PBL 385 82
dB
Battery feed
Regulation:
48V, 2 • 200Ω
48V, 2 • 400Ω
48V, 2 • 800Ω
R13
∞
∞
∞
R14
∞
∞
180k
R6
c.
b.
47Ω
75Ω
100Ω
a.
High limit
No regulation:
Set for low gain
All feedings
Set for high gain
Set for high gain
∞
18k
22k
<22k
∞
∞
47 - 100Ω
47Ω
75 - 100Ω
I
L
Low limit
Figure 9. Gain regulation principle.
What is balancing the side
tone?
To understand that side tone balancing
is to counteract the signal, that is
transmitted via the microphone and trans-
mitter to the line, returning to the earphone
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
in the ear as when not talking in a telephone.
The need to lower the side tone level
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
impedance of the line and the signal that
enters the ear acoustically directly from the
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.
To understand that the side tone network
can be trimmed to form a veritable
”distortion analyser”, so that the distortion
that is present from the microphone, will be
the only signal entering the earphone and
this signal even being small will sound very
bad. It is better to induce some of the
fundamental frequency back by making
the balance less perfect at that frequency.
This is valid for a network that is trimmed to
only one frequency. It is to strive to trim the
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
result in a very distorted signal entering the
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.
Telephone
set side
Line side
a).
1
A short guidance for under-
standing the side tone
principle. (See fig. 10.)
Assuming the line impedance to be 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 to appear across R6.
PBL 385 82
17
Tx
2
15
Rx
18
b).
R7
Z2
Z1
c).
R8
R6
C5
}
R10
C4
R11
Zbal
R12
R9
Figure 10. The side tone suppression principle.
7
ERICSSON [ ERICSSON ]
