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参数资料
| 型号： | PBL388131SO |
| 厂商： | ERICSSON |
| 英文描述： | Voice-switched Speakerphone Circuit with built in loudspeaker amplifier |
| 中文描述： | 语音交换与内置免提扬声器放大器电路 |
| 文件页数： | 16/16页 |
| 文件大小： | 299K |
| 代理商： | PBL388131SO |
16
PBL 388 13
will help the balance in the speech switching
a lot if the telephone is placed in a noisy
surrounding. It can not be required that the
other party has to know that he is talking
with somebody with a handsfree telephone
in a noisy environment and thus has to
shout to get through.
The circuit has no corresponding
function in the receiver channel in fear that
it would only worsen the performance. The
reason for this is that various tone signals
on the line are difficult to detect and to
separate because of the big level
differencies. A normal behaviour would be
that when one receives a high noise level
from the loudspeaker one automatically
rises ones own voice and compensates for
the noise in the other end thus functioning
as a noise compensation for the receiver.
There is a risk that the loudspeaker volume
would be turned down but in that case it
would be difficult to hear the other party
from the noise.
Something that can be tried in a
”sophisticated” handsfree telephone is, to
let the volume control influence the gain
slightly also at the input of the receiver.
The circuit does not contain any
automatic volume controls ( type AGC).
These kind of functions can of course be
included externally to the inputs of the
receiver and transmitter but it is very difficult
in this way to better the performance. The
speech switching is based to feel
differencies in signal levels where again
the automatic volume controls are working
to keep the levels constant. This results in
almost unsolveable problems with time
constants if these two systems are
combined. It is not even certain that
automatic volume controls are desirable. If
one stands on the other side of the room,
where the telephone is placed, facing it,
one automatically rises ones voice the
same way as one would do when speaking
with somebody standing further away. On
the receiver side we have a volume control
to set the desired level.
Loudhearing:
By setting the CTR control input high
with a resistor to +supply the circuit will go
into half speech control mode. The
amplifiers in the other half of the signal
paths G3 and G4 will be set into maximum
gain constantly. This does not alter anything
in the speech control function because the
hysteresis function is set by the other two
controlled amplifiers. The purpose with
this is to lead the signal from the handset
microphone via the speech control trans-
mitter channel and deconnect the
”handsfree function”. If the loudhearing
mode is active with the loudspeaker on,
there will be no oscillation when the hand-
set is placed close to the loudspeaker
which would be the case in normal mode
when lifting and returning the handset.
Because the microphone in the handset
has lower sensitivity related to the
handsfree microphone, the 25 dB speech
control that is used, is enough to counteract
oscillation. There are other solutions to this
problem but none has the same speech
quality than this one. This speech control is
needed so that the party in the other end of
the telephone line will not be disturbed by
the echo of his own voice, which can be
extremely disturbing.
The efficiency of the
loudspeaker power amplifier.
The PBL 388 13 has an extremely
high efficiency when it comes to convert
the existing line current to loudspeaker
output power. It is possible to make a
telephone line fed ”handsfree” telephone
with just under 10 mA of line current.The
current that is taken for the loudspeaker
power amplifier supply is set by resistor at
pin RE. The value of this resistor should
not be made so low that the speech circuit
will at any time ”current starve” as this
would cause high distortion on the line.
Because this kind of current feed system is
a co-operation between the speech circuit
and the power amplifier of the ”handsfree
circuit”, it will only function properly with
Ericsson speech circuits exept circuits
PBL3726/21 or PBL3853. (The two last
named circuits could feed the power
amplifier from the special supply they are
both providing). The voltage increases with
increasing line current across the resistor
RE, which results in, that optimum current
is taken at all line currents. The current is
fed into a reservoir capacitor between -C
and +L. The power amplifier is grounded at
the positive rail, this to avoid that the gro-
und would have a small level shift in case
the -L is used for ground. A level difference
in the ground between the circuits can
cause serious trouble in regard of RFI.
Everything is ground related to the two
possible points, those being the two
telephone wires. The reservoir capacitor
is chosen between 470 - 2200
μ
F dependent
on price contra efficiency. Because the
speech has a highly varying amlitude a big
capacitor will save energy to the real high
amplitude peaks. The power amplifier is a
simple output stage in order to render
maximum efficiency. A balanced output
stage would only lead to much increased
loudspeaker impedance, which is already
with a simple stage in the highest order.
The optimum loudspeaker impedance is
dependent on many factors like the
available voltage and current, if the
optimization is done agaist RMS value or
more towards speech like low RMS value
but with some high peaks. The optimum
loudspeaker impedance for RMS calculus
will be round 50 ohms, for speech ( music
power ) a 25 ohm loudspeaker is more
optimal and if it can be considered that it is
long time between the peaks, even a 16
ohm loudspeaker can be used.
Ericsson Components AB
S-164 81 Kista-Stockholm, Sweden
Telephone: (08) 757 50 00
Specifications subject to change without
notice.
1522-PBL 388 13/1 Uen Rev.A
Ericsson Components AB
January 1998
Information given in this data sheet is believed to be accurate and reliable. However no responsibility is assumed
for the consequences of its use nor for any infringement of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Ericsson
Components AB. These products are sold only according to Ericsson Components AB's general conditions of sale,
unless otherwise confirmed in writing.
Ordering Information
Package
Plastic DIP
Plastic SO
Plastic SO
Temp. Range
-20 to 70
°
C
-20 to 70
°
C
-20 to 70
°
C
Part No.
PBL 388 13/1N
PBL 388 13/1SO
PBL 388 13/1SO:T (Tape and Reel)
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| PBL388131SOT | 制造商:ERICSSON 制造商全称:Ericsson 功能描述:Voice-switched Speakerphone Circuit with built in loudspeaker amplifier |
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