8

PBL 388 13

Figure 15. Control modes as function of voltage applied to gain dynamics control

input CTR I

Figure 13. Transmit and receive gain as a function of V

Figure 14. Timing of the transmitter and receiver channels at the CMP-input.

Transmit

gain =

Receive

gain =

dB

dB

40

30

20

10

0

-40

-60

-20

0

20

40

60

V

-V

CMP

REF

____

30

20

10

0

-10

-20

mV

---------

Full transmit level

Full recieve level

Rxdet

Txdet

CMP

B

D

E

F

G

C

A

The comparator will increase the gain in the

transmitter channel and decrease it in the

receiver channel

accordingly. Signal at

Rxin will do the same but vice versa. The

voltages RxDetout and TxDetout control

thus the gain setting in respective channel

through the comparator using the CMP

input as a summing point. The attack and

decay times for the signals RxDetout and

TxDetout

are controlled by individual

external RC-networks. The attack time in

the receiver channel is set by C2 together

with C1 and by the maximum current capa-

bility of the detector output. The time cons-

tant is altered best by altering the value of

C2. The transmitter channel works likewise.

See fig. 7.

The decay time in the receiver and

transmitter channels is set by C2 and C3

respectively. The resistor in the time cons-

tant is formed by an internal 100k

Ω

resistor.The text above describes the case

when only one channel is open at a time

and there is a distinctive pause between

signals at receiver and transmitter channel

inputs so the circuit will have time to reach

its idle state. See fig.14 A) to E). If one of

the channels gets an input signal

immediately

after

the

signal

has

disappeared from the other channel input

the effective decay time, as the CMP input

sees it, will be shorter than in the first case.

See fig.14 F) to G). The capacitor C1 at

CMP - input sets the speed of the gain

change in the transmitter and receiver

channels. The capacitors C2 and C3 should

be dimensioned for a charging time of 0.5

- 10ms and for a discharge time of 150 -

300 ms. The question of switching times is

a highly subjective proposition. It is to a

large part dependent of the language being

spoken in the system, this because of the

varying sound pressure pattern in the diffe-

rent languagues.

A hysteresis effect is

achieved in the switching since the level

detectors sense the signals after F2 and F5

respectively (F2 and F5 are affected by the

gain setting). For example: If the transmit-

ter channel is open (maximum gain), a

signal to keep the transmitter channel open

is smaller than the signal that would be

needed to open the channel when the

receiver channel is open. The output swing

of the level detectors is matched for

variations in the supply voltage. The

detectors have a logarithmic rectifier

characteristic whereby gain and sensitivity

is high at small signals. There is a break

point in the curve at a level of

± 200mV from

the internal reference voltage (

≈2V), where

the sensitivity for increasing input signals

1

23

0

Disable

Mode

(V)

V

ref

Mute

50 dB speech

control

25 dB speech

control