AN1500A
The side tone is probably one of the most important parameters, if not the most important parameter. It determines
very much the overall instinctive performance of a telephone since it has direct influence on other parameters. In a
subjective test the two most inherent parameters are distortion and side tone, or other parameters directly
influenced by those two, e.g. echo, acoustic stability, clearness, etc.
During the design of the SA2532K family considerable effort was put into the system definition of the side tone
cancellation. It was clear that even the highest side tone cancellation could give an unpleasant harsh distortion at
very large signal levels if no efficient limiter was provided. This led to the designing of what turned out to be the
best “soft clipping” circuit yet developed, surpassing any solution used in a telephone so far.
The encounter is a side tone which is easy to calculate and which gives a well defined cancellation virtually
independent of other parameters like return loss (ac impedance) and dynamic range (absolute input signal levels).
12.1 Dual Soft clipping:
The dual soft clipping circuit prevents harsh distortion and acoustic shock in both directions by limiting the
maximum output of the Tx/Rx amplifiers at a level which still maintains a non-distorted signal (see VAGC levels in
data sheet).
If the output of the amplifier is already at the soft clip level and the input level is further increased, then the
amplifier’s gain is reduced, so that the output level remains non-distorted at the soft clip level. Even fast input signal
peaks are detected because of the fast attack time (see tATTACK) of the soft clip circuit. Instabilities are prevented by
using a long decay time (see tDECAY in data sheet).
12.2 Side tone balance network:
The side tone balance network is simply calculated as:
Z
= Z
x 10
LINE
BAL
12.3 Example:
For a line termination of
270Ω+750Ω//150nF
the resulting sidetone balance network would be: 2k7 + 7k5//15nF
Within the AN2201 application, this would apply to R13= 2k7
R14= 7k5
C4 = 15nF
13 The Double Wheatstone principle
The family of SA2532K single chip telephones are using a double Wheatstone bridge for return loss and side tone
cancellation. This unique configuration makes it very easy and uncomplicated to set the side tone network.
Fig. 5 explains the principle of the Double Wheatstone bridge: For easier understanding the block diagram is split
into two separate diagrams, which only show the relevant components.
The components shown in the block diagram represent the following components in the DB1500I schematic:
ZLine
Z1:
:
the PTT´s AC impedance (external)
R11 = 30Ω
Z2:
R12 = 300Ω
Zsyn
:
Q3
Zbal:
Rref,Zref:
the sidetone network
internal resistors
11/27
sa m e s
SAMES [ SAMES ]
