PSTN

5 Articles

Retrotechtacular: Rebuilding A Fire-Ravaged Telephone Exchange

January 11, 2024 by 15 Comments

Those who haven’t experienced the destruction of a house fire should consider themselves lucky. The speed with which fire can erase a lifetime of work — or a life, for that matter — is stunning. And the disruption a fire causes for survivors, who often escape the blaze with only the clothes on their backs, is almost unfathomable. To face the task of rebuilding a life with just a few smoke-damaged and waterlogged possessions while wearing only pajamas and slippers is a devastating proposition.

As bad as a residential fire may be, though, its impact is mercifully limited to the occupants. Infrastructure fires are another thing entirely; the disruption they cause is often felt far beyond the building or facility involved. The film below documents a perfect example of this: the 1975 New York Telephone Exchange fire, which swept through the company’s central office facility at the corner of 2nd Avenue and 13th Street in Manhattan and cut off service to 300 blocks of the East Village and Lower East Side neighborhoods.

Continue reading “Retrotechtacular: Rebuilding A Fire-Ravaged Telephone Exchange” →

Homebrew Telephone Exchange Keeps The Family In Touch, In The House And Beyond

January 14, 2023 by 4 Comments

It doesn’t happen often, but every once in a while we stumble upon someone who has taken obsolete but really cool phone-switching equipment and built a private switched telephone in their garage or basement using it. This private analog phone exchange is not one of those, but it’s still a super cool build that’s probably about as ambitious as getting an old step-by-step or crossbar switch running.

Right up front, we’ll stipulate that there’s absolutely no practical reason to do something like this. And hacker [Jon Petter Skagmo] admits that this is very much a “because I can” project. The idea is to support a bunch of old landline phones distributed around the house, and beyond, in a sort of glorified intercom system. The private exchange is entirely scratch-built, with a PIC32 acting as the heart of the system, performing such tasks as DTMF decoding, generating ring voltage, and even providing a CAN bus interface to his home automation system.

The main board supports five line interface daughterboards, which connect each phone to the switch via an RJ11 jack. The interface does the work of detecting when a phone goes off-hook, and does the actual connection between any two phones. A separate, special interface card provides an auto-patch capability using an RDA1846S RF transceiver module; with it, [Jon Petter] can connect to any phone in the system from a UHF handy-talkie. Check out the video below for more on that — it’s pretty neat!

We just love everything about this overengineered project — it’s clearly a labor of love, and the fit and finish really reflect that. And even though it’s not strictly old school, POTS projects like this always put us in the mood to watch the “Speedy Cutover” video one more time.

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Wood And Carbon Rods Used For This Handsome And Effective Microphone

April 18, 2020 by 20 Comments

Anyone who was active in the phreaking scene or was even the least bit curious about the phone system back in the Ma Bell days no doubt remembers the carbon capsule microphone in the mouthpiece of many telephone handsets. With carbon granules sandwiched between a diaphragm and a metal plate, they were essentially sound-driven variable resistors, and they worked well enough to be the standard microphone for telephony for decades.

In an attempt to reduce complicated practices to their fundamentals, [Simplifier] has undertaken this surprisingly high-fidelity carbon microphone build that hearkens back to the early days of the telephone. It builds on previous work that was more proof of concept but still impressive. In both builds, the diaphragm of the microphone is a thin piece of wood, at first carved from a single block of softwood, then later improved by attaching a thin piece of pine to a red oak frame. The electrical side of the mic has four carbon rods running from the frame to the center of the diaphragm, where they articulate in a carbon block with small divots dug into it. As the diaphragm vibrates, the block exerts more or less pressure on the rods, varying the current across the mic and reproducing the sound. It works quite well, judging by the video after the break.

Congratulations to [Simplifier] for another great build and top-notch craftsmanship. We’ve seen homebrew vacuum tubes, conductive glass, and solar cells from him before, which sort of makes him the high-tech version of Primitive Technology. We’re looking forward to whatever comes next. Continue reading “Wood And Carbon Rods Used For This Handsome And Effective Microphone” →

Books You Should Read: The Cuckoo’s Egg

October 19, 2017 by 62 Comments

The mid-1980s were a time of drastic change. In the United States, the Reagan era was winding down, the Cold War was heating up, and the IBM PC was the newest of newnesses. The comparatively few wires stitching together the larger university research centers around the world pulsed with a new heartbeat — the Internet Protocol (IP) — and while the World Wide Web was still a decade or so away, The Internet was a real place for a growing number of computer-savvy explorers and adventurers, ready to set sail on the virtual sea to explore and exploit this new frontier.

In 1986, having recently lost his research grant, astronomer Clifford Stoll was made a computer system admin with the wave of a hand by the management of Lawrence Berkeley Laboratory’s physics department. Commanded to go forth and administer, Stoll dove into what appeared to be a simple task for his first day on the job: investigating a 75-cent error in the computer account time charges. Little did he know that this six-bit overcharge would take over his life for the next six months and have this self-proclaimed Berkeley hippie rubbing shoulders with the FBI, the CIA, the NSA, and the German Bundeskriminalamt, all in pursuit of the source: a nest of black-hat hackers and a tangled web of international espionage.

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In-Band Signaling: Dual-Tone Multifrequency Dialing

September 5, 2017 by 43 Comments

One late night many decades ago, I chanced upon a technical description of the Touch-Tone system. The book I was reading had an explanation of how each key on a telephone sends a combination of two tones down the wire, and what’s more, it listed the seven audio frequencies needed for the standard 12-key dial pad. I gazed over at my Commodore 64, and inspiration hit — if I can use two of the C64’s three audio channels to generate the dual tones, I bet I can dial the phone! I sprang out of bed and started pecking out a Basic program, and in the wee hours I finally had it generating the recognizable Touch-Tones of my girlfriend’s phone number. I held the mouthpiece of my phone handset up to the speaker of my monitor, started the program, and put the receiver to my ear to hear her phone ringing! Her parents were none too impressed with my accomplishment since it came at 4:00 AM, but I was pretty jazzed about it.

Since that fateful night I’ve always wondered about how the Touch-Tone system worked, and in delving into the topic I discovered that it’s part of a much broader field of control technology called in-band signaling, or the use of audible or sub-audible signals to control an audio or video transmission. It’s pretty interesting stuff, even when it’s not used to inadvertently prank call someone in the middle of the night. Continue reading “In-Band Signaling: Dual-Tone Multifrequency Dialing” →