AS2520 austriamicrosystems AG, AS2520 Datasheet - Page 7

• Manufacturer Part Number: AS2520
• Description: Telephone Speech Circuit with Loudhearing and Handsfree
• Manufacturer: austriamicrosystems AG

Preliminary
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).
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
discrimination, etc.) can not be changed or adapted
to the actual conditions by the user.
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.
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.
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.
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.
The concept of the AS252x, however, does not
exclude
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.
In more technical terms, the AS252x works in the
following manner:
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.
Rev. 5.1
the
(thresholds,
human
factor,
time
but
constants,
provides
noise
the
Page 7
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.
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 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 (V
figure 5).
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.
AGC
(mV)
300
250
200
150
100
TH
50
0
Figure 5: Channel Control System
0.00
) is 15 mV assuming that V
Figure 6: Speech Compression
0.25
DETECTOR
PEAK
0.50
Microphone Input Signal
Sidetone Cancellation: 11 dB
Volume Control: 0 dB (neutral)
0.75
V
TH
AS2520/21/20B/21B
1.00
± 10 dB
1.25
VOL
(mV)
Z
AC
1.50
RX
> V
May 1999
2/4
V
V
TX
TX
RX
(see
V
LINE