Noises in phone line when no phones are in use

Hi!

I wonder--do you have any cordless phones on the line?

The two chirps you're hearing remind me of what I heard when I listened to my 900MHz phones on a scanner. Each time one hit the base (e.g. put back in the base for charging purposes), there would be a brief burst of communication in the form of tones.

From what I've found out, this is the handset and base agreeing upon a security code and communicating it to one another. If you're not hanging up phones, perhaps someone with a similar phone nearby is doing so and your cordless phone base can hear it. Or perhaps your phones rotate codes randomly or in the presence of interference. In a heavily enough populated area, there might not be enough frequencies and security codes available to give each phone a unique pair.

There's only on reason I can think of that your phone line might have conversations on it when nobody is using it...and that would be the possibility that your cordless phone receiver drops into something of a "listening" mode on any valid frequency...and, again, that there are lots of cordless phones running on the same band in your area.

Not sure about the 60Hz hum, but it might be a ground loop problem. As for the ringing type sounds, I remember someone somewhere saying that phone companies did sometimes put test signals on the line. Do you see these signals on a regular timetable?

Considering that mixers and phone lines really don't go together, you may be inducing some of these strange noises when you connect the two.

William

Your phone line is always 'live' and connected to the SLIC at the exchange. These days this is a bus-connected electronic card, and for sure, there will be leakage out onto the line, of supervisory tones and other exchange related control stuff that you would not normally hear. Also, your copper pair is coming possibly miles from your exchange, and will pass by all sorts of EM interference sources, which will induce whistles, pops and so on into the line, which in theory is balanced, so shouldn't respond, but in practice, seldom is, so does.

You should also consider that although the exchange equipment and modern phones are now all electronic, the actual transmission and ringing system is legacy, which means that it runs at the comparitively high voltage that the original electro-mechanical phones needed to get them going. The upshot of this is that whilst a modern phone could actually work with just a few mV of incoming audio, in order to maintain compatibility between it, the system, and legacy phones, it's input is deliberately attenuated such that it too needs volts of signal to produce audio. A side effect of this, is that low level phone company electronic signalling, which is deliberately placed on the line by the phone company, and line induced interference, are also attenuated by the same amount, so become insignificant at only a few mV at your line entry.

Now consider the mixer desk that you are glueing on the end of your line. That is intended to work with low level signals from musical instruments, or other audio equipment, and will be sensitive to those otherwise unheard signals, so suddenly they become audible to you.

I don't know exactly how you are isolating the line and connecting it to this mixer, but it strikes me that you could be dicing with a big problem here. For a start, there are several tens of volts across the line in its idle state. This increases to around 90v peak when a ring signal appears across the line. The SLIC at the exchange detects an off-hook condition by looking to see when the line has been looped by a phone, and a DC current starts to flow. If your mixer has got an electrolytic capacitor at the iput that you are using, when you hit it with the 90v ring signal, there will likely be a substantial leakage current through the input circuit. The SLIC may well see this as a looped line condition, and remove the ring signal, hence the reason that you get one ring and no more. When the ring signal is removed, the current in the input circuit of the mixer, will cease again, and the SLIC will drop the call, believing that the 'phone' has been replaced on hook.

I think that you should seriously consider what you are doing here. The large levels of voltage that are present in the phone system, particularly the ring signal, are highly likely to ultimately cause damage to the mixer input circuitry. Also, it is causing a problem with your phone calls. Another possibility is that the exchange's routiners, which scan the subscriber lines for faults, may well pick up the "odd" impedance that this set up is like to be placing across your line, as a fault, that may get flagged as one for a serviceman to come and check. Phone companies also frown on unauthorised connections to their lines, which might result in an unbalance, and they get particularly upset about connection of non-galvanically isolated equipment that runs from household power, due to the potential danger that it could represent to their linemen, in the event of a failure that placed household voltage on the phone line.

Arfa

Arfa is right, be careful what you do to the phone company line. I designed several central office test line circuits in my years at Bell Labs. One circuit was used to detect false power crosses and grounds on subscriber lines, and put fairly high voltages out onto the subscriber lines when the central office was suspicious of a line problem. Also the 90VAC ringing signal has a peak voltage of about 125 volts and can give you a very nasty surprise if you happen to be touching the line when the phone is either ringing or perhaps being tested because they sense something abnormal about the line.

H. R.(Bob) Hofmann

Thanks for the replies.

To be clear, I was NOT hearing conversations EXCEPT when the phone was IN use. However, the NOISES I was hearing were when the phone was NOT in use.

I do have cordless phones but I believe the chirping tones were still was happening even if I disconnected those phones from the phone jacks.

Since this was interfering with the ability of the phone to ring, I've abandoned the idea of connecting the phone line itself DIRECTLY to an audio mixer, and decided to go to Radio Shack and invest $30 in a "Multi-Phone Telephone recorder." This device, so far, appears to do the job of letting me INDIRECTLY connect the phone line to the mixer, or audio recorder, and the device says it complies with FCC specs. The device has a small output cord with an 1/8" plug that will plug into an audio recorder or mixer, and a

Presumably the use of the Radio Shack device should be much safer/reliable way to connect the phone line to the mixer than simply taking the red and green phone wires and directly connecting them to an audio cable with a 1/4" plug and then plugging that into the mixer, which is the method I was using up until I bought the Radio Shack device.

Thanks,

J.

Oops...to be clear, the Radio Shack device is not a recorder, but a controller used to connect the phone line to a recorder.

J.

I was concerned about ringer voltage when I was connecting to the mixer directly, but the ringer signal did not fry the mixer. Maybe it's because I had so many phones connected to the line that they absorbed some of the voltage. But, obviously there WAS a problem with my setup, since the ringing was being stopped (maybe by the phone company) after the first ring.

Thanks,

J.

Yikes, the first time the phone rings while plugged into the mixer you'll probably be buying a new mixer, any method is better than connecting it directly like that.

Yes, that's the way to go. This device probably has a REN of less than 1, and will only be seen by the phone company as another electronic phone, if they can see it at all. It will have a "phone-like" front end which provides the isolation out to the phone line from the mixer, and also the other way to the mixer. It will also contain circuitry which absorbs the ring voltage ( for that device alone ), making use of it to provide the cassette recorder control output, and finally, an attenuator to provide the correct level for a low-level input device such as a mixer or amplifier.

Arfa

Remeber, it is illegal to record a teleone message without the consent of all prties to the conversation unless you have a court warrant to do so. You can directly listen to the conversation(eavesdrop) but cannot repeat what you have heard.

H. R. Hofmann

Remeber, it is illegal to record a telephone message without the consent of all prties to the conversation unless you have a court warrant to do so. You can directly listen to the conversation(eavesdrop) but cannot repeat what you have heard.

H. R. Hofmann

Arfa,

Nice summary. I agree with nearly everything you wrote, except, that in the US, the ringing voltage can actually be considerably higher than 90V peak since the legacy Telco spec calls for "less than 100VAC RMS" (more typically 40-75V RMS as you have stated).

This translates to a "peak" of 141 volts across the incoming tip and ring. Also, since there is always a nominal -48VDC (-52VDC maximum) bias/battery, the absolute peak voltage relative to earth ground can actually reach -193 volts (-52VDC - 141 volts peak)!

Bob

Bob, you are quite right - fingers running, brain out of gear - I must be getting old ... !! 90 volts ring signal was the figure in my head, but that is of course, as you say AC RMS max spec, not peak. Thanks for putting me straight d;~}

Arfa

Hello Bob,

The line isolation circuit card that you designed wasn't the ALIT (5ESS Circuit Packs TN138, TN139, and TN140 as I recall) was it? I just have to ask since I was a very young Western Electric factory engineer way back when the Bell System actually existed and manufactured their own stuff. I wrote the factory functional circuit tests for that card! As I recall, it was one of the first to use the (then brand new) Intel 16-bit 8086 microprocessor, but still used a whole lot of mechanical relays to connect the Metallic Service Unit to the customer line under test in the Line Unit.

Bob Shuman - Who is one of the few still working at the Network Software Center/Indian Hill complex over a quarter of a century and three company names/corporate logos later!

I designed the power cross detector for Nos 1 and 2 ESS, also the line switching frame and ferrod scanners for 1 and 2 ESS way back in the

H, R.(Bob) Hofmann