pbl38573 ETC-unknow, pbl38573 Datasheet - Page 9

• Manufacturer Part Number: pbl38573
• Description: Speech Circuit Constant Current Feeding Systems
• Manufacturer: ETC-unknow

What is balancing the side
tone?
is to counteract
transmitted via the microphone and trans-
mitter to the line, returning to the earphone
via the receiver.
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
Figure 14. The side tone suppression principle.
To understand that side tone balancing
Battery feed
Regulation:
48V, 2 • 200
48V, 2 • 400
48V, 2 • 800
No regulation:
All feedings:
Set for low gain
Set for high gain
That presence of a strong side tone
R6
Tx
b.)
1
2
a.)
the signal, that is
C5
R7
Sweden apply for spec. application
PBL 38 573
R8
450k
75k
R15
c.)
Zbal
R9
R10
12
R16
22k
R12
C6
Rx
R11
where no balancing has been done is in
the order of 6 - 12 dB.
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.
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
To understand that the side tone network
13
14
47 - 75
47 - 75
To understand that the side tone is
75
75
47
R6
Telephone
set side
Z2
Figure 13. Gain regulation principle.
dB
Line side
Z1
a.
c.
Low limit
b.
A short guidance for under-
standing the side tone
principle. (See fig.14.)
Assuming the line impedance to be 600 .
( theorethical value )
Z1 = Line impedance
Z2 = The telephone set impedance 600
Z1//Z2 = 300
R6 will have a certain value 39 - 100 to
give the telephone a specified DC-
characteristic and overcurrent protection.
Assuming that
requires R6=60 , 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.
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.
signals entering
from either direction,
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
amplitude of the fundamental frequency.
To understand that if one of the two
High limit
this DC-characteristic
the balancing system
PBL 385 73
ten times the
direct from
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