If the public switched telephone network is digital, are landline phones technically modems?


I read that the PSTN or POTS has been digital since the 1980s. It uses 8 KHz, 8 bit sampling = 64 kbps. So does this mean that digital information is sent through telephone lines? (other than DSL).

Does this mean that your landline phone is technically a modem or a has a modem in it? And circuitry to decode the digital signal?

In: Technology

Not really — a modem, conventionally, is a thing that takes digital data and encodes it into analog signal so you can get the digital data back (the same digital data).

A phone, even an ISDN phone, takes your sound (analog signal) and encodes it into digital data, which then goes on the line.

You could consider the part of the phone that takes the digital data containing the samples of your sound and sends it over the line a modem, probably. Not sure it’s a useful description.

A landline phone is not a modem. It’s an A/D D/A converter and works in the opposite order of a modem.

A modem turns a digital signal from a computer into audible sound that can be transmitted through an analog phone line; and turns audible sound received through the phone line into a digital signal to be fed into a computer. It does that by modulating a carrier wave (a tone). This is why it is called a MOdulator/DEModulator.

Your digital landline phone does the opposite. It converts an analog sound (your voice) into a digital signal that can be transmitted through the digital phone line; and converts the digital signal it receives from that line into sound that you can hear.

You can still hear noise on a phone line because phones are cheap. The analog portion of the circuit uses cheap mics and speakers, cheap discrete electronics, cheap wires, cheap insulation, cheap power conditioning… All of these things will introduce noise into the signal before it gets digitized, and this noise is happily transmitted to the other end together with your voice.