SAN2202

4/9

sames

sames

sames

sames

Fig. 1 shows the block diagram of the speech paths: the transmitted signal is fed to the line via the 2-4 wire

interface and the receive signal coming from line is fed to the Rx amplifier. The same Rx signal passes an

attenuator for volume control and is then amplified by the loudspeaker amplifier.

Transmit path

Receive path

manual volume control

LS

Amplfier

TX

Amplfier

RX

Amplfier

2-4

wire

interface

Z ACr

Line

Fig. 1: speakerphone block diagram

6.1 how to avoid howling

As can be seen on Fig.1, there is an acoustic loop from

loudspeaker

⇒ acoustic feedback (determined by the mechanical distance between loudspeaker and handset

microphone

⇒ Tx amplifier ⇒ sidetone ⇒ LS amplifier.

Howling will occur, when the total loop gain is

≥1 ( equal to ≥ 0dB) . Therefore the following conditions will have an

effect in loop gain:

Parameter:

measure to reduce

howling

Note:

Transmit gain

reduce gain

must meet the SLR limits

Sidetone

increase sidetone

cancellation

can be done by optimizing the sidetone network, the

better the sidetone cancellation, the more gain can be

given to the loudspeaker

Loudspeaker volume

decrease loudspeaker

volume

for good performance, a maximum of loudspeaker

volume is desirable, therefore this should be the least

step

Loudspeaker

frequency response

limit bandwidth

howling will occur with the frequency of the highest

amplification. Therefore the frequency response curve

should be as flat as possible and limited to the speech

band (

≈ 300..3400Hz)

acoustic coupling

between

loudspeaker and

handset microphone

increase distance between

loudspeaker and handset

microphone

In normal operation, the distance can be down to a

few centimeters, when the handset is put onto or off

the cradle. The loop should be adjusted such that in

normal speech mode there is no howling at distances

>5..10 cm. Additionally, the loudspeaker can be

mounted away from the handset microphone position,

like on the side or on the rear, if possible.

6 Block diagram