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I'd like to connect my landline tellephone to my computer's soudcard, and I was about to do this when somebody warned me that the voltages are all wrong and that I will blow my soundcard. To what voltages must I change the line, and how do I do that? Also, the telephone line has two wires. How to I split them into input & output for my soundcards's ports? |
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You can get a transformer from an damaged modem, like that:
Reminder: In my country (Brazil) it is forbidden to record a phone conversation without the consent of another person on the phone. |
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If the goal of this action to record the call or to use computer headset for telephoning, it is more safe solution to buy a voice modem. Costs 10-20 USD and there are plenty of software to record sound, make answering machine, etc. It is right that you cannot connect the phone line directly to your sound card. There is around 80-100 V AC in the phone line when it is ringing! And for legal reasons you cannot connect any device to the line which is not approved by local agencies. However, you can use an audio transformer to isolate the sound component from the dc voltage, but you have to make some protection circuit in case of ringing. |
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Basically sounds me to like you need a way to make your computer pretend it behaves the way a conventional POTS phone does. Traditionally, you interface with the phone line via a modem on the PC. Some modems allow for voice calls via PC, sometimes called a voice modem. If your'e cool with writing software or using a Hyperterm window, you can try controlling the modem in voice mode using AT commands and a software package. You can do pretty much anything to a phoneline by sending the right combination of AT commands to the modem. This assumes your'e cool with experimenting with the software. For this use, I'd use a modem rather than implementing a hardware-based solution. One, because it is cheaper, and two, because your'e likely to get significantly better results with the debugged PC<->PhoneLine interface that the modem provides. You can make a hardware solution, too, but it may take a while, be expensive to figure out, and may not work reliably - but you'll learn a whole lot more. Depends on what you want to do. |
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It's a little bit tricky. Indeed, there's a DC voltage present when the phone is on the hook (usually 48V) and the ringing signal is high-voltage AC, as in old times it used to physically drive a motor to ring a bell. The two wires you refer to are commonly referred to as 'tip' and 'ring', which again dates to olden times when operators used to manually switch calls by moving plugs around - one wire was to the tip of the plug, the other was behind the insulating ring (look at a headphone plug if you need a visual aid). I believe both incoming and outgoing voice are mixed and sent on one wire with reference to the other. A "safer" way (but also most likely illegal) to do this would be to simply hook the sound card line input to the earpiece of an old handset that you don't mind butchering. You'll have to take that phone off the hook to make the speaker active, but you'll hear both what's being said on your end plus the remote end (since there's a small amount of audio feedback from the talker sent to the talker's earpiece). You'll also (hopefully) not be exposed to the -48V and the ringer voltage - you may wish to put a small capacitor in series with the soundcard connection to isolate any DC on the speaker side. Good luck! |
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This may be oldschool, but how about a (small) balancing inductor (transformer)... |
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If you want to record the telephone line, here's a cable on eBay. |
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