Preliminary
AS2520/21/20B/21B
This approach has some disadvantages. It requires a
high degree of discipline, since the three state
channel control gives a very distinct half duplex with
a relative high switching time constant to avoid
chopper effects. Furthermore, the system is very
sensitive to the environment,- noise, line conditions
and acoustics (echo).
The advantages of using the transmit state as the
static (idle) state are that the B subscriber hears an
open line (the line is not dead), does not miss the
initial word of a sentence when the A subscriber
starts talking, and hears the level of the background
noise at A´s end which will actuate her/him to speak
up accordingly.
Apart from keeping a distinct discipline, the user can
not do anything to minimise the effect of these
constraints, since the parameters of the voice
switching (thresholds, time constants, noise
discrimination, etc.) can not be changed or adapted
to the actual conditions by the user.
When the A subscriber starts talking, the circuit
remains in the static state.
The dynamic state of the voice switching can only be
activated by the receive signal. Applying a receive
signal above a certain level will cause the circuit to
enter the dynamic state.
The dynamic voice control system of the AS252x
have been designed to overcome the above
constraints. The basic philosophy behind the AS252x
is that telephone circuits should not have any
automatic regulations preventing the user from
having all information about the actual conditions
which should enable her/him to act accordingly, i.e.
to comply with the given constraints.
VTX
ZAC
VLINE
PEAK
DETECTOR
AGC
2/4
VTH
Now, assuming subscriber A has a handsfree
telephone and is calling subscriber B, who has a
normal telephone. The B subscriber does not
necessarily know that A is using a handsfree
telephone and will therefore not automatically comply
to the discipline of a half duplex conversation. Hence,
the disadvantages by using half duplex should apply
to the A subscriber only.
VRX
± 10 dB
VOL
Figure 5: Channel Control System
The signal for controlling the channel attenuation is
taken after the sidetone amplifier. With the volume at
0 dB (neutral) the threshold for entering the dynamic
state (VTH) is 15 mV assuming that VRX > VTX (see
Secondly, if A is in a noisy environment, the B
subscriber should hear it, so that he speaks up to
increase the signal-to-noise ratio at the A subscriber.
figure 5).
The traditional 3-state switching system has two
major drawbacks: first of all, when no one is talking,
the circuit is in idle state and the B subscriber gets
the feeling that the line is dead, since the background
noise does not activate the voice switching.
Secondly, the B subscriber does not speak up, since
she/he does not hear the background noise.
In the dynamic state the channel attenuation is
controlled by a voltage controlled amplifier. The
attack time is 4 ms/6 dB and the hold time is 200 ms.
A speech compression is activated when a transmit
signal with a high amplitude reaches a level
corresponding to approximately 460 mV on the line.
The concept of the AS252x, however, does not
exclude the human factor, but provides the
information about the actual conditions to the user
and allows her/him to act accordingly, i.e. to speak
up, to change the volume, etc.
300
(mV)
250
200
150
Sidetone Cancellation: 11 dB
In more technical terms, the AS252x works in the
following manner:
Volume Control: 0 dB (neutral)
100
50
0
When no signal is applied neither from the line nor
from the microphone, the circuit is in the only static
state, which is transmit channel full open and receive
channel attenuated by up to 30 dB.
0.00
0.25
0.50
0.75
1.00
1.25
1.50
(mV)
Microphone Input Signal
Figure 6: Speech Compression
Rev. 5.1
Page 7
May 1999
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