PBL 385 71
dB
Battery feed
Regulation:
R1
R2
R6
c.
b.
∞
450k
∞
∞
∞
∞
47Ω
75Ω
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
75k
∞
Figure 13. Gain regulation principle.
where no balancing has been done is in
the order of 6 - 12 dB.
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.
What is balancing the side
tone?
To understand that the side tone is
influenced by other factors like, the
impedance of the line and the signal that
enterstheearacousticallydirectlyfromthe
mouth and from the mouth through the
material in the handset. The signal that
enters the microphone from the earphone
acoustically will also influence the return
loss factor to the telephone line.
Tounderstand thatthesidetonenetwork
can be trimmed to form a veritable
”distortion analyser”, so that the distortion
thatispresentfromthemicrophone,willbe
the only signal entering the earphone and
thissignalevenbeingsmallwillsoundvery
bad. It is better to induce some of the
fundamental frequency back by making
To understand that side tone balancing
is to counteract the signal, that is
transmitted via the microphone and trans-
mitter totheline, returningtotheearphone
via the receiver.
That presence of a strong side tone
signal is disturbing in a way that one quite
instictively lowers ones own voice level
thus lowering the signal level for the other
party. But again, if the balance is too good
(seldom the case) the earphone will feel
”dead”. In practical terms what is expected
is the same amplitude of ones own voice
intheearaswhennottalkinginatelephone.
The need to lower the side tone level
A short guidance for under-
standing the side tone
principle. (See fig.14.)
Telephone
Line side
set side
a.
1
15
16
PBL 38 571
Assumingthelineimpedance tobe 600Ω.
( theorethical value )
Z1 = Line impedance
Z2 = The telephone set impedance 600Ω
Z1//Z2 = 300Ω
Tx
b.
Rx
2
14
Z1
Z2
C6
c.
R10
R7
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.
R11
R8
R6
R12
Zbal
R9
C5
Figure 14. The side tone suppression principle.
9
ERICSSON [ ERICSSON ]
