# Multiplexing, narrowbanding

1. Aug 4, 2014

### trupha

1)what are multiplexing and narrowbanding,basically and how do they arise the need for modulation?

2)how actually ,practically (not mathematically) that carrying of baseband signal by the carrier signal happens?

2. Aug 4, 2014

### Staff: Mentor

Welcome to the PF.

Could you please provide some background and context for your question? Where have you seen these terms used? Can you provide some links?

What is your background in math? Are you currently in University? Is this for schoolwork?

3. Aug 4, 2014

4. Aug 4, 2014

### davenn

Hi trupha
welcome :)

very basically
multiplexing is combining multiple input signals onto a single say phone line, radio carrier frequency etc
narrow banding --- is to do with the band width of the signal. for example the bandwidth of a radio transceiver
may be 7.5kHz, 12.5kHz or 25kHz. These would be considered narrowband

on the other hand wide band signals say a commercial FM stereo radio station can have a bandwidth of 75kHz or more or a TV transmission can be 7MHz or more

The baseband signal is usually the audio or data signal that is used to modulate the carrier signal.
I have seen cases where the baseband signal is a modulated lower frequency RF signal which can be used around a studio etc for easy monitoring and then it can be mixed with the, much higher frequency, main transmitter carrier signal.

That way a more indepth answer can be provided

Dave

5. Aug 5, 2014

### skeptic2

Expanding a little on what davenn posted, narrowbanding is a process the 2-way radio users should have completed by Jan 2013. Before that radio channels 25 kHz wide were allowed. With the Jan 2013 deadline 2-way radios can use only 12.5 kHz or less per channel. So if 2 channels are multiplexed onto 1 carrier, they are allowed a 25 kHz bandwidth. Amateur radio and pagers are exempted from narrowbanding.

The FCC will no longer renew any license with wideband emissions on it (e.g. 20K0F3E, 16K0F3E).

6. Aug 5, 2014

### trupha

yes,im in university.
no,its not for homework.

actually Im studying btech electronics&communication,and I found these terms there,where they explained the basic need for modulation,they were size of antenna,narrowbanding,multiplexing....so the later two I could not understand,what they are and how they arise the need for modulation.

for the second one,i wanted to know how actually,the modulation is done,practically..

sorry for the indefinite question,but I hope I`m clear now :)

7. Aug 5, 2014

### Staff: Mentor

The wikipedia entry on Modulation is pretty helpful:

http://en.wikipedia.org/wiki/Modulation

8. Aug 7, 2014

### meBigGuy

modulation is simply the process of varying the carrier in some way that can be detected. You can vary the strength (amplitude modulation) by changing the gain of a carrier generator. You can vary the phase of a carrier by varying the phase via a filter. You can vary the frequency of the carrier's oscillator. Lots of other techniques to vary phase and or amplitude.

9. Aug 7, 2014

### sophiecentaur

You can multiplex signals together in several ways. The simplest analogue multiplexing methods are frequency multiplexing, which involves using AM with a number of different carrier frequencies, to separate the channels in frequency (discussed above). This requires at least nB of bandwidth, where n is the required number of input signals. n receivers, tuned to the n multiplexed channels, demodulate the n different signals. This is just like the MF radio bands - but it all goes down a wire.
Also you can time division multiplex. Basically, this involves the use of a switch that selects short bursts (samples) of each of a number of input signals (imagine a rotary switch with the inputs arriving at the edges and a selection arm whirring round in the middle). You then send the train of samples (one after the other) down a line. The line, of course, has to be able to cope with very fast changes, from sample to sample and the bandwidth needed is only of the same order as that used by FDM (about nB). At the other end, a similar (synchronised) switch selects the interleaved signals in sequence and joins up the samples, delivering them into the wanted output channel.

There are dozens of sites where you can read about the basics of Amplitude Modulation (and other forms of mod). In many ways it is better to seek them out for yourself as you will be able to find one that makes best sense to you, personally. (But you need to check the content against other sites to test its accuracy).