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walid

telephone related question #4

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hi, i'm comig back ..... i love you all
my question is : All DTMF ICs used in telephone circuits have pin called
MUTE (pin 13), in HM9102 IC datasheet i read the following "This output is
an inverter normally at low state when there is no key entry. During
outdialing it changes to high state and is used to mute the speech network."
Assuming that low state = ground and high state = VDD = 4.3V, and looking to
Fig.3 i think that the inverse must be done, that is:"normally at high state
when there is no key entry. During outdialing it changes to low state and
is used to mute the speech network".
when it in low state, the two terminals of MIC are grounded so no voicesig
canout from MIC, thismust be done during dialing. is thisture?
thanks. 

post-2833-14279142296915_thumb.jpg

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Hi Walid,
The 2.4V on the schematic is proof that the mute pin is not high (about 4.3V) nor low (about 0V) normally when there is no key dialing. Maybe it is an open-drain P-channel Mosfet that conducts during dialing which shorts the mute pin and microphone to 4.3V, muting the microphone. ;D

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MIC has 2 pins one grounded (the -ve) and the other connected through a resistor to the power supply, here in our circuit, the +ve MIC terminal is connected through R17 + R15 to 7.4 Vdc. To MUTE this MIC, the +ve terminal of it should connected to Ground, if it connected to 2.4Vdc as shown in the diagram, i see that equivalent to replace the 7.4Vdc by (7.4-2.4 = 5Vdc), so i think that this pin (#13) gives two outputs:
(a) When dialing ====> Ground to mute audio network
(b) When no dialing ===> 2.4vdc to compensate the 7.4v to 5v.
is this true ?????????????

Secondly: you said: "Maybe it is an open-drain P-channel Mosfet that conducts    during dialing ....."
          I read about something like this in many datasheets but they said about open collector, please what this mean?
thanks

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Hi Walid,
I can't find a datasheet so I don't know if the mute pin has a PNP transistor's collector, an NPN one , a P-channel Mosfet's drain or an N-channel one.

"Open collector" is just the bare collector of a transistor without a resistor load. When it is turned-off it floats, so the circuit it is connected to can work normally.

The mute pin doesn't provide 2.4V. The divider from 7.4V of R17, R15 and the current through the microphone to ground determines the voltage. ;D

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Very good, this is what i looking for, floating i understand it, but please tell me:
(1) What happen when it turned on.
(2) What the difference between a PNP transistor's collector, and NPN one.

(3)Are you mean that the MIC acts as a resistor and the voltage drop across it =2.4Vdc 

thanks

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(1) What happen when it turned on.

Hi Walid,
If its collector load has the correct polarity, then it conducts, creating a low voltage between its collector and emitter.

(2) What the difference between a PNP transistor's collector, and NPN one.

A PNP transistor's collector has a negative supply to its load, and an NPN transistor a positive one. A PNP's collector pulls up and an NPN pulls down in a circuit with a positive supply.

(3)Are you mean that the MIC acts as a resistor and the voltage drop across it =2.4Vdc

The microphone draws a small current and produces a voltage drop because it is an electret type with an N-channel junction field-effect transistor inside that uses and modulates the current. ;D

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As shown in the schematic, the MIC signal travel from MIC to C7 (0.04uF) to the base of Q4 for pre amplification.
I read somewhere that part of MIC sig is fed back to the speaker driver (Q5) so that the user can hear himself, if they don't do this the user shouting.
Now is this part of  MIC sig passthrough:
(1) C5 directly without amplification to R20 then to base of Q5, if not why C5 placed here?
(2) Collector of Q5 (after amplified) then into R20?
(3) Both (1) & (2).
Thank you.

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Hi Walid,
A telephone feeds part of the mic signal to its earphone. A speakerphone needs to eliminate the mic signal from its speaker to avoid local acousical feedback howling. The transmitted mic signal is reflected back to its speaker at the telephone line, so frequently negative feedback is use to cancel it. I think C5 cuts high frequencies from being transmitted.

The mic and speaker signals are usually switched one-at-a-time in speakerphones to avoid local and telephone line feedback. The switching is usually done by detecting the level from the mic or with a digital echo canceller circuit. ;D

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Hi audioguru
Your answer above needs more explanation in the following points:
(1) I'm sure that I read some where that the tel. designers make part of MIC sig. fed back to speaker network and they do so to prevent the user of tel. from shouting. This is true, if you can't hear yourself you may think that there are some troubles in the device. On the other hand you tell me the inverse that they design circuits to eliminate this by negative feedback. Where is the truth?
(2) I understand you when you say: "The transmitted mic signal is reflected back to its speaker at the telephone line" I know that this is due to impedance mismatching, but please tell me where this negative feedback circuit in the companion diagram?
(3) You said: "I think C5 cuts high frequencies from being transmitted." tell me please, The MIC sig come from MIC and through C7 to B of Q4 for amplification. When and where C5 takes place in cutting high frequencies from being transmitted?
(4) You said:" The mic and speaker signals are usually switched one-at-a-time in speakerphones to avoid local and telephone line feedback". Is this done inside tel. device or in the central office? 
Thanks.

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(1) I'm sure that I read some where that the tel. designers make part of MIC sig. fed back to speaker network and they do so to prevent the user of tel. from shouting. This is true, if you can't hear yourself you may think that there are some troubles in the device. On the other hand you tell me the inverse that they design circuits to eliminate this by negative feedback. Where is the truth?

Hi Walid,
In a telephone handset the mic signal is fed to the earpiece so that the user can hear himself. This is called sidetone and you can look at this definition in Google. Speakerphones usually try to cancel as much mic signal from being fed to the speaker as possible, to avoid local acoustical feedback howling. Look at Motorola's MC34018 voice-switched speakerphone IC for a good discussion about cancelling feedback:
www.datasheetarchive.com

(2) I understand you when you say: "The transmitted mic signal is reflected back to its speaker at the telephone line" I know that this is due to impedance mismatching, but please tell me where this negative feedback circuit in the companion diagram?

I think that the R19 and R20 voltage divider cancels mic signals from the speaker.

(3) You said: "I think C5 cuts high frequencies from being transmitted." tell me please, The MIC sig come from MIC and through C7 to B of Q4 for amplification. When and where C5 takes place in cutting high frequencies from being transmitted?

C5 causes negative feedback from Q4's collector impedance of about 600 Ohms (the telephone line) to the base of Q4 that has a fairly high impedance. The feedback cuts high frequencies from being transmitted to the telephone line.

(4) You said:" The mic and speaker signals are usually switched one-at-a-time in speakerphones to avoid local and telephone line feedback". Is this done inside tel. device or in the central office?

This is done inside inexpensive speakerphones, see MC34018 speakerphone IC above.
The central office and expensive speakerphones and boardroom tele-conferencing and video-conferencing systems use digital echo cancellers instead of switching to avoid long-distance acoustical feedback howling and echoes. The digital echo cancellers are so good that you can have the mic directly in front of a speaker and it won't cause acoustical feedback howling. Polycom is the most popular speakerphone with a built-in digital echo canceller. ;D

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