Was it possible to send data over the phone-line without a modem?


  • Was it possible to send data over the phone-line without a modem, either by connecting computers directly to the phone line, or, by connecting cassette-recorders/players directly to the phone line
  • It must not require the use of a commercially available modem, but allow only a bit of extra wiring and maybe some resistors, and, or, audio-speaker and microphone .
  • This is for the scenario where two people may want to share a program or data, and they don't want to waste money on the modem .
  • I'm assuming the answer is no, or else people would not have bothered buying modems.

I suspect that either it may have worked for some computers and phone-lines, or, that it may have been easy for the computer manufacturers to design the specifications so it would function well without a modem, however, I suspect it was strongly discouraged in order to avoid damage to computers or to phone-lines :

  • Only commercially available modems were legally certified to be connected to the phone-line, not the computers themselves, to prevent people damaging the line .
  • And connecting a computer straight to the phone line may risk damaging not only the line, but also the computer .


  • It must not require the use of a commercially available modem, and only a bit of extra wiring and maybe some resistors .
  • Initially, this question is not about the use of any external memory devices like cassette-recorders etc.
  • Or if required, this question is about computers that had access to cassette-tape-recorders, not disk-drives, but information about disk-drive machines may also be useful.
  • For the common affordable home-computers in the years approximately 1978 - 1984 (a few years before, or after, is ok)
  • NOTE - By plugging the cable straight from the transmitting computer and into the telephone, the cable that the transmitting computer normally used to save to cassette-tape, to the microphone-jack of the telephone (did that exist?).
  • Or, by attaching an audio-speaker to the cable that the transmitting computer normally used to save to cassette-tape, and then placing that audio-speaker next to the telephone-microphone.
  • (Theoretically, could you just have left a program on a cassette-tape-answering-machine, unless there were copyright problems in those days, or data quality loss because of the cassette tape)
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    One person reads a program off the screen, and the other person types it into a keyboard? – supercat Feb 15 at 3:42
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    Don't know if it was done over phone by anyone, but at least in 2 countries (UK and Sweden) public radio or television sent prigrams over the air. – UncleBod Feb 15 at 4:13
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    Even in the @supercat case, one might argue that the person reading the program and speaking into the phone is "modulating" the information, and the person listening on the other phone and typing in the program is "demodulating" the information. This requires a very broad definition of what is meant by modulating and demodulating. Something like "changing to a form suitable for transmission over an acoustic voice link" and back. – Walter Mitty Feb 15 at 12:42
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    I think this is basically asking if computers can, without special equipment, modulate/demodulate data suitably for transmission down a POTS line? Which is clear enough I suppose. This seems relatively on-topic, so I'll reserve my close vote for now. It may be unclear because the OP isn't clear themselves on what a modem actually does for us? – user12 Feb 15 at 14:28
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    With these updates, an even deeper hole is dug ... ready to bury this. Serious, legality needs also to clarify the area the claim is made to. And claims about damaging the computer will need an near endless list of computers and parts involved to make any useful criteria. A bit more and Australia will be reached. – Raffzahn Feb 15 at 22:14

TL;DR If it acts like a modem, it IS a modem.

What is a modem?

a combined device for modulation and demodulation, for example, between the digital data of a computer and the analog signal of a telephone line.

What are you trying to accomplish? send digital data (a program) from one computer to another over the analog signal of a telephone line. So whether it is an older modem with an acoustic coupler or a Hayes Smartmodem or anything in between, including something you just come up with and experiment with (violating telephone company rules along the way), you are using a modem when you transfer digital data over an analog phone line. An analog tape interface, as was the case with all microcomputer tape drives of that era (mainframe tape drives were different) does the same thing - convert digital data to audio tones. Connect that tape interface to a phone line and it is now a modem. DTMF created from a computer is a fancy (and slow) way of converting digital data to audio tones - send it over a phone line to be decoded by a computer and you have created a modem.

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    @Tommylee2k It wouldn't take much to adapt an RJ11 jack to 3.5mm phono connector. – snips-n-snails Feb 15 at 10:08
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    "you are using a modem when you transfer digital data over an analog phone line" says it. you're spinning in circles, using a self-made modem still is using a modem. – Tommylee2k Feb 15 at 11:50
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    A tape drive interface connected to a phone line would be a half duplex modem. – manassehkatz Feb 15 at 16:53
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    @manassehkatz: A typical tape drive interface doesn't actually modulate or demodulate anything itself, but instead transmits pulses generated directly by code that modulates data, or makes pulses available to code that can decode them into data. The combination of the interface and the code to drive it could be a half-duplex modem, but the hardware itself wouldn't be. – supercat Feb 15 at 19:01
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    @supercat Then the computer itself is the modem and so the question contradicts itself. – snips-n-snails Feb 15 at 19:24

"Hello, Jeff?"


"I hear you got your new Radio Shack Model 3?"


"Is it on?"


"type the number ten and a space, then type print quote hello world quote and press enter"

"Okay now what?"

"Type run and press enter"


Sorry, I couldn't resist.

  • supercat beat you to it: One person reads a program off the screen, and the other person types it into a keyboard? – manassehkatz Feb 15 at 16:33
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    Rats. All the other answers looked more like serious attempts at an answer. – Bill Hileman Feb 15 at 17:34
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    Is that you, Dad? – DrSheldon Feb 16 at 1:59
  • Could be a 1 instead of 10 to make it even easier. – Thorbjørn Ravn Andersen May 31 at 18:50

Not an answer just points to be connected...

  1. Phone system means analogue dial up network for voice (sound) communication.

  2. Sound is transmitted between terminals (phones) as electric signal

  3. Phone systems are designed to transfer electric signals with frequencies of 300 to 3000 Hz (*1)

  4. There is no galvanic (direct) connection.

  5. Audio transformers are used to separate lines and systems.

  6. Their frequency response is designed to support this range

  7. Frequencies outside the range (300..3000Hz) will be dampened.

  8. Filters may be applied in addition to guarantee these limits.

  9. Phone systems do only offer a/this single (user side) channel with these properties.

  10. System signaling may be in or out of band (time or frequency diverted) but is not available for user side content transfer.

Derivates points:

  1. No DC signal can be transmitted.

  2. No level (voltage) based signal can be transmitted.

  3. No current signal can be transmitted.

  4. Only signals made up from frequencies within the limits can be transported.

In more than 100 years of application of these systems, many special to purpose devices have been created and named by their inventors and/or users. So, Question: What would be the basic term for a devices that transforms user signals into a collection of frequencies for transmission and back?

*1 - Overall simplification, some systems may use slightly different. These limits are not just for interoperability between systems, but also to ensure higher level of transportation (like multiplexing on lines between interchanges)

  • Not upvoting because "not an answer". But valid points that support my premise quite well, thank you. – manassehkatz Feb 15 at 14:43
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    @manassehkatz :)) Ouch, according to one source, I should be devastated now about the missed points. Let's check tomorrow if I survived the resulting hangover:)) I do not think the Quesion is really asking for an answer, it seams more like a pile of made up rules to prove some point I fail to see. That's why I restricted myself to list a few of the basics involve (And to be honest, I didn't vote for you as well (I almost did), as the answer (albeit perfectly good) written trips into the missleading parts of the question - at least in part.) – Raffzahn Feb 15 at 15:10
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    Thank you, that was the freqency bandwith I wanted to find. So, if the cassette interface did keep itself within those limits (300 to 3000 Hz) the audio could in theory be sent over the phone line without any more change to the signal. I assume that is what the question is about. – UncleBod Feb 15 at 15:39
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    @UncleBod But then you have just turned a tape interface + extra wires into (TA DA!) a MODEM! – manassehkatz Feb 15 at 15:59
  • Isn't the frequency range 300-3400 Hz? (see eg Voiceband on wikipedia) – JeanPierre Feb 16 at 8:30

Yes, it's possible. For example, systems like the X10 fr home automation can be controlled via a phone line using DTMF (touch tone) dialing. While such systems are often designed to be used by humans, there have been instances of direct machine-to-machine communication too.

For example, the user could call the X10 receiver connected to an IBM compatible PC running the DOS based X10 software, and use DTMF tones to reprogram the system, altering timers and controlling devices, i.e. sending programming.

Unfortunately these systems are so old that I can't find any good references on the net. We are talking early DOS days. The best I can do at the moment is this LGR video with a demonstration of X10 control over the phone.

Note: The DFTM encoder and DTMF decoder devices are not modems. Modem is short for "modulator-demodulator", i.e. a device that incorporates both modulation and demodulation for two way communication. The X10 system is one way, one end is a pure modulator and the other is a pure demodulator, thus neither are modems.

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    DTMF is a modulation scheme that can represent 16 symbols and the decoder at the far end is a demodulator. That makes the whole setup a modem, albeit a simple one. – Blrfl Feb 15 at 13:34
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    I was going to write something similar at first, saying that you basically can't send any digital data over a POTS line without modulation. But the question does say a "modem", and a DTMF decoder is not generally considered to be a modem. It was designed for human-machine interaction, so calling it a modem is a bit like saying someone typing in a program from a book makes the keyboard a data storage device. – user Feb 15 at 14:10
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    @user Of course it's a modem (or at lease the MO part). It modulates user interaction (data) into sound - which sound frequencies then get turned into electric frequencies. So perfect example for a modem - and just because the frequency points used are the same as for in band signaling doesn't change the fact - beside, that's exactly the standards way phone systems have used for data exchange between them (no soudn involed) since the 70s. – Raffzahn Feb 15 at 14:59
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    @manassehkatz Yep. Have Alexa talk to Siri over the phone... – tofro Feb 15 at 17:52
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    And that differs in what part from my comment? And no, a modem doesn't have to incooperate both parts. That's only useful for bidirectional communication, isn't it? Beside, I'd say, trying hard to be negative isn't a good idea and wast of time. – Raffzahn May 31 at 12:15

In 1985, Your Computer magazine had a software download service called TELSOFT. Audio files of (I presume) recorded MODEM tones were played down the phone line in a continuous loop. It was supposed to be used with a MODEM and special software that you typed in, but I did not have a MODEM for my ZX Spectrum so I tried receiving it directly through the Spectrum's 'EAR' socket.

Back then a toll call from New Zealand to the UK cost ~$10 per minute, and I wasn't sure how well my program would work, so I recorded the audio on a tape recorder connected to the telephone's receiver insert. I could then play the recording back as many times as I needed until I got the software working properly.

After a few hours of debugging the software I finally managed to get a good data dump, only to discover that it was for a Commodore 64! I had forgotten that magazines took 3 months to get from the UK to New Zealand, so of course they were transmitting files 2 months ahead of what was in my magazine. :(

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    That's a "software modem" - a program acoustically coupled to the phone, which MOdulates and DEModulates digital signals to and from "sounds" that can actually be transmitted by the telephone. It doesn't really matter if you buy a "modem in a box" or use an existing computer as a modem - it's still a modem. (Which is the problem with this question). – dirkt May 31 at 11:40
  • No, it's not a MODEM because it only does DEModulation, not MOdulation. A MODEM has to be capable of doing both. – Bruce Abbott Jun 1 at 0:42
  • It's only half a modem, then. But adding the other part is not hard. (And that wasn't the point - the point was that it doesn't matter if you do it in software or in hardware, and it doesn't matter if you do it on a dedicated device or a general-purpose computer - it's the same thing. In fact the modems you could buy at later stages are "computers" with a microcontroller and hardware to offload some or most of the modulating/demodulating work from the main CPU, and leave the CPU to deal with somewhat higher protocol levels.) – dirkt Jun 1 at 4:30
  • Half a MODEM isn't a MODEM, no matter how much you try to redefine it as one. A tape player plugged into the ear socket of a ZX Spectrum is not a MODEM, and no amount of software can turn it into one. The OP specifically asked about using a tape player with "only a bit of extra wiring" instead of a commercially available modem, which is precisely what I implemented. Saying 'no you can't do that because then it is a MODEM' is not only wrong, it's silly. – Bruce Abbott Jun 1 at 9:49

When I was a student in late 90's, I did something like this as a research / hobbyist project. The hardest part was the hardware interface to the phone line which had to be completely galvanically separated from the computer, and that also was tasked with amplifying the analog signal coming from the phone line so that it could be reliably detected by the built-in comparator chip.

As for transmitting data, one could use a Manchester code or simple frequency modulation but the transfer rates would be limited to something like up to 600 baud, I guess.

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    You built a modem. Homemade non standard but still a modem. – manassehkatz May 31 at 13:25
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    @manassehkatz depends on what you call a modem :) The external hardware I built did not actually perform any modulation or demodulation - it was merely an analog adapter to connect the computer's tape recorder port to a phone line. To be precise, of course it was a bit more complex than "only a bit of extra wiring and maybe some resistors", but the additional complexity was purely intended to achieve full galvanic isolation and avoid any damage. – DmytroL May 31 at 13:52
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    If you only recorded an audio signal to tape using the computer as a controller, with no interpretation/decoding of the signal at all * then arguably this wasn't a modem. But if you did all that and then played the tape back and decoded *that signal, you have basically made a delayed-action modem. In the end, it sounds like the goal was the same - take a digital signal and transform it to analog and back to digital in order to transfer data from one computer to another via a phone line. That's a modem. – manassehkatz May 31 at 13:59

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