Telephone wire is a lowpass chaneel so the process looks like below. (Maybe right?)

Voice(analog signal) → Sampling(digial data) → Coding(digial signal) → transmit

There's no need to convert digital signal to analog signal because it is not a bandpass channel. Meanwhile, Wikipedia states that the digital signal which is converted from digital data of computer can not be transmitted well if it is sent immediately. And that's what modem does. (Modulates the signal according to the characteristics of the transmission line)

The strange thing is, it also states that the modem converts digital signal to analog signal so that it can be transmitted through telecommunication line. Can't the digital signal be transmitted without converting to analog signal like how our voice is transmitted through the line?

  • \$\begingroup\$ The phone line has limited low-freq and high-freq responses. Picking a proper modulation (64-QAM) allows 6 bits per symbol, at about 3,000 symbols per second, or 18,000 bits per second. If lots of signal-processing methods are brought to bear, you can up that bit rate to 60,000+ bits per second, likely limited by the codecs used by ATT. \$\endgroup\$ Sep 19, 2018 at 4:00
  • \$\begingroup\$ Originally, POTS (Plain Old Telephone System) was analog all the way between the two phones involved in a call. With modern telephone systems, the call will probably be handled digitally most of the way. \$\endgroup\$ Sep 19, 2018 at 5:34

3 Answers 3


I agree that you are confused. A telephone line is a strictly analog connection. The typical connection is shown in the picture below (from https://pbxbook.com/other/trunks.html#4 )

Unless you are using a digital telephone, voice is not converted to digital until it gets to the Central Office (CO). A a modem modulates digital signals into analog signals that fit within the frequency bandwidth of the analog line. Another modem at the other end demodulates the analog back into digital.

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  • \$\begingroup\$ Then is the voice itself as an analog signal transmitted through the line until it reaches CO? Do I understand correctly? \$\endgroup\$ Sep 19, 2018 at 4:29
  • 1
    \$\begingroup\$ Yes, traditional phones are an all analog system. \$\endgroup\$
    – pjc50
    Sep 19, 2018 at 10:34

Telephone systems are effectively AC-coupled and bandpass-filtered; you can't send DC levels over them. The filter has a passband of roughly 300Hz to 3kHz.

It's a single loop - a shared medium between transmit and recieve. So you need some means of discriminating between bits going in one direction and bits going in the other.

It's also often an extremely bad line. It picks up all sorts of noise as clicks and crackles. The system needs to not interpret these as "bits".

This leads to the very basic modulation scheme used by early modems, FSK: bit value “0″ at 1850 Hz and bit value “1″ at 1650 Hz.

(In practice, whenever a signal leaves a PCB and travels over a long cable it should always be thought of as analog, because it's the analog properties that are going to degrade its reception. It might not be modulated, but it's no longer a completely clean set of 1s and zeroes.)


Can't the digital signal be transmitted without converting to analog signal like how our voice is transmitted through the line?

By one definition of analogue and digital, all physcially transmitted signals are analogue (the analogue domain, continuous in time and amplitude, and that includes square waves because of course a real square wave is never a perfect constant amplitude). All signals in computer memory are digital (the digital domain, discrete in time and amplitude).

But the more common definition of analogue and digital signal are as follows: When someone says digital signal they mean an analogue signal in which digital data is encoded, and the receiver decodes the signal into a binary data packet. An analogue TV displays the received signal where the actual received voltages are used to control the brightness of the electron gun for each colour directly. This is what people usually mean by analogue. There's no ADC that samples the signal converting it into digital data points to be processed digitally and classified into bit sequences and packets of data, which can contain further error correcting. Similarly, the VGA interface is analogue because the voltage signal directly drives the electron gun in the analogue domain after manipulation only from analogue components that treat the signal as a voltage signal as opposed to a microcontroller that treats it as encoding binary data and samples it and places it into memory (digital domain) for processing. VGA was built for CRT monitors but became a standard, and LCD displays were made backwards compatible with it by containing an ADC – indeed people still calling it analogue becomes questionable in this case because the analogue voltage signal no longer directly drives the display, and given there's also a DAC on the transmitting end, it's just effectively a digital signal (digital data encoded in an analogue signal) now transmitting amplitude modulated amplitude samples, so you just have to remember that the VGA signal is still able to drive analogue displays directly, which is why it's still called analogue.

It's the same thing with a modem in a router. It converts the data in the digital domain (the router memory) to an analogue wave transmitted on the wire (waves can only be analogue). The diagrams that show digital between the PC and modem but analogue between the modems is misleading. Any physically separate device requires an analogue transmission (with digital data encoded in it), so it is analogue between modem and PC. Using the definition of digital signal in the previous paragraph however, the diagram should surely show digital between PC and modem, and digital between modem and modem, because they're both digital signals, but instead the diagram shows analogue between the two modems.

I think the reason for this is because it's assumed to not be a physically point to point link between 2 digital nodes (the 2 modems, that will interpret the signal digitally), but instead passes across analogue switches that treat it as a purely analogue signal, and apply analogue processing it it as if it were the analogue voice signals it was designed for. It could also be because it considers the PC, router, modem as the digital domain (they could all be the same device accessing the same memory) and the analogue transmission line is indeed analogue and transmits digitally encoded signals. It could also be that transmission via local serial cables (ethernet, rs232) between the devices tend to baseband transmissions that resemble square waves i.e. what people associate with a digital signal, whereas the transmission over the phone line is modulated up and looks like what people associated with an analogue wave.

To recap, I'd say that an analogue signal is one that is created by an analogue source (microphone) and the intended and standardised output device is an analogue one to be directly driven by the signal and it is able to do so, even though this signal of course can actually being digitally sampled at the receiver. An analogue signal is also a signal that is digitally generated using a DAC but the intended and standardised output device is an analogue one to be directly driven by the signal and it is able to do so, such as a VGA signal is intended for and is able to drive a CRT. If there is an ADC it does not change the fact that it is an analogue standard and therefore an analogue signal. A digital signal is one that is created in the analogue or digital domain and it is intended for a receiver that extracts digital information from it to memory. Of course the signal could power an LED or something, but that is not the intended receipt of the signal, and it does not change the fact that it is a digital signal, until you define your own separate standard driving an analogue LED and then it would be an analogue signal.


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