As I have understood the concept of modems, modems translate digital signal to analog ones and in reverse.

Well and then I've read the definition of a modem (technical book):

A modem converts digital direct current signals to digital alternating current signals and the other way around.

Well how does the word "analog" fit in there!? How can I get a picture of it? Can you explain a bit how "direct current" and "alternating current" fits in there?

  • \$\begingroup\$ The quotation does not make any sense to me. Is it copied verbatim? \$\endgroup\$ – Eugene Sh. May 30 '18 at 16:28
  • \$\begingroup\$ I meant "converted". The original text is in German and I have translated it to Englisch. \$\endgroup\$ – watchme May 30 '18 at 16:35
  • \$\begingroup\$ I afraid you haven't translated it correctly. Or the book is very wrong. \$\endgroup\$ – Eugene Sh. May 30 '18 at 16:36
  • \$\begingroup\$ Ok now it should make sense^^ \$\endgroup\$ – watchme May 30 '18 at 16:37
  • 1
    \$\begingroup\$ Modem is MODulator - DEModulator. That is it is modulating a signal to be put on a carrier signal and vice versa. Regardles of the carrier type. \$\endgroup\$ – Eugene Sh. May 30 '18 at 16:41

A very simple explanation is as follows:

  • Most digital systems use "logic level" signals to signal a binary 0 or 1. This is typically 0 V for '0' and 5 V for a '1'.
  • The system may have to transmit long strings of zeros or ones. This is, in effect, a steady DC signal.
  • The telephone system is designed to transmit voice signals and has a bandwidth of about 300 Hz to 3000 Hz. DC can not be transmitted through it.

enter image description here

Figure 1. A modem connection between two computers. Source: MyStudentSite.

  • To work around the problem a simple two-tone system (think of two musical notes with a reasonable interval between them but not harmonically related) is used. To transmit a 0 we can send a low tone. To transmit a 1 we then use the high tone.
  • The transmitting modem MOdulates the digital data into analog audio. The receiving modem DEMmodulates the audio tones back into digital signals. MO+DEM = MODEM.
  • \$\begingroup\$ thanks, very clear answer! I don't know if you know this but: when somebody says "ISDN is when the 'last mile' got digitalized" he doesn't mean that there is a digital signal, but a analog signal which indicates if there is a 1 or a 0, right? \$\endgroup\$ – watchme May 30 '18 at 16:43
  • \$\begingroup\$ I'm afraid that is a different question so I will not answer it here. Ask another question but read about ISDN first. Don't listen to "somebody". ISDN is digital all the way. \$\endgroup\$ – Transistor May 30 '18 at 16:50
  • \$\begingroup\$ But when there are just two tones (high tone 1, low tone 0), how can ones and zero be transmitted so fast? It apparently doesn't matter if the audio signals interfere...right? \$\endgroup\$ – watchme May 30 '18 at 17:35
  • \$\begingroup\$ They are transmitted sequentially - one after another. You don't transmit ones and zeros simultaneously. \$\endgroup\$ – Transistor May 30 '18 at 17:43

A modem modulates a waveform (either in phase, amplitude or frequency. Or a combination to send even more data) to transmit ones and zeros. The modem waveform shows the analog signal (frequency shift keying) above and the corresponding reconstructed digital signal below.

Comm theory explains how these signals are transmitted and received and how to build systems that can transmit messages in the presence of noise.

enter image description here


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