, pushbutton set designs, voice
identification, and transmission quality evaluation._
CASE HISTORY NO. 6
How Can We Repair an Orbiting Satellite?
E. Jared Reid
_Electrical Engineer—Member of Staff, Satellite Design Department_