Frequency division multiplexing

In summary, telephone signals are multiplexed into a group signal with 48kHz bandwidth, using subcarrier frequencies from 420-612kHz in 48kHz increments.
  • #1
gnome
1,041
1
Apparently a standard FDM scheme for telephone signals is to multiplex 12 voice channels, each occupying 4kHz bandwidth, into a group signal with 48kHz in the range 60-108kHz, using subcarrier frequencies from 64-108kHz in 4kHz increments, and using only the lower sideband of each modulated signal. This all seems to make sense -- the arithmetic works.

Five of these group signals are then multiplexed together into a "supergroup", again using only the lower sideband of each modulated signal, to produce a combined signal with a 240kHz which occupies the 312-552 kHz band. So far so good. The problem is, supposedly the subcarriers have frequencies from 420 to 612kHz in 48kHz increments. How does a signal with 48kHz bandwidth, modulated on a 612 kHz subcarrier, end up below 552kHz? There seems to be a downward shift of 60kHz but I can't find an explanation of this anywhere.

I read this in William Stallings' Data and Computer Communications (7e), and it's repeated in numerous places on the web. Some of them are definitely just taking it from Stallings; others, well, they may be taking it from him too, but they don't make that clear. In any case, none of them explain this shift.

Is this just an error that's being copied over and over, or is there a good explanation that everybody is simply failing to mention?
 
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  • #2
That does sound fishy. Could you post a couple links?
 
  • #3
Look at pages 7 and 9 of this one:
http://ee.hannam.ac.kr/~ckang/index...tes/lecture.files/datacomm.files/[new]ch8.pdf

or slide 8 of this one:
http://www.csse.monash.edu.au/courseware/cse4882/lects/lect07.pdf

along with page 17 of this:
http://www.utdallas.edu/~cantrell/ee6345/lectures/pstn.pdf

This one looks as if it might not come straight out of Stallings:
http://dar.ju.edu.jo/mansour/723/Auxiliary/01Introduction/Basics of Communication Technology.doc

Here's one that definitely does NOT come from Stallings. It confirms the 312-552 kHz band, but I can't find anything in it regarding subcarrier frequencies. See e.g. pg. 5-16 (section 5.2.5):
http://www.iowatelecom.com/PDFs/FCC 1 Section 5A -- Special Access092804.pdf
 
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1. What is frequency division multiplexing (FDM)?

Frequency division multiplexing is a method of transmitting multiple signals simultaneously over a single communication channel by dividing the channel into several subchannels, each using a different frequency band. This allows for more efficient use of the channel and increases the overall data transmission capacity.

2. How does FDM work?

FDM works by separating the available bandwidth of a communication channel into multiple subchannels, each with its own frequency range. Each subchannel is then used to transmit a different signal, and the signals are recombined at the receiving end by using a device called a demultiplexer.

3. What are the advantages of FDM?

One of the main advantages of FDM is its ability to allow multiple signals to be transmitted simultaneously over a single communication channel. This leads to increased efficiency and higher data transmission capacity. FDM is also relatively simple and cost-effective to implement.

4. What are the limitations of FDM?

One limitation of FDM is that it requires a wide bandwidth to be effective, as each subchannel needs its own frequency range. This can be a challenge in situations where bandwidth is limited or expensive. Additionally, if one of the subchannels experiences interference or noise, it can affect the overall quality of the transmission.

5. How is FDM used in practical applications?

FDM is commonly used in various communication systems such as cable television, radio broadcasting, and telephone networks. It is also used in data communication systems, such as DSL and satellite communication, to increase the data transmission capacity. FDM is also used in scientific research and instrumentation to transmit multiple signals from sensors and devices over a single channel.

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