Frequency division multiplexing

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SUMMARY

The discussion centers on Frequency Division Multiplexing (FDM) for telephone signals, specifically the multiplexing of 12 voice channels, each occupying a 4kHz bandwidth, into a group signal of 48kHz within the 60-108kHz range. This group signal is further multiplexed into a "supergroup" signal with a bandwidth of 240kHz, occupying the 312-552kHz band. A critical issue raised is the downward frequency shift of 60kHz, which is not adequately explained in the literature, including William Stallings' "Data and Computer Communications (7e)." The lack of clarity on subcarrier frequencies, particularly the transition from 612kHz to below 552kHz, raises questions about potential errors in the referenced materials.

PREREQUISITES
  • Understanding of Frequency Division Multiplexing (FDM)
  • Knowledge of bandwidth allocation in telecommunications
  • Familiarity with modulation techniques and subcarrier frequencies
  • Basic principles of signal processing
NEXT STEPS
  • Research the concept of subcarrier frequency modulation in FDM systems
  • Investigate the implications of bandwidth allocation in telecommunications
  • Explore William Stallings' "Data and Computer Communications" for further insights
  • Examine case studies on frequency shifts in communication technology
USEFUL FOR

Telecommunications engineers, signal processing specialists, and students studying communication technology will benefit from this discussion, particularly those focused on FDM and its applications in voice transmission.

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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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That does sound fishy. Could you post a couple links?
 
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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