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)
  • 16
    One person reads a program off the screen, and the other person types it into a keyboard?
    – supercat
    Commented Feb 15, 2019 at 3:42
  • 6
    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
    Commented Feb 15, 2019 at 4:13
  • 3
    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. Commented Feb 15, 2019 at 12:42
  • 3
    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
    Commented Feb 15, 2019 at 14:28
  • 3
    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
    Commented Feb 15, 2019 at 22:14

10 Answers 10


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.

  • 3
    @Tommylee2k It wouldn't take much to adapt an RJ11 jack to 3.5mm phono connector. Commented Feb 15, 2019 at 10:08
  • 6
    "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
    Commented Feb 15, 2019 at 11:50
  • 5
    A tape drive interface connected to a phone line would be a half duplex modem. Commented Feb 15, 2019 at 16:53
  • 3
    @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
    Commented Feb 15, 2019 at 19:01
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    @supercat Then the computer itself is the modem and so the question contradicts itself. Commented Feb 15, 2019 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? Commented Feb 15, 2019 at 16:33
  • 2
    Rats. All the other answers looked more like serious attempts at an answer. Commented Feb 15, 2019 at 17:34
  • 1
    Is that you, Dad?
    – DrSheldon
    Commented Feb 16, 2019 at 1:59
  • 1
    Could be a 1 instead of 10 to make it even easier. Commented May 31, 2019 at 18:50
  • 1
    Puzzling, this is how I entered my first computer program.
    – Janka
    Commented Aug 25, 2019 at 12:31

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. Commented Feb 15, 2019 at 14:43
  • 1
    @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
    Commented Feb 15, 2019 at 15:10
  • 1
    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
    Commented Feb 15, 2019 at 15:39
  • 1
    @UncleBod But then you have just turned a tape interface + extra wires into (TA DA!) a MODEM! Commented Feb 15, 2019 at 15:59
  • Isn't the frequency range 300-3400 Hz? (see eg Voiceband on wikipedia)
    – JeanPierre
    Commented Feb 16, 2019 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.

  • 10
    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
    Commented Feb 15, 2019 at 13:34
  • 3
    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
    Commented Feb 15, 2019 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
    Commented Feb 15, 2019 at 14:59
  • 1
    @manassehkatz Yep. Have Alexa talk to Siri over the phone...
    – tofro
    Commented Feb 15, 2019 at 17:52
  • 1
    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
    Commented May 31, 2019 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. :(

  • 1
    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
    Commented May 31, 2019 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. Commented Jun 1, 2019 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
    Commented Jun 1, 2019 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. Commented Jun 1, 2019 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.

  • 2
    You built a modem. Homemade non standard but still a modem. Commented May 31, 2019 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
    Commented May 31, 2019 at 13:52
  • 1
    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. Commented May 31, 2019 at 13:59

It was quite common to communicate to a computer without a modem, teletype interfaces worked over the phone lines and did not used modems. They switched up to 20 mA on up to a 48V phone line. The fact that old phone lines were DC-coupled (your handset was powered from the local exchange) was one of the reasons the phone companies tried to keep strict control over all equipment you could attach to the phone socket, making it illegal to connect uncertified devices and in many cases they insisted on renting you the equipment. Up through to the late 1970s the typical console of a computer was a teletype machine.

I used an IBM System/7 in 1973 to receive data from rotary phones in a factory. Touch tone phones did not exist yet, and they put dials along a Ford production line so that workers would dial in the number of an approaching chassis so we could send them a print out (teletype, outgoing lines only) of the options to be added.

The S/7 had a number of options for digital and analog I/O in factory and laboratory work, and in this case I was using a module which could report open/closed circuit which was a variation on the teletype interface which could be programmed with different timings. The dialing mechanism was much slower, if I remember correctly the pulse rate was about 10 per second and you just counted from 1 to (1)0, then a longer pause indicated the end of the digit.

Early time sharing systems used teletypes over the phone system, essentially just generalizing the adjacent console mechanism to support multiple phone lines each of which had a teletype at the other end. I also used those in 1973 to run APL on a time-sharing service, remotely in another city.

I also used signalling like this in early 1981 to connect a Victor 9000 to a brand new IBM PC (we had them talking the same day the PC was unboxed) and although the outputs were only toggling 12V the circuits were most likely compatible with the telephone voltages, as that was a common signalling standard at the time. I suspect some folks may have programmed PCs to work with teletypes including answering the phone lines. Modems were still expensive and rare, while many teletypes were in use, almost every medium business had a teletype machine (like more recently they would have a fax machine).

  • Was it really possible to attach a current-loop mode teletype directly to a public phone line? I had assumed that the phone companies would put a stop to that. If this was possible, which teletypes for example were certified to be connected directly to the phone line? What about the equipment on the host side, was that also directly connected?
    – dirkt
    Commented Aug 25, 2019 at 6:35
  • Looking it up, of course you could connect teletypes to the Telex-network or similar services, but those were normally separated from the (voice) phone line network. However, the Bell Teletypewrite Exchange Service shared line with the (voice) phone network (POTS), but was "set up with a special Class of Service to prevent connections from POTS to TWX and vice versa. " Given that the point of a modem is to transfer data over POTS, I'd assume that Telex-like connections don't count for this question.
    – dirkt
    Commented Aug 25, 2019 at 7:03
  • Another way of connecting a computer to the Telex network was to punch the word processor to papertape. The computer output via Telex machine over telephone lines generated papertape for a remote system. Lawyers needed to reliably transmit documents sent by Telex without typos. A solution was to punch Baudot code from CP/M and Apple II systems. While working for a S100 system reseller I built interfaces and wrote code so that the punch was effectively a printer. Don't remember what model of papertape punch was used and certainly never kept any of the company's circuit diagrams or code.
    – PDP11
    Commented Sep 11, 2023 at 5:04

As the question specifically states that this is about common home computers of the early 1980s, the answer has to be no.

To interface with the analogue telephone system, a modem is required (internal or external). Provision of an internal modem would add to the cost (both in bill-of-materials and in testing and regulatory compliance), and networking was such a minority interest at the time that this would not be justified by the demand. On many systems, even a standard serial interface is an optional extra.

The nearest you could get without a proper modem would be to abuse a cassette interface to drive an acoustic coupler (but even between a pair of like systems, you'd need to write your own driver, as the bandwidth of a telephone line is much narrower than that of cassette tape).

It turns out that the lack of provision of internal modems was probably a blessing in disguise - separate modems connected by RS-232 cables allowed innovation by modem manufacturers to advance transfer speeds without significant changes to the home computers driving them.


The kind of circuitry needed/used to drive a tape recorder that only offers audio connections, for purposes of storing digital data from a computer or similar device ... IS A MODEM. Even if is not a (purpose built) telephone modem (which is usually implied when people speak of modems these days).


A direct electrical connection between a phone line and a computer would immediately destroy the computer, because the phone line carries a 50V DC supply to power telephones connected to it.

It's possible to get over that hurdle, of course, by inserting an isolating audio transformer or an acoustic coupler into the connection. You must then contend with the fact that a phone line will typically carry only signals in the 300-3000 Hz range, so you must arrange for the signals to be emitted from the sending computer to fall into that range. This is, in fact, precisely what a modem does.

If you do not have a modem designed for use on telephone lines, but you do have a tape-cassette interface on both machines and a suitable electrical-isolation device as noted above, then it is likely that the tape-cassette interface will work sufficiently well as a modem. Not only is the problem of storing signals on tape reasonably similar to the problem of sending signals over a phone line, but in many home computers the very same techniques were used. The 1200/2400Hz FSK signal used by the Kansas City standard is nearly identical to that used by half-duplex TTY-modems of the period (Bell 202 or ITU V.23).

If you do not have suitable hardware, then it is possible to encode the data in ways that are easy to read out loud, or send by hand in Morse Code, requiring only a proficient operator at the far end to transcribe these voice or CW signals back into the computer.

For example, 5-bit digits can be represented using Base32 encoding and printed out in four-digit groups, two such groups conveying five 8-bit bytes; Base32 is specifically designed to resist transcription errors by humans, as similar-looking letters and numerals are mapped to the same meaning. Similarities in the pronunciation of letters (or differences between national conventions for same) should be disambiguated by using the NATO phonetic alphabet, if reading the digits out loud.

  • You are aware that many older computers (particularly those using valves rather than transistors) had circuitry well in excess of 50 volts, right? As normal I/O, without any damage. Commented Aug 9, 2019 at 12:42
  • @TobySpeight Among the parameters in the question is "For the common affordable home-computers in the years approximately 1978 - 1984 (a few years before, or after, is ok)" - so that means NMOS and CMOS chips running at about 5V levels, which would not withstand 50V for even a moment. But most of them did have a tape interface.
    – Chromatix
    Commented Aug 9, 2019 at 16:05
  • Ah, sorry, I missed that bit. Commented Aug 9, 2019 at 16:07
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    I'm confused over why this is getting so many downvotes. If you do so, please leave a comment explaining why.
    – Chromatix
    Commented Aug 12, 2019 at 5:09
  • I didn't down vote this answer but do have reservations over the opening statements. For example, 20mA style interfaces for computer systems even predate the IBM PC. It is possible to connect a computer to a 48V line without it being immediately destroyed. Therefore your opening statement in Bold characters about immediate destruction would instantly attract downvotes. My guess is a number of readers would not be able to get past that bold, initial statement in your answer. As I said, I didn't downvote your answer and this is just my best guess as to the thoughts of others.
    – PDP11
    Commented Sep 11, 2023 at 5:20

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