# telephone recording ?

Discussion in 'General Electronics Chat' started by Mathematics!, Dec 8, 2009.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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OK , I measured the voltage across the red and green wires it is about 47 volts dc

Then I measured the voltage when the phone is off the hook it is about 7.6 volt dc

Then I measured the current flowing when the phone is off the hook it measures around 37.5mA dc

I did this by attaching my multimeter to the red wire could have used the green but whatever....

Note this was all with my multimeter in dc mode.

Then I tried it in AC mode

To see if I could get the 90 volts 20 hz ac ring but to much my suprise is that when somebody called me I got the voltage going from 0 to 56 ish volts in ac mode. That means if this is the rms value to get the peak I would have around 80 volt ac that is 10 volts differents seems like alot.
Is it common to have this much difference?

Also I am wondering the current is 37.5mA when the phone is off the hook
when the phone is on the hook it has 0 current.
If I start talking on the phone while I am measuring the current/voltage I would think I should see the current/voltage flucation but to much my suprise I don't see this it stay's pretty fixed when it is on the hook it is 47 volts dc when off the hook it is 7.6volts dc.

When I put it into ac mode I don't know where to adjust it to see the voltage/current voice signal changes or at least some fluctation???

Anyway I am trying to convert the voltage/current of the phone into a proper voltage/current for a sound cards mic input or line in input so I can record my phone conversation.

I believe the mic port is around 3mV like an electric mic would give out with out any amplification...etc

Maybe to see these signals my multimeter can't proccess this quickly dun ?

Either way if I use a ton of resistors to drop the voltage down to mV level I would also be droping the current as well as the voltage at the same time... I would think this would drop the current even to low for the mic port on a soundcard.
(series drops both current and voltage )

I have an audio transformer could this be used ?

Thanks for any help

Last edited: Dec 8, 2009
2. ### S_lannan Active Member

Jun 20, 2007
247
2
did you just put the current meter across the two telephone wires?

3. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
well orignal I did it the wrong way by conneting the ends of the red and green wires to the ends of the 2 probes on my DDM to get the 37.5mA

How ever if I cut the red or green wire and attach the DDM between both sides of the wire will this be the way to go for measuring the current. When I do that I get 0mA of current when it is on the hook and when it is off the hook again I get around 37.5mA in dc mode.
Should I have my DDM in ac or dc mode ?
And will I even beable to see the tiny fluctation of current because maybe the DDM doesn't respond fast enough for the 20 to 20khz voice signal ???

Also I am curious why the voltage drops to around 7.6 volts dc when the phone is off the hook? If the telephone company is supplying you with +- approx 48 vdc then how does taking the phone off the hook drop the voltage so much? ( it must be adding more resistance in some way??? )

Thanks for any help

4. ### jpanhalt Expert

Jan 18, 2008
6,940
1,449
So why are you doing this?

You seem to have more than enough projects started to keep you busy. Messing with the telephone system in the USA is illegal. Even though Maine is a one-party state, any device you attach to the line must be approved.

John

5. ### Externet AAC Fanatic!

Nov 29, 2005
1,320
167
Hi.
Part of the telephone central office circuit is similar to

(+)---------uuuu----------------------\/\/\/\/-------------------- A
50V
(-)----------nnnn----------------------\/\/\/\/---------------------B

Being A & B at your home end
The coils uuunnn being actually relays
The resistance being the copper pair from the CO to your home

Typical values are 200Ω each coil ; 500 Ω each wire

Nothing is going on if there is no current between A & B, you see 50V

When a telephone is connected to A&B, current flows and the relays engage

The voltage at A&B in seized condition is around 7V ---> Ohm's law
Total resistance = 1400Ω
50V ÷ 1400Ω = 35mA Got it ?

The audio in normal speech is about 0.2 VRMS AC (A to B) riding on top of the DC.

To connect the audio from the telco line to any recorder, the DC has to be isolated and the balance kept. You need an audio output transformer, with the 8Ω winding in SERIES to any ONE of the wires.
The primary can feed the recorder, or amplifier, by "line" in.

-----------------------------------------------------A
----------------------uuu--------------------------B
primary-------->nnnnnn
1 diode----------> |<|
another---------> |>|

Works also in reverse, to inject "line" level audio to the telco line, it is safe to you and the telco, does not impair the line, and is not detectable, can be left permanently connected BUT protection must be used against ringing! with counterparalleled diodes at the primary or your recorder circuit will die.

Miguel

Last edited: Dec 8, 2009
6. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I got the current calculation

From this I am confused how you got the 7V for phone off hook?

if the current is 35mA then the sum of the 2 relays and the wire resistance times the current (KVL should hold?)

gives 35mA * 200 = 7vdc
35mA * 500 = 17.5 vdc

adding the 2 relays and wires gives
7+7+17.5+17.5 + phone voltage = 50vdc

so the phones voltage should only be 1vdc using KVL but I measured about 7.6vdc off hook??????

I am confused alittle bit about the nnnn uuuu thing in your diagram maybe that is what is screwing up my calculations.
Why would you ever need those 2 relays infront of the battery? I mean I have used relays in stuff like controling high voltage circuits with low voltage ones also protecting components in HVAC circuit but cann't see why use them in this situation? Maybe it protection against the customer connecting or charging a car battery with it or something duno

Also is there any meaning to your VVVV or are you just using this to begin the wires ?

As for this

Cann't under stand your diagram ? I do get The audio in normal speech is about 0.2 VRMS AC (A to B) riding on top of the DC though.

the primary wires of the transformer is on the input to the soundcard side?
Where do I place this diode thing and how do I protect against the ring voltage ,...etc?

The diode block current flowing in one direction while lets it flow in the other direction but I don't see how this stops the high plusating dc voltage when the ringing occurs ???

Just alittle unclear on some of what you have here.

Last edited: Dec 9, 2009
7. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Can anybody answer these last questions. I am egar to understand this.
Maybe Externet is out their some where.

Thanks

Sep 26, 2009
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9. ### SgtWookie Expert

Jul 17, 2007
22,201
1,806
Before you proceed with this project, you need to be aware that there are laws regarding the recording of telephone conversations. The laws may change from time to time. There are Federal laws, and there are also State laws. You need to be in compliance with the Federal laws and any State laws, or you may find yourself in a great deal of trouble.

Here is a helpful resource written in layman's terms, primarily for news reporters: http://www.rcfp.org/taping/index.html

A quick reference by state: http://www.rcfp.org/taping/quick.html

Another resource: http://www.callcorder.com/phone-recording-law-america.htm

Note that the FCC requires a distinct periodic "beep" tone to notify both parties that the conversation is being recorded - just one of the requirements.

10. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Thanks but I am still curious on my last big post questions.

Also can I substitute
for the 1X 600 ohm to 600 ohm modem transformer
with radioshack 273-1380 audio output transformer 1kohm center tapped to 8 ohms. ( I don't have the ability to get a modem transformer and I don't have any old modems that I want to destroy to get this part ...?)

Also I don't see in the circuit diagram how the 2 zener diodes are going to
prevent anything won't the 90volts 20hz ring still go around the zener bridge and reach the RCA out/in jack?

The diode back to back would only be preventing current from flowing across the 2 diodes not around the outer loop to the RCA out/in? I don't see how this is even nessary and what it is preventing

Thanks

11. ### BMorse AAC Fanatic!

Sep 26, 2009
2,675
241
No you can not swap an isolation transformer for an audio transformer, not the same thing.

and also, no,

12. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Why does it matter on blocking the dc the transformer only works on ac so I don't see why the major concern in blocking it out. I guess I don't know how dc effects a transformer? I would think only pulsating dc could effect the transformer?

4.7K resistor limits the current of the 90 ringing signal OK get this but won't it also reduce the current/voltage of the ac voice signal and if you reduce this signal to much then it must be amplified? ( I am unsure about how much you can reduce the voice signal before it can't be used 4.7K seems pretty arbitrary?)

The zener diodes clamp the 90 volt ringing signal and other transient spikes to protect your recorder
I don't see how 2 diode back to back can do anything in this circuit current is still going to bypass it around the big loop and it would be like not having that 2 diode line anyway?

The 10K potentiometer is used to adjust the level to the tape recorder
Ok got this since providing more resistance you drop more voltage/current and that makes the volume lower and if you use less resistance the volume is louder.

Thanks for clearing up any of these questions.
And yes I am fully aware of the laws I don't plan on using it in a bad way.

Last edited: Dec 9, 2009
13. ### Externet AAC Fanatic!

Nov 29, 2005
1,320
167
Hi.

Those 35mA are shorting A to B.
In reality, a telephone connected at A & B presents about 200Ω. Voltage across it showed in your case ~7V.

...Why would you ever need those 2 relays infront of the battery?...

The relays click when you lift the handset to enable the dialtone, cut ringen off, and voice path is routed away (I said it is a partial circuit for you to understand step by step)

... I am confused alittle bit about the nnnn uuuu thing in your diagram...

uuuu ; nnnn = coils schematic symbol
\/\/\/\/ = resistor schematic symbol

...radioshack 273-1380 audio output transformer 1kohm center tapped to 8 ohms...

That is a good one for the application. NO 600Ω.

...the primary wires of the transformer is on the input to the soundcard side?...

Yes.

...Where do I place this diode thing and how do I protect against the ring voltage...

Parallel to the primary winding, as kind of shown.

...Why does it matter on blocking the dc the transformer only works on ac...

Because you want only audio AC and need to keep the DC path for the telephone intact. The audio will show up at the primary, and the DC and AC will pass trough the secondary to the telephone.

Miguel

Last edited: Dec 9, 2009
14. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Then from this how do you calculate the 7 ish vdc that you get when the phone is connected to the Green and red wires and off the hook?

Because when I use KVL it must be a voltage drop of 1vdc between the phone? Unless your saying the current is much less then 35mA when the phone load is provided and of the hook? (but I measured the current when the phone was of the hook and it measured around 35mA so with this the phone must drop 1vdc and the resistance of the phone must be 1/0.035A = 29ish ohms

Anyway I still don't see how the zener diodes are preventing anything.
I am more into understanding the circuit then building it.
Building it is the easy part.

If you remove the zener diode line won't the circuit be the same because no current is flowing thru the diode line.... don't see how this is doing anything against protection against the ring????

ALso why do we need this transformer doesn't the capacitor and resistor take care of blocking the dc while filtering and lowering the ac voltage/current down to a suitable level?

I just don't get why the need for the transformer and diodes.
Essentially you should beable to go from capacitor right to the pot (varible resistor.

Also I am curious about how they determined what capacitance to use and resistance values to use? their must be a reason.

15. ### Externet AAC Fanatic!

Nov 29, 2005
1,320
167
Connect a 200Ω between A&B and use Ohm's law.

Remember I said typical values. Every house is different.

Where did I mention Zeners? Diodes conduct above 0.7V 'shorting' the ring.

Where in my responses I mention any capacitor ?

To isolate DC and to clamp high ringing voltage

NO. will destroy the line balance introducing noise.

Who is they ? What capacitance ?

Miguel

16. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4

http://www.solorb.com/elect/phone/tap/

Why capacitance of 0.22uf was chosen?
Why resistance value of 4.7Kohm 1/2Watt ?
Why transformer rating 600ohms to 600ohms?

I get the 10Kohm pot for the volume

AND MY BIGGEST QUESTION
Why the 2 zener diodes back to back I still don't see how this is preventing any voltage/current when the 90vac at 20 hz is induced into the transformer it will flow thru the outer loop the inner loop won't do anything except block voltage/current since you have 2 diodes back to back in that line????

Last edited: Dec 9, 2009
17. ### BMorse AAC Fanatic!

Sep 26, 2009
2,675
241

Lets start with the zener diodes.... read this so you might get a better understanding on how it is used to "clamp" to prevent an AC voltage from exceeding a specified value... http://sound.westhost.com/appnotes/an008.htm

My .02

18. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Ok, I understand the zener diodes are different then regular diodes in that they will have a breakdown voltage in which case they will conduct backwards as well as forwards....etc

But then 10K pot is really in parrell with the zener diodes so you would have to have same voltage 90 across both places.

All though the current would change.

But I would think 90ac would be way to high for the input/output of a soundcard regardless of what the current is?

19. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Never mind about the diodes I should have done more research and read the allaboutcircuits chapter on diodes.... anyway figured that out.

But I am still stuck on what this isolated transformer is needed for?
This transformer has the same coil ratios so it is not steping down or up current or voltage in anyway. All it is doing is seperating the phone line circuit from the recorder side. But I would think it would work if I just used the capacitor and resistor and just add another resistor that was 600ohms that would be the equivalent of the 600ohm transformer or close to it.???

As for the resistor and capacitor values why did they choose those???

Thanks

Jul 17, 2007
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