PBL 385 41
dB
Battery feed
Regulation:
R1
R2a
R2b
R6
c.
b.
}
700k
∞
∞
∞
∞
600k
∞
75Ω
47Ω
75Ω
48V, 2 • 200Ω
a.
48V, 2 • 400Ω
48V, 2 • 800Ω
∞
∞
High limit
Sweden, apply for spec. application
No regulation:
All feedings
Low limit
Set for low gain
Set for high gain
∞
∞
22k
∞
47 - 75Ω
47 - 75Ω
I
L
∞
75k
Figure 13. Gain regulation principle.
the balance less perfect at that frequency.
Thisisvalidforanetworkthatistrimmedto
only onefrequency. Itistostrivetotrimthe
network such that it will attenuate the fun-
damental and the harmonic frequencies
alike throughout the different line
combinations.
To understand that if one of the two
signals entering the balancing system
from either direction, direct from
microphone or via the line is clipped, will
resultinaverydistortedsignalenteringthe
receiver amplifier and thus the earphone.
Further , to remember that side tone is a
small signal that is the difference of two
large signals and that the amplitude of the
distortion can be up to ten times the
amplitude of the fundamental frequency.
where no balancing has been done is in
the order of 6 - 12 dB.
To understand that the side tone is
influenced by other factors like, the
What is balancing the side
tone?
To understand that side tone balancing impedance of the line and the signal that
is to counteract the signal, that is enterstheearacousticallydirectlyfromthe
transmitted via the microphone and trans- mouth and from the mouth through the
mitter totheline, returningtotheearphone material in the handset. The signal that
enters the microphone from the earphone
That presence of a strong side tone acoustically will also influence the return
signal is disturbing in a way that one quite loss factor to the telephone line.
via the receiver.
instictively lowers ones own voice level
Tounderstand thatthesidetonenetwork
thus lowering the signal level for the other can be trimmed to form a veritable
party. But again, if the balance is too good ”distortion analyser”, so that the distortion
(seldom the case) the earphone will feel thatispresentfromthemicrophone,willbe
”dead”. In practical terms what is expected the only signal entering the earphone and
is the same amplitude of ones own voice thissignalevenbeingsmallwillsoundvery
intheearaswhennottalkinginatelephone. bad. It is better to induce some of the
The need to lower the side tone level fundamental frequency back by making
A short guidance for under-
standing the side tone
principle. (See fig.14.)
Telephone
Line side
set side
a).
1
17
PBL 385 41
Assumingthelineimpedance tobe 600Ω.
( theorethical value )
Z1 = Line impedance
Z2 = The telephone set impedance 600Ω
Z1//Z2 = 300Ω
R6 will have a certain value 39 - 100Ω to
give the telephone a specified DC-
characteristic and overcurrent protection.
Assuming that this DC-characteristic
requires R6=60Ω, hence it will be 1/5 of
the Z1//Z2. This will in transmitting mode
result that 1/5 of the ac-signal that is on the
line appear across R6.
Tx
b).
Rx
18
2
15
Z1
Z2
C6
R11
c).
R10
R7
R8
R6
R12
Zbal
R9
C5
Figure 14. The side tone suppression principle.
9
ERICSSON [ ERICSSON ]
