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G8MNY > TECHNI 24.08.20 16:50l 123 Lines 4861 Bytes #-1527 (0) @ WW
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Subj: What is Multiplexing?
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
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 ³:::::::³
is on different \:::::/Cable
conductors. '---'
X Band X Band X Band Another example would
Sat A Sat B Sat C be Satellites, where
\' . |'ú. /ú bands of frequencies
\ |ú /ú.'. or channels are reused
\ | /' .'ú:. on satellites on
Different 'Same different beam headings.
Dish Angle Routes e.g. each 1.5ø 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.
³ch1 ch2 ch3 ch4 ch5 ch6 ch7 ch8 ch7500
ÀÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁ/ /ÄÁÄÄÄ
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.
76kHz
Left ÄÄo Switch
\,_______________________ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄ¿
RightÄÄo /|\ ³ Add ÃÄÄ>Ä´ 53kHz ÃÄÄÄ> to Tx
³ ÚÄÄ¿ ÚÄÄÄÄÄÄÄ¿ ÀÄÄÂÄÄÙ ³Low Pass³
76kHz >ÄÄÁÄÄÄ´ö4ÃÄÄÄ´ 19kHz ÃÄÄÄÄÄÙ-20dB ³ Filter ³
ÀÄÄÙ ³Filter ³ ÀÄÄÄÄÄÄÄÄÙ
ÀÄÄÄÄÄÄÄÙ
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 ³ _________
---------´ Code ³ DIGITAL ³Coloured ³ _____________
---------´ModulationÃÄÄÄÄÄÄÄÄÄÄ´ Light ³FREQUENCY ³Multi Strand ³HIGH
---------´ & Time ³ >´FrequencyÃÄÄÄÄÄÄÄÄÄÄ´ Fibre Optic ³CAPACITY
---------´ Division ³ Other >´Division ³ Other >´Cable. Space ³TRUNK
---------´Multiplex ³ Inputs>´Multiplex³ Inputs >´Div Multiplex³ROUTE
ÀÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
Then huge capacities are possible with all 3 in use.
CONCLUSION
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.
QUESTION.
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 )||(____ ____)||(
___)||(____³_______________________³____)||(______
³ ³
___ ____³ ³____ ______
Phantom )||( Balanced )||(
Circuit ___)||(____ Quad ____)||(______
³ Cable ³
___ ____³_______________________³____ ______
Circuit 2 )||(____³ ³____)||(
___)||(_________________________________)||(______
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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