PBL 385 70
A short guidance for understan-
ding the side tone principle
Making any of the impedances
unnecessary high will make the circuit
sensitive to RFI. All values given here are
approximate and serve as starting entities
only.Thefinaltrimmingofsidetonenetwork
is a cut and try proposition because a part
of the balance lies in the acoustical path
between the microphone and earphone.
What is balancing the side tone ?
•
Tounderstandthat sidetonebalancing
is to counteract the signal, that is
transmitted via the microphone and
transmitter to the line, returning to the
earphone via the receiver.
(See fig.13.)
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
resultthat1/5oftheac-signalthatisonthe
line appear across R6. Note that the sign-
als at points a. and b. are 180 degrees off
phase.
•
That presence of a strong side tone
signal is disturbing in a way that one
quiteinstictivelylowersonesownvoice
level thus lowering the signal level for
theotherparty.Butagain,ifthebalance
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.
Reverse side tone network.
This type of side tone balancing will help
when of some reason there is a need to
make the R6 low < 47Ω and thus the
signal for balancing gets small across R6.
Byplacingthebalancingnetworklikeshown
in fig.14. the possible signal level is 6 dB
higher than in the first case and it will also
help in case that a volume control is added
to the receiver.
10 x R6 ≈ R7 + Zbal
R7 ≈ Zbal
Note #1
Note#2
•
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.
The ac-signal at point c. is now 1/10 of
the signal on the line because it is further
divided by two from point b. (R7≈Zbal).
Hence 10 x R10 ≈ R11 to satisfy the
balancing criteria. R12 is to set the recei-
ver gain ( can also be a volume control
potentiometer).
PBL 385 70
Note #1 These values ensure that the
frequency behaviour of the transmitter is
not influenced. With the ratio 1/10 the
influence is 1 dB, and with ratio 1/20 it´s
0.5 dB.
Note #2 If the R7 is made low ohmic
compared with Zbal, it will load the latter
and resultinabadsidetoneperfomannce,
again if the R7 is made high ohmic
compared with Zbal will result in a low
signal to balance the side tone with and
make the balancing difficult.
15
2
C6
•
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
evenbeingsmallwillsoundverybad. It
is better to induce some of the funda-
mental frequency back by making the
balance less perfect at that frequency.
This is valid for a network that is trim-
med to only one frequency. It is to
strivetotrimthenetworksuchthatitwill
attenuate the fundamental and the
harmonicfrequenciesalikethroughout
the different line combinations.
R10
+Line
R11
Z
bal.
R6
R12
*
C
* To give receiver flat
frequency response
Figure 14. Sidetone network with
complex R11.
Telephone
Line side
set side
a.
1
17
18
PBL 385 70
•
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
enteringthereceiveramplifierandthus
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.
Tx
b.
Rx
2
15
Z1
Z2
c.
C6
R11
R10
R7
R8
R6
R12
Zbal
R9
C5
Figure 13. The side tone suppression principle.
9
ERICSSON [ ERICSSON ]
