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I've been doing home improvement projects for a while and one thing that I've made use of the most is taking advantage of existing cabling in the walls that isn't being used. The project that did this the most was my "video-over-coax" setup, which uses a composite-video-to-cable modulator and a coax switch to show some video output on the whole coax network, for example the security camera monitor. This thing works by tuning into a specified channel on the TV that the coax is attached to.

I want to get access to my landline phone signal from my garage/office. As of now, there are only a couple of connections from the network closet (where the main networking stuff is, most importantly here the phone line switch) to the garage, which are a coax line and an Ethernet line (which can't be tampered with because it's needed for an internet connection).

Simply running new cable from the closet to the garage is unfortunately impractical, since it would need to be underground (the garage and main house are separated from each other) and we can't do that.

skip to here to ignore background info ramble

The only solution I can come up with is somehow modulating the voice signal over the existing coax line and demodulating it at the other end (and vice versa because voice is two-way), which I have no idea how to do (or if it's even possible). The main problem here is that my current modulation setup is one-way, video-->coax-->TV, where the TV never interacts back with the modulator or video source. This is obviously incompatible with voice (and IP, etc), which means I would need some sort of modem on both ends rather than a modulator on one and a demodulator on the other. Regardless, if it is indeed possible, how could I go about running voice over coax?

Expanding on this I'm also curious if I can do "general purpose" modulation, wherein I could run stuff that's completely unrelated to what coax is usually used for, like USB or SPI (for Arduino), over the existing cable. I've ran USB over Cat5 cable before with success (the 4 USB wires connected to 4 of the 8 Cat5 wires on either end), which is what brings up this idea.

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    Wait a minute. Aren't 4 of the wires on Ethernet unused? – Harper - Reinstate Monica Feb 5 at 3:41
  • I thought so too for a moment, but that's only the case for old Ethernet. Modern Gigabit Ethernet uses all 8, so the Cat5e connection can't really be added onto at all (that I know of, anyway). – BoomBoomPowe Feb 5 at 3:45
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    But Voice Over Internet Protocol (VOIP) will happily carry voice as ethernet packets without messing up your other ethernet packets (it's just one more stream in the network traffic, and not a particularly heavy one at that.) – Ecnerwal Feb 5 at 4:24
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    I can appreciate the challenge of using what one has available, but is a cordless phone out of the question? They're pretty darn reliable these days. – HikeOnPast Feb 5 at 5:00
  • Keep in mind that telephone lines have 48v DC current on them, with a 90vac spike when the phone rings. It's not just a straight audio connection. – JPhi1618 Feb 5 at 14:48
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Assuming you don't already have ethernet available in the garage, you COULD use a pair of MoCA adapters to use the coax for ethernet.

Beyond that, the two magic terms you'll need to Google for are "FXO" and "FXS". A FXO VoIP gateway is basically a VoIP server that's connected to a telephone line. A FXS VoIP interface is basically a VoIP client that uses a regular telephone as its input and output device. In theory, any device capable of being a FXO server should be able to be a FXS client as well, because the interface hardware is basically the same... but if you can find a FXS adapter, it'll probably be cheaper than buying two FXO servers.

Don't take my word for it without further research (it's been about 10 years since I last looked into it), but I think at least one member of the Linksys SPA-3xxx family can be used for this purpose if you buy a pair of them... basically, each box has an ethernet port, a FXO jack, and a FXS jack. You'd configure one of them as a FXO gateway and VoIP server with local LAN IP address & connect its FXO jack to the phone jack, and configure the other one as a FXS client that connects to the first & plug the phone into its FXS jack.

Be careful with the SPA-3xxx (and SPA-21xx?) devices, though... some of them are service-locked, especially if you buy one that isn't brand new. From what I remember, the only way to know for sure whether it's locked to a specific service is to try accessing its configuration menu... if it's locked, you won't be allowed to change the SIP or FXO settings. Also, there are two passwords... one of which is necessary for "advanced" settings and can NOT be factory reset. I remember reading about how a bunch of people who bought cheap ones on eBay to use with Asterisk & discovered that they were basically paperweights due to service locks or lost passwords.

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POTS (analog phone) is very undemanding, so long as you respect its nature.

It's dependent on a DC connection, so any sort of "transformer" like converter will screw it up. Odds are excellent that simply connecting one phone wire to the center and the other to the shield will work, despite the cable being unusual. This is, of course, also the inexpensive option.

Your more modern approach would be to use VOIP to add the phone conversation to your other data streams on the existing data connection, or to use a different sort of converter/modem to send ethernet signals over the coax.

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    yes, if the coax to the garage is currently unused, it can be used to carry the telephone signal unaltered. – Jasen Feb 5 at 5:06
  • Do make sure though that the coax cable is not routed through some splitter, patch panel or distribution panel with other coax cables. Such devices may be connecting the shields of all the coax cables together and possibly to a common ground. The analog phone line is a balanced pair current loop all the way back to the central office switch and shorting one side to ground or other circuitry can introduce a lot of power line noise. The 90 VAC ringing signal on the phone line could also damage circuitry associated with the other coax cables. So as long as the (continued) – Michael Karas Feb 5 at 14:49
  • (continued from above) coax cable is known to be continuous end to end without any intermediate diversions or interruptions then it should work just fine to connect up as an extension of the analog phone line. It may seem a bit unconventional should work just fine. – Michael Karas Feb 5 at 14:53
  • Definitely worth a shot before I'm able to get VoIP equipment; had no idea that coaxial could be used in this way. – BoomBoomPowe Feb 9 at 16:48
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Depending upon what you want to do, there are several ways to accomplish your goal.

You can get an ethernet to coax adapter which will allow you to use the cable to connect between two adapters. You will then have ethernet in the garage, and can choose from a variety of devices to stream TV, music, provide internet, etc. You will likely also want a small ethernet switch to help you interface to more than one device.

If audio is your only desire, a simple balun will match two balanced wires to the coax unbalanced line.

There is a plethora of other devices to help you match your signals to a coax line.

My suggestion is the ethernet to coax adapter, and that will give you the ability to interface to a variety of devices. Yes, you can have a big screen TV in the garage for your buds when they come over.

Depending upon which way you want to go, you may be able to find the adapters from sources like Amazon. If you want two audio channels over coax, without ethernet, that can be done, but finding the adapter and matching it to your coax may be more complicated.

It will be helpful to know what kind of coax you have. Some are 50 ohm, and others are 75 ohm. You may be able to read the type of coax off the cable and look it up. Knowing the impedance will help you select the right devices to interface to the coax.

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