Output voltage of the telephone line

[tuanvoi]

Hi all,
I've been trying to measure the AC voltage coming out the phone line when I talk, but I don't know for some reasons I've got noise at the output. Could you please tell me the range of this output (it would be good if you show me on the Oscilloscope). And also what chip should I use to amplify this AV signal? Thank you
Tom

 

[Mikebits]

Take a look at this link.
Telephone interfacing circuits

 

[Externet]

The audio level is from 0.2 to 0.7 VRMS for ranges of 'normal' speech loudness.
Higher and lower levels are still achievable and intelligible. DTMF tones are usually in the louder range.
Any audio chip can be used to amplify, but direct connection to the line will create noise.
The correct way to extract/inject audio from/into the telephone line is with an audio output transformer, the lowest impedance is highly preferred.
As its 8Ω winding (uuuu) in series to one of the telco wires. The higher impedance winding (nnnn) is your in/out.

L1------------------------------------------------------------L1
L2--------------------------uuuu---------------------------L2
............................A-----ññññ-----B

A and B are the points to handle. Ignore the dots.
Extra stuff... your post title could say "audio voltage" ; not "output voltage"
Miguel

 

[Willbe]

It's the current variation that carries the speech.

 

[crutschow]

The audio level is from 0.2 to 0.7 VRMS for ranges of 'normal' speech loudness.
Higher and lower levels are still achievable and intelligible. DTMF tones are usually in the louder range.
Any audio chip can be used to amplify, but direct connection to the line will create noise.
The correct way to extract/inject audio from/into the telephone line is with an audio output transformer, the lowest impedance is highly preferred.
As its 8Ω winding (uuuu) in series to one of the telco wires. The higher impedance winding (nnnn) is your in/out.

L1------------------------------------------------------------L1
L2--------------------------uuuu---------------------------L2
............................A-----ññññ-----B

A and B are the points to handle. Ignore the dots.
Extra stuff... your post title could say "audio voltage" ; not "output voltage"
Miguel

You need to add a resistive load between A and B otherwise the transformer will act like a choke and suppress the phone signal.
-----------
Abolish the deciBel?? You must be friends with Duffy.

 

[tuanvoi]

Thank you all, but then what is the correct answer? Is it a current variation(if so, how do I change this current to voltage) or voltage rms from 0.2 to 0.7 rms? Because I want to do ADC for this audio voltage. Could you please help me step by step? I'm new to this kind of stuff.
Tom

 

[crutschow]

Yes, you will see a varying voltage across the telephone line due to the varying signal current since the telephone power source is not zero impedance but it will tend to be noisy and distorted. The signal information is accurately contained in the current, so to properly monitor the signal you need to view the current, using a series transformer connection (as Externet showed) to convert it to a voltage across a resistor.

(You could also conceivable monitor the signal across a small resistor in series with the phone line using a differential amp, but that would require a diff amp that will tolerate the 48VDC and 80VAC ringing voltages of the telephone line. It's generally simpler to use a transformer for this purpose.)

 

[Externet]

Thank you all, but then what is the correct answer? Is it a current variation(if so, how do I change this current to voltage) or voltage rms from 0.2 to 0.7 rms? Because I want to do ADC for this audio voltage. Could you please help me step by step? I'm new to this kind of stuff.
Tom

The voltage available at terminals A and B will depend on the number of turns the nnnn winding has, and to the audio current in the telco line. The audio voltage in the telco line is in general words, 0.2 to 0.7 VRMS
Choose a transformer that can handle at least 50mA DC in the ~8Ω uuuu winding, the secondary winding nnnn can be any low DC current rating and try to find a low impedance or number of turns.

Edited/added: ---->Audio Output Transformer - RadioShack.com

The ringing voltage can put out over 100 Volts AC in the terminals A and B; if the transformer is inserted between the telco and the ringer.

A couple of inverse parallelled diodes to those terminals can limit the spikes, ringing or audio amplitudes to a safer 0.7V PP

If transformer insertion point is protected by a hook switch, the terminals A and B will not be exposed to ringing voltages and can feed your ADC safely and directly.

=====================================================

I do not understand the
... "You need to add a resistive load between A and B otherwise the transformer will act like a choke and suppress the phone signal."...

Resistor o no resistor, the transformer will always present inductance to the telco current; and in order to 'choke' the audio, it would have to be a huuuuuge impedance of thousands of ohms; which is not the case here.

Miguel

 

[crutschow]

I do not understand the
... "You need to add a resistive load between A and B otherwise the transformer will act like a choke and suppress the phone signal."...

Resistor o no resistor, the transformer will always present inductance to the telco current; and in order to 'choke' the audio, it would have to be a huuuuuge impedance of thousands of ohms; which is not the case here.

Miguel

The inductance may always be there but the impedance is a function of the load on the other winding. The inductive impedance of a transformer can be very high if the output is unloaded. A transformer reflects the impedance from primary to secondary and vice versa. If one winding is open circuit the other winding will also appear to be open circuit (equal to the open circuit excitation inductance of the transformer and the frequency.) That can indeed be henrys of inductance and thousands of ohms at audio frequencies.

 

[Externet]

OK, Carl. Thanks.
Yes, a load placed in AB will lower the impedance in the 8Ω side.

Assuming a typical 8Ω to 1KΩ (AB) transformer @ 1KHz, how to calculate the 'altered' 8Ω rated impedance winding if AB is left open ?

Tuanvoi: If a specific range of audio voltage is preferred for your ADC, a potentiometer across AB can attenuate it to your needs; or amplify it if a higher level is preferred. I would say a very low-still useable level at the telco line (not! at the AB terminals!) is 10mV. At the AB terminals the level will depend on the AB winding and the load placed to it.

Miguel

 

[crutschow]

Assuming a typical 8Ω to 1KΩ (AB) transformer @ 1KHz, how to calculate the 'altered' 8Ω rated impedance winding if AB is left open ?

You need to know the open circuit (magnetizing inductance) of the transformer, which is sometimes hard to determine from data sheets. Then you simply calculate the impedance of that inductance at the frequency of interest.

In this mode of operation the transformer is acting as a current transformer. Thus the telephone signal current (perhaps a few milliamps) is transformed by the inverse of the turns ratio to the output winding. This current is then transformed to a voltage by the resistance across the transformer secondary.

You might check this link Telephone interfacing circuits. The section entitled "Philips recording interface" shows a recording interface to a telephone line.

 

[mvs sarma]

the best pint is is to tap the energy of Receiver of the phone. when you speak, a pert of your speech is also made available at the receiver, called side tone. this would be exactly the same amount that you hear you speech acoustically at the other ear. thus you can measure it.
the speech as seen at the line point(a,b) will be higher, as it is meant for traveling across to reach the other party.

 

[tuanvoi]

Thank you all! Could you please tell me how to amplify this output voltage so that the max voltage is 5V and min voltage is 0V as reference for my chip "ADC0808" A to D conversion. What else do I need? Also I need to connect this chip with my PIC18F4550, do I need an interface code in order for them to be compatible with each other? Thank you again!

 

[mvs sarma]

Thank you all! Could you please tell me how to amplify this output voltage so that the max voltage is 5V and min voltage is 0V as reference for my chip "ADC0808" A to D conversion. What else do I need? Also I need to connect this chip with my PIC18F4550, do I need an interface code in order for them to be compatible with each other? Thank you again!

The average level of incoming speech may be low depending on the distance of the connection from the C.O.
the average level may be around -13 to -18Dbm.

0dbm across 600 ohms (telecom standard impedance is 0.775Vrms)
thus you might need to amplify the signal.
perhaps any audio amplifier can do your job and later whether you need DC or you can convert it as it is to Digital?
you may try out. i feel you don't need 5V. even 1Vrms level would do.

 

[crutschow]

Thank you all! Could you please tell me how to amplify this output voltage so that the max voltage is 5V and min voltage is 0V as reference for my chip "ADC0808" A to D conversion. What else do I need? Also I need to connect this chip with my PIC18F4550, do I need an interface code in order for them to be compatible with each other? Thank you again!

The ADC0808 has a 0 to 5V input. To convert an AC voice signal you will need to offset the input so that the DC level is 2.5VDC with no AC input. That way you can convert the +/- excursions of the AC voice signal. An op amp can be used to generate this offset.

Alternately you could use an A/D converter that can convert plus and minus signals (bipolar input).

 

[gelled01]

When your phone is off the Hook its on 50 volt out put and when on hook its around 30 volt.

 

[crutschow]

When your phone is off the Hook its on 50 volt out put and when on hook its around 30 volt.

Why are you responding to a nearly four year old thread?