SAN2202
6 Block diagram
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.
RX
Amplfier
Handset
TX
Amplfier
Transmit path
Z ACr
Acoustic feedback
Sidetonet
2-4
wire
interface
Line
LS
Amplfier
manual volume control
Receive path
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:
Transmit gain
Sidetone
measure to reduce
howling
reduce gain
increase sidetone
cancellation
decrease loudspeaker
volume
limit bandwidth
Note:
must meet the SLR limits
can be done by optimizing the sidetone network, the
better the sidetone cancellation, the more gain can be
given to the loudspeaker
for good performance, a maximum of loudspeaker
volume is desirable, therefore this should be the least
step
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)
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.
Loudspeaker volume
Loudspeaker
frequency response
acoustic coupling
between
loudspeaker and
handset microphone
increase distance between
loudspeaker and handset
microphone
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