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参数资料
| 型号： | PBL388141SO |
| 厂商： | ERICSSON |
| 英文描述： | Voice - switched 2-channel Circuit with loudspeaker amplifier |
| 中文描述： | 语音-开关2声道扬声器放大器电路 |
| 文件页数： | 13/14页 |
| 文件大小： | 239K |
| 代理商： | PBL388141SO |
13
PBL 388 14
possible. If a simple telephone casing is
considered, it could be a box with a large
hole for the loudspeaker and a small hole
for the microphone. This would normally
not function. The acoustical coupling would
be much to high. Three different acoustical
signal paths are apparent. The first through
the air outside the casing, damped best
by observing that the signal has no direct
path or can be reflected for ex. by a hard
table surface from the loudspeaker to the
microphone. The second path inside the
casing can be best minimized by designing
both the loudspeaker and the microphone
into individual compartements only open
to the outside world. The third path would
be the one through the material of the
casing. The simplest counter measure is to
mount the microphone in soft shock and
sound absorbing material , the same goes
also for the loudspeaker. There are a
number of other, besides these, principal
requirements on the acoustical coupling
between loudspeaker and microphone.
One being to make the microphone
sensitive for the user so that the gains in
the paths G1 and G3 can be made low,
furthermore to get it such that the room
acoustics do not disturbe. The speech
switching helps in this regard quite a bit by
having the loudspeaker damped in the
transmitting mode and the microphone
damped in the receiving mode which ma-
kes that the other party at the other end of
the telephone line will not get disturbed by
hearing his own voice.
Dimensioning of signal paths G1
to G6.
The +input of the receiver channel is
connected to the receiver signal output at
the sidetone network either via a capacitor
or a filter. Signal path G2. The sesitivity is
made to suit directly. If clipping of signal is
experienced in the channel the signal must
be attenuated at the input. A high sesitivity
is desired to have the speech switching
working at low signal levels thus being
inaudible, where at the same time the
receiver input has to function with high
dynamic range. The differencies in input
signal levels can be 20 dB or more.
The maximum receive gain is set by a
resistor in series with the ac. volume control.
This ends the dimensioning of the path G4.
The signal from the microphone is
coupled via a capacitor to the transmitter
channel +input. The wanted sensitivity in
the signal path G1 is set by the current
feeding resistor to the microphone. A
balance between the signals in both
channels reaching their detectors should be
attained. This can be studied with a two
channel oscilloscope one channel attached
to each ”handsfree” channels detector out-
put. The volume control should be at maxi-
mum setting and the study should be made
with different signal levels and insignals at
both microphone and from the line.
The final study should take place when
even the signal from the transmitting channel
with suitable attenuation is coupled to the
speech circuit transmitter. This completes
the signal path G3 and sets the transmitting
gain from the microphone to the telephone
line. What can be studied here is, that the in
signal at the receiver causes in many cases
a signal at the transmitter detector. This is
the signal path G5. In a good design this
signal path must be well damped. If the
signal G5 itself reaches to same level of
outsignal as the insignal there is a risk that
the system switches itself to transmitting
instead of receiving which results in a pulsa-
ting tone. In a good quality ”handsfree”
telephone this kind of behaviour must be
solved by decreasing the acoustic coupling
between loudspeaker and microphone. In a
budget type of telephone other solutions
may have to be considered like lowering the
maximum gain in the receiver by means of
higher series resistor with the ac. volume
control or to unbalance the detectors slighly
with lower gain in G1 (naturally with less
attenuation to the transmitter of the speech
circuit in order to keep the G3 constant).
Same kind of crosstalk exists also in the
opposite case ( signal path G6) but the
sidetone balancing can normally be made
that good to prevent this signal path to cause
problem.
Dimensioning of filter:
The inputs of transmitter and receiver
amplifiers ought to have simple filters
according to the application in order to be
able to set and limit the frequency behaviour.
More complex filters can be applied at the
detector inputs. In the application used are
Only low frequency limiting coupling
capacitors are used in the application, this is
adequate in most of the cases.
Dimensioning of time constants:
The charging time of the detectors
(negative for the transmitter, positive for the
receiver) is determined by the drive capacity
of the rectifier and the size of the external
capacitor. The speed of the charging (at-
tack) is highly due to a personal feeling, also
somewhat dependent of the language at
hand and can be set by the capacitor at the
respective detector output. Even the dis-
charge (decay) time can be altered by
high ohmic resistors from the respective
detector output to + supply or to ground.
The values in the application serve as a
good starting point. The capacitor at the
comparator input that sets the switching
speed can also be varied one or two
values up or down in order to get a good
”feeling” for the system. The question of
the system quality is an extremely
subjective proposition and is based on
subtle differencies. What is right or wrong
in the end is hard to tell.
Transmitter or receiver
priority:
There is sometimes a requirement
of either transmitter or receiver priority of
the speech switching. This means that
the speech switch will not rest at idle
position, in (no signal in either channel)
condition, but is biased towards either of
the channels. This requirement is usually
coupled to some special features but is
also used in ”primitive” handsfree phones
where the transmitter priority will make it
to sound better for the other party and
saves him from suffering that the first
party has a bad handsfree phone. The
reason for receiver priority is more difficult
to comprehend, maybe that the buyer will
be given a feeling that he got more value
for his money by hearing the other party
better. Priority is an unwanted feature
while ruining the speech switching
balance, it can be introduced in lesser or
greater degree on the PBL 388 13. A high
ohmic resistor from +supply to the
comparator input will move the system
towards receiver priority where a high
ohmic resistor from the comparator input
to ground will move the system towards
transmitter priority.
Background noise
compensation:
There is a detector at the transmitter
rectifier that senses continuous signals
like fan noise or noise from many people.
In case the function it is not required the
external components at its output are
simply omitted. In case the function is
required an integration capacitor is
coupled from the output to ground and a
resistor from the output to comparator
input. This resistor determines the amount
of compensation. Care has to be taken in
order not to over compensate by making
the resistor too small, it can result in
hook-up fenomena. By setting the sys-
tem in slightly under compensating mode
will help the balance in the speech
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相关代理商/技术参数 |
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| PBL388141SOT | 制造商:ERICSSON 制造商全称:Ericsson 功能描述:Voice - switched 2-channel Circuit with loudspeaker amplifier |
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