1 - History of Ericsson - History of Ericsson

Fig. 3 X 8460
Isopreference diagram
for a 3000 c/s low pass circuit
The diagram is split in the following opinion regions:
Excellent >74 db
Good 74-66 db
Fair 66-58 db
Poor 58-50 db
Unsatisfactory < 50 db
(Taken from a paper written by Munson & Karlin)
deviations-whicb, however, is impossible-or that we have to increase the
performance of local switching equipment which is engaged also on
long distance calls, in order to meet future subscriber demands.
Speech Level and Noise
As an example we can study the influence of speech and noise levels on
the quality of a call. On the basis of investigations carried out by many
administrations, CC1TT has made recommendations for the total reference
equivalent and its distribution over the different parts of a telephone network
in order to ensure a sufficient speech level for the listener.
We have also to consider, however, that noise can jeopardize intelligibility
even if the speech level in itself is sufficient. For cheap local calls this may be
of less importance, but becomes more so on expensive long distance calls.
Room noise the subscribers themselves can get rid of, but the reduction of
circuit noise-such as noise from relay contacts or switches, intelligible or
unintelligible crosstalk, intermodulation or singing in carrier circuits, echo in
2- to 4-wire switching points-must be the task of the administrations.
For maintenance and supervision people it is essential to have an idea how
a combination of speech and noise levels will influence the transmission. For
that purpose we can use the isopreference contours-presented by KARLIN 3
fig. 3. On the coordinate axes the relative levels of speech and noise are expressed
in decibels. In the coordinate plane are drawn a number of "isopreference
contours" based on opinion tests. Every contour represents a combination
of speech and noise levels for which the listeners have the same
preference. The figures beside the contours express relative "preference levels"
in db. It is said that a relative level of about 75 db corresponds to good performance,
but 50 db will be too poor. As the opinion test was carried out with
a restricted number of persons under specific conditions, one has to be careful
in drawing conclusions, but in spite of that we may say that:
• A high preference level corresponds to better performance.
• All isopreference contours have maximum points, indicating maximum
permissible noise level for a certain performance.
• A line through the maximum points, starting at 82 db speech level,
splits the plane into two regions:
10 20 30
NOISE SPECTRUM LEVEL
21