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SL34118

Voice Switched Speakerphone Circuit

System Logic Semiconductor 

SL34118

ATTENUATOR CONTROL BLOCK

The Attenuator Control Block has the seven

inputs described above:

- Tthe output of the comparator operated by RLO2

and TLO2 (microphone/speaker side) - designated

C1.

- The output of the comparator operated by RLO1

and TLO1 (Tip/Ring) side) - designated C2.

- The output of the transmit background noise

monitor - designated C3.

- The output of the receive background noise

monitor - designated C4.

- The volume control.

- The dial tone detector.

- The AGC circuit.

The single output of the Control Block controls

the two attenuators. The effect of C1-C4 is as follows:

Inputs

Output

C1

C2

C3

C4

Mode

Tx

Tx

1

X

Transmit

Tx

Rx

Y

Y

Fast Idle

Rx

Tx

Y

Y

Fast Idle

Rx

Rx

X

1

Receive

Tx

Tx

0

X

Slow Idle

Tx

Rx

0

0

Slow Idle

Rx

Tx

0

0

Slow Idle

Rx

Rx

X

0

Slow Idle

X = Don’t Care; Y = C3 and C4 are not both 0

A definition of the above terms:

1) “Transmit” means the transmit attenuator is

fully on (+6.0 dB), and the receive attenuator is

at max. attenuation (-46 dB).

2) “Receive” means both attenuators are

controlled by the volume control. At max.

volume, the receive attenuator is fully on

(6.0 dB), and the transmit attenuator is at max.

attenuation (-46 dB).

3) “Fast Idle” means both transmit and receive

speech are present in approximately equal levels.

The attenuators are quickly switched (30 ms) to

idle until one speech level dominates the other.

4) “Slow Idle” means speech has ceassed in both

transmit and receive path. The attenuators are

then slowly switched (1 second) to the idle

mode.

5) Switching to the full transmit or receive modes

from any other mode is at the fast rate (30 ms).

A summary of the truth table is as follows:

1) The circuit will switch to transmit if: a) both

transmit level detectors sense higher signal levels

relative to the respective receive level detectors (TLI1

versus RLI1, TLI2 versus RLI2), and b) the transmit

background noise monitor indicates the presence of

speech.

2) The circuit will switch to receive if: a) both

receive level detectors sense higher signal levels

relative to the respective transmit level detectors, and

b) the receive background noise monitor indicates the

presence of speech.

3) The circuit will switch to the fast idle mode if the

level detectors disagree on the relative strengths of

the signal levels, and at least one of the background

noise monitors indicates speech. For example,

refferring to the Expanded Logic Diagram (Figure 8), if

there is sufficient signal at the microphone amp

output (TLI2) to override the speaker signal (RLI2),

and there is sufficient signal at the receive input

(RLI1) to override the signal at the hybrid output

(TLI1), and either or both background monitors

indicate speech, then the circuit will be in the fast idle

mode. Two conditions which can cause the fast idle

mode to occur are a) when both talkers are attempting

to gain control of the system by talking at the same

time, and b) when one talker is in a very noisy

environment, forcing the other talker to continually

override that noise level. In general, the fast idle mode

will occur infrequently.

4) The circuit will switch to the slow idle mode

when a) both talkers are quiet (no speech present), or

b) when one talker’s speech level is continuously

overriden by noise at the other speaker’s location.

The time required to switch the circuit between

transmit, receive, fast idle and slow idle is determined

in part by the components at the CT pin (Pin 14). A

schematic of the CT circuitry is shown in Figure 4 and

operates as follows:

- RT is typically 120 kΩ, and CT typically 5.0 µF.

- To switch to the receive mode, I1 is turned on (I2 is

off), charging the external capacitor to +240 mV

above VB. (An internal clamp prevents further

charging of the capacitor.)

- To switch to the transmit mode, I2 is turned on (I1

is off) bringing down the voltage on the capacitor

to -240 mV with respect to VB.

- To switch to idle quickly (fast idle), the current

sources are turned off, and the internal 2.0 kΩ

resistor is switched in, discharging the capacitor

to VB with a time constant = 2.0 KΩ x CT.

- To switch to idle slowly (slow idle), the current

sources are turned off, the switch at the 2.0 kΩ

resistor is open, and the capacitor discharges to

VB through the external resistor RT with a time

constant = RT x CT.

SLS

System Logic

Semiconductor

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