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