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G8MNY  > TECH     09.07.18 10:33l 112 Lines 4533 Bytes #999 (0) @ WW
BID : 28252_GB7CIP
Subj: What is Multiplexing
Sent: 180709/0820Z @:GB7CIP.#32.GBR.EURO #:28252 [Caterham Surrey GBR]

By G8MNY                            (Updated Dec 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

What is Multiplexing
This is where several circuits or channels use the same route/system.

There are only 3 types of Multiplexing or MUX.
1/ Space Division
2/ Frequency Division
3/ Time Division

1/ Space Division Multiplexing
This is not obvious, but is the simplest, it is where the circuits occupy
different physical space.

Examples are multi            .---.
conductor cable              /:::::\  : = Circuit pairs 
where each circuit          │:::::::│
is on different              \:::::/Cable
conductors.                   '---'

 X Band  X Band  X Band         Another example would
   Sat A  Sat B  Sat C          be Satellites, where
       \' . |'˙. /˙             bands of frequencies
        \   |˙  /˙.'.           or channels are reused
         \  |  /' .'˙:.         on satellites on
       Different    'Same       different beam headings.
      Dish Angle    Routes      e.g. each 1.5° for comms Sats.

2/ Frequency Division Multiplexing
This is where the channels are split up on different frequencies like on a
radio. e.g. they all share the same aerial. Cable TV & old cable SSB telephone
system cram 1000s of channels down a coax this way.

 │ch1 ch2 ch3 ch4 ch5 ch6 ch7 ch8     ch7500
 └───┴───┴───┴───┴───┴───┴───┴───┴/ /─┴───
     4k  8k 12k 16k 20k 24k 28k 32k .... 30MHz

Some high capacity Fibre optics systems still use it with different coloured
light to multiply the fibre capacity 1000s of times.

3/ Time Division Multiplexing *
This is where channel is swapped for another normally in sequence. Simplest
example would be day & nighttime TV by 2 different stations. Further example is
Stereo Radio, where the left & right channels are swapped over at 76000 times a
second. Note a synchronisation system is needed to maintain channel separation.

Left ──o  Switch
        \,_______________________┌─────┐    ┌────────┐
Right──o  /|\                    │ Add ├──>─┤ 53kHz  ├───> to Tx
           │   ┌──┐   ┌───────┐  └──┬──┘    │Low Pass│
  76kHz >──┴───┤÷4├───┤ 19kHz ├─────┘-20dB  │ Filter │
               └──┘   │Filter │             └────────┘
A switched audio system like this used to be used for busy transatlantic phone
calls, only switching in the those people who where actually talking.

More complex example is DAB/Digital TV where quality/digital statistical
multiplexing determines which channel get sent in the data stream next.

All 3 at once!
It is possible to get confused if more than one multiplex method is used.

Channels  __________
---------┤  Pulse   │           _________
---------┤   Code   │ DIGITAL  │Coloured │           _____________
---------┤Modulation├──────────┤  Light  │FREQUENCY │Multi Strand │HIGH
---------┤ & Time   │         >┤Frequency├──────────┤ Fibre Optic │CAPACITY
---------┤ Division │   Other >┤Division │   Other >┤Cable. Space │TRUNK
---------┤Multiplex │   Inputs>┤Multiplex│  Inputs >┤Div Multiplex│ROUTE
         └──────────┘          └─────────┘          └─────────────┘

Then huge capacities are possible with all 3 in use.

Quite often several of the above are used at once all invisibly computer
controlled. The advantage is more efficiency use of a route, but as a cost of
far more complexity & reduced reliability. E.g. a DAB Radio is running the
equivalent of a pentium processor to decode the bit stream.

Now just ponder this:- Take two balanced 600ohm lines (diagonals of a quad
cable) & using balanced transformers (or op amps) make a 3rd circuit using the
centre tap phantoms...

          ___    _________________________________    ______
Circuit 1    )||(____                         ____)||(
                     │                       │
          ___    ____│                       │____    ______
Phantom      )||(            Balanced             )||(
Circuit   ___)||(____          Quad           ____)||(______
                     │         Cable         │
          ___    ____│_______________________│____    ______
Circuit 2    )||(____│                       │____)||(

How is this 3rd Phantom circuit multiplexed, Space, Frequency or Time?

* See my TECH bul on "FM Stereo Radio Principles" 

Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

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