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G8MNY > TECH 24.05.08 08:46l 78 Lines 2775 Bytes #999 (0) @ WW
BID : 56811_GB7CIP
Read: GUEST
Subj: What is Multiplexing
Path: IZ3LSV<IQ0LT<IW2OHX<I0TVL<ED1ZAC<GB7CIP
Sent: 080523/2207Z @:GB7CIP.#32.GBR.EU #:56811 [Caterham] $:56811_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Jan 08)
(8 Bit Graphics use code page 437 or 850)
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 where each circuit
is on different conductors.
.---.
/:::::\ : = Circuit pairs
³:::::::³
\:::::/Cable
'---'
Another example would be Satellites, where bands of frequencies/channels are
reused on satellites on different beam headings e.g. each 1.5ø for comms sats.
X Band X Band X Band
Sat A Sat B Sat C
\ | /
\ | /
\ | /
Different
Dish Angle
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 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.
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.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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