Is the telephone signal digital or analogue?

Question

Hi, I'm a little confused. Im trying to determine if a telephone signal is digital or analogue. The early telegraph signal is analogue, but with a mobile phone, being a digital device in terms of data on it, is the telephone signal still analogue?

Answer 1

The telegraph system was actually digital, despite being developed back in the 19th century. Digital doesn't necessarily mean the use of computers, it just describes the way that data is transmitted. The telegraph system used Morse Code, which is a digital coding system (dots and dashes).

Telephone systems were originally entirely analogue, but are largely digital these days.

Mobile phone systems are now entirely digital (the old analogue networks for the likes of Vodafone were phased out several years ago).

The land line telephone system uses an analogue signal between the phone and the exchange (this aspect of the telephone system has not changed in over 100 years), but a digital signal is now used for transmission between exchanges.

Despite being intended for transmission of analogue audio signals, telephone cables can also be used for digital services such as ADSL, which "piggy back" on top of the normal analogue telephone signal.

Answer 2

Actually, there is no straight yes or no answer to this question.  

In part because you are asking about different modes and mediums of transmission and methods of modulation - mixed.  

The differentiation between analog and digital isn't always black and white.

Early telegraph (Morse Code) was essentially used over two different mediums:

- Copper Wire

- Radio Waves

Telegraph and Morse Code are essentially a method of "encoding" information, or data, onto a "carrier" signal.

In the case of the old copper wire telepgraph, a Direct Current signal (a steady DC Carrier) was modulated, or interupted at an rythmic interval known as Morse Code.  So, the DC carrier current on the line could be considered Analog, but the Morse Code can be considered as Digital because of its on-off nature.

In the case of Radio Wave transmissions, a steady uninterupted radio wave signal is known as a "carrier" signal, or continuous wave (CW).

Radio waves always have been, and always will be - Analog - regardless of how you modulate the radio wave.

But, the analog radio wave can be modulated (encoded with information) by Analog or Digital means.

Telegraph (Morse Code) over radio waves is really just the act of modulating the radio waves with Morse Code encoding known as dots and dashes.  In the early days, dots and dashes were implemented by turning the transmitter On and Off to correspond with the Morse code.

This was in essence digitally encoding a RF (radio frequency) carrier by digital means.

But, later other methods of encoding (or modulating) of Radio Waves became available and common.  

For example, Voice over radio was initially implemented by using Amplitude Modulation (AM).

AM is an Analog method of encoding and modulation.  AM radio is still in common use today.

AM also became another alternate method of modulating and encoding radio waves for Telegraph and Morse Code transmissions.

In this case, the RF CW analog carrier wave is modulated with a CW analog Audio wave.  The CW audio is turned on and off at the rate of Morse Code data.  Morse code data is digital in nature.

So now I ask you, is this Analog or Digital mode of transmission?

Technically, it is Both at the same time.  The RF carrier is Analog.  The RF carrier is modulated with a analog audio tone.  But, the audio tone is also modulated with Morse Code - digital encoding.

Analog PSTN Telephone Lines

The old fashioned telephne lines still in use today (POTS - plain old telephone service) are by nature Analog.

The carrier for your voice is the steady DC voltage and current always presint on the line (~ -48VDC).  When we talk on the phone, we Modulate the carrier voltage with our analog voice signal.  So, in essence this is a pure analog medium of transmission.

But, what about when you hook up a Fax machine on your POTS line?

Fax machines transmit digital data.  But the digital encoded data is modulated onto Audio Tones (analog) that are frequency shifted at the encoding rate which is of a digital nature.  So, now once again we are integrating digital information onto an analog system.

Cell phones used to be all analog.  The radio waves were (and still are) analog.  The voice modulation (AM or FM) was also analog.

But, now days cell phones are becoming predominately digital in nature.  The voice data is encoded and packetized into digital data.  This data modulates the RF radio waves with this digital information.

So, it is still a mixed bag of tricks.  The radio waves (the transmission medium) are analog (and always will be - it's the nature of RF) but the information being modulated onto those RF signals are of a digital nature.

Answer 3

telegraph is digitial, only because it is on-off keying.

telephone is analog up to the neighborhood SLC bank, where it is converted into digital and MUX'ed into a broadband channel and sent to the ESS Central Office, then it is sent to the destination as a digital signal to be decoded at the SLC bank at the destination. SLC or "Slick" is a Subscriber Line Concentrator.

Cell phones convert in the phone, sent digital to the cell site, then routed to a central cell CO where it is routed to another cell or to the landline CO.

older cell phones were analog and could be monitored with a simple receiver, new cell phones as well as interoffice land phones are digital and need specialized equipment to monitor.