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.Theacousticalcouplingwould
bemuchto high.Threedifferent acoustical
signalpathsareapparent.Thefirstthrough
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
casingcanbebestminimizedbydesigning
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.Thesimplestcountermeasureisto
mount the microphone in soft shock and
sound absorbing material,thesamegoes
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.
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
eventhesignalfromthetransmittingchannel
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
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
subtledifferencies.Whatisrightorwrong
in the end is hard to tell.
Transmitter or receiver
line. What can be studied here is, that the in priority:
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
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
thechannels.Thisrequirementisusually
coupled to some special features but is
alsousedin”primitive”handsfreephones
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
reasonforreceiverpriorityismore difficult
tocomprehend,maybethatthebuyerwill
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
greaterdegreeonthePBL38813.Ahigh
DimensioningofsignalpathsG1 opposite case ( signal path G6) but the ohmic resistor from +supply to the
to G6.
sidetone balancing can normally be made
thatgoodtopreventthissignalpathtocause
problem.
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.
The +input of the receiver channel is
connected to the receiver signal output at
the sidetone network either via a capacitor Dimensioning of filter:
or a filter. Signal path G2. The sesitivity is
made to suit directly. If clipping of signal is
experiencedinthechannelthesignalmust
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 inputs of transmitter and receiver
amplifiers ought to have simple filters Background noise
according to the application in order to be compensation:
abletosetandlimitthefrequency 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.
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
Dimensioning of time constants: required an integration capacitor is
The maximum receive gain is set by a
resistorinserieswiththeac.volumecontrol.
ThisendsthedimensioningofthepathG4.
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
The charging time of the detectors
(negative for the transmitter, positive for the
receiver)isdeterminedbythedrivecapacity
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-
coupled from the output to ground and a
resistor from the output to comparator
input.Thisresistordeterminestheamount
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-
teminslightlyundercompensatingmode
will help the balance in the speech
13
ERICSSON [ ERICSSON ]
