1 - History of Ericsson - History of Ericsson

trend of the isopreference and isoopinion contours is the same, but the slope
of the latter is less for low noise levels. The explanation is that the talker
during natural conversation adjusts his voice according to what he hears
from the far end, as has been confirmed by investigations at L M Ericsson.
From BOERYDS 5 investigations we have taken the diagram in fig. 5, showing the
relation between SyRE and outgoing speech volume VU, when 50 db
room noise but no circuit noise is present. It is interesting to note
that VU has a minimum about 17 db. which is the starting point for
the splitting line in fig. 4. We may therefore assume that the splitting
line represents the most convenient system reference equivalents from
the users' point of view. If the SyRE is too low, the users probably try to
outtalk each other and raise their voices. If SyRE is higher than 17 db, the
talker also raises his voice, thereby partly compensating for the SyRE
increase. At a SyRE of 34 db we thus have a compensation of 4 db or
about 25 % of the increase. We have here an example of a lucky interaction
between man and machine. But it must be noted that the speaker does not
react to his own speech but to what he hears from the far end. It is
obvious that the desired adjustment of the voices will be jeopardized if the
transmission conditions differ too much in the two directions, and this must
be kept in mind when we later discuss long distance calls via a number of
4-wire links with repeaters.
From fig. 4 we may draw some conclusions from the isoopinion contours
in the lower region and the isopreference contours in the upper:
• If we assume that isoopinion contour Y — 2.23-about 15 % risk of poor
or bad calls-is the lower limit for satisfactory service, we may have
SyRE = 35 db when no noise is present. On intercontinental calls CCITT
permits a noise level of- 50 dbmp all the time, and we find that SyRE has
to be reduced to 27 db if the same mean opinion score is to be maintained.
It is obvious that supervision and maintenance people have to reduce the
noise as far as possible and hunt for all noise sources, or we have to reduce
the permissible SyRE, which has to be done in the 2-wire networks in
order to ensure satisfactory transmission on long distance calls. A reduction
of SyRE means increased investment costs in the local networks,
and we have here a possibility of evaluating the inconvenience of circuit
noise in terms of money.
3 The isopreference contours in the upper region show that too high a
level or too low a SyRE is also inconvenient to the listener. He can,
however, protect himself by moving the telephone away from his ear,
which one must do on a large number of calls between subscribers with
short lines connected to the same exchange. If the user moves the receiver
away from his ear, he moves the microphone away from his mouth and may
develop unsound habits when he has to talk on long distance calls with
poor transmission conditions. In fig. 3 we have approximately split the
diagram into opinion regions, and if we assume that preference levels below
58 db should be considered as poor, we find that SyRE should never be
better than - 4 db. Nowadays we have the technical means of attaining
this, if we install modern telephone instruments with line equalizers at
the subscribers' premises. It is, of course, not economical to replace all
old instruments without line regulation, but a lot could be gained if this
were done when subscribers complain about poor performance on long
distance calls and if we teach them how to use the new instrument in the
proper way and how to talk.
With respect to long propagation times and interlinked echo suppressors
we have to inform our users-not being telephone experts-that they cannot
intervene in each other's speech in the usual way on calls over very long
distances but have to give the other party an opportunity to answer in order
to avoid confusion, maybe resulting in "hallo"-calls and prolonged conversation
time.