digital vs analog transmission ?

Discussion in 'Wireless & RF Design' started by Mathematics!, Nov 12, 2009.

1. Mathematics! Thread Starter Senior Member

Jul 21, 2008
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Ok , I know comcast ,...etc switch to broadcasting in digital over the air .

digital = square waves (discrete wave)
analog = sine waves (continous waves )

My question is how can you ever broadcast completely square waves threw the air? Won't interference or noise change the square waves into looking like more like continous wave?

At least when broadcasting a 1 where the voltage is other then zero you would not have a flat constant voltage at all times. Some fluctation will occur???

Thanks for any help

2. studiot AAC Fanatic!

Nov 9, 2007
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513
Most detection systems are 'edge triggered'. This means that they work on the transition, not the whole wave. Transitions may be positive going or negative going.
For a big enough transition (change) it will not matter if the waveshape is untidy.

3. davebee Well-Known Member

Oct 22, 2008
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Digital information is broadcast not by trying to broadcast an actual square wave, but rather by having the original digital signal do some sort of switching or modulation to sine waves before they are broadcast.

Over the years, engineers have come up with all sorts of ways to digitally modulate a signal, from Morse code, where the sine is just turned on and off (called CW, for continuous wave (even though the wave really isn't continuous) or also known as on-off keying, or OOK), to many other types of amplitude, frequency and phase modulation techniques with acronyms like QFSK, FSK, ASK, PSK, QAM, MSK, CPM, PPM, TCM. OFDM, DTMF, CPFSK, GMSK, TCM, and I'm sure many more. These all just do something to the carrier being transmitted.

In any case, the receiver has to have been designed to detect the change of the carrier signal and translate that back into the original digital signal.

4. Papabravo Expert

Feb 24, 2006
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1,757
This assumption on your part is completely misinformed. One of the digital methods is called quadrature amplitude modulation, or QAM. No need to Google it just yet, I'd be willing to bet that the explanation will sail right over the heads of most people including many long time members. It is truly complex stuff.

http://www.complextoreal.com/chapters/mod1.pdf
http://zone.ni.com/devzone/cda/tut/p/id/3896

A lot to swallow -- isn't it
Any complex waveform can be expressed a Fourier sum of harmonic components.

5. Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I guess I always thought of digital waves as being square waves.

I usually picture analog waves as FM/AM sine waves.

Have done avr chip to wire to avr chip to computer, programs sending ascii characters using PWM. Thats what I picture as digital waves.

So I guess I have to generalize my outlook on what digital waves and analog waves look like.

MY MAIN QUESTIONS
ARE IS THEIR SOME DEFINING FACTOR THAT DISTINGUISH THE GRAPH OF A DIGITAL WAVE FROM THE GRAPH OF A ANALOG WAVE?
(WHAT I WAS THINK IS THAT DIGITAL WAVES MUST EITHER HAVE DISCONTIUNOUS AT POINTS ON THE GRAPH OF ITS WAVE OR MUST BE PIECEWISE CONTINOUS BUT NOT FULL BLOWN CONTINOUS DIFFERENTIABLE ,...ETC AT ALL POINTS ) WHERE AS ANALOG IS CONTINOUS AND SMOOTH I.E DIFFERENTIABLE AT ALL IT POINTS. (<-SETTING ASIDE NOISE )

MAYBE THOSE ARE THE DEFINING FACTORS???

TAKE THE MORSE CODE EXAMPLE IF YOU USE AM OR FM CARRIER WAVES AND TURN THE TRANSMITTER ON THEN OFF THIS WOULD BE AN EXAMPLE OF USING ANALOG WAVES TO CREATE A DIGITAL WAVE.
WHEN YOU STOP THE TRANSMITTER YOU GET YOUR DISCONTINUTIES IN THE ANALOG WAVE WHICH MAKES IT DIGITAL.
SO THE FIRST CW MORSE CODE BROADCAST WAS A FORM OF DIGITAL BROADCAST.

MY OTHER QUESTION
IS OTHER THEN THE GRAPHS OF DIGITAL AND ANALOG WAVES WHAT IS THE DIFFERENCE BETWEEN THE TWO?
IS THE ONLY DIFFERENCE IS ONE CAN ONLY HAVE A DISCRETE NUMBER OF SYMBOLS/STATES DETECTED BY THE DEMODULATOR AND THE OTHER ONE CAN HAVE A INFINITELY CONTINOUS AMOUNTS OF DIFFERENT SYMBOLS/STATES . BUT MAYBE THEIR IS MORE AND OBVIOUSLY THIS DEPENDS ON THE DEMODULATOR.

ALSO I HAVE READ THAT ANALOG CHANNELS TAKE UP MUCH MORE BANDWIDTH THEN A DIGITAL CHANNEL.
WHY IS THIS? AND HOW MUCH BANDWIDTH DO YOU NEED FOR A TYPICAL ANALOG OR DIGITAL CHANNEL?

THANKS FOR ANY HELP.

Last edited: Nov 13, 2009
6. Papabravo Expert

Feb 24, 2006
10,021
1,757
All real waveforms are analog. The digital waveforms in and out of your Arduino are both continuous and differentiable. If you look at the waveshaping of a good CW transmitter you will notice the exponetial rise and fall to avoid key clicks.

I've been doing all this for a long time and it comes down to levels of abstraction. If using the abstraction of voltages representing 1's and 0's then it doesn't matter from a digital perspective what voltage levels represent those values. If the information is encoded in the frequency content of a signal then the digital 1's and 0's abstraction no longer applies. If the information is encoded in the continuous variation of the amplitude then the digital 1's and 0's abstraction does not apply. If we are talking about an uncompressed WMA file at 24/96 then the digital abstraction applies until we do the D/A conversion and suddenly we have a 1:1 correspondence between the digital world and the analog world.

Do you get my drift?

7. blueroomelectronics AAC Fanatic!

Jul 22, 2007
1,758
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What's with the all CAPS?

8. Papabravo Expert

Feb 24, 2006
10,021
1,757
Nobody told him it is the verbal equivalent of yelling!

9. Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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Sorry about the all caps. I just wanted to get my point across.

I thank you for all your help but as I dive further and further into it I cann't really see any major defining factors that seperates digital/analog waves/transmission ?

Question why does analog channels take up more bandwidth then the digital? ( I am think of the 62 analog TV channels to the hundreds of different digital channels that can be supported as an example)

Also why is digital less prown to interference then analog ? (I have heard this being said but maybe this depends more on the modulation/demodulation scheme. ( For example FM is not as prown to interference as AM partly because phase is harder to interfer then amplitude of a wave but as for digital modulation don't really know how much better then analog? ...etc)

10. Papabravo Expert

Feb 24, 2006
10,021
1,757
I see no evidence that you understand the difference between VSB and QAM. Without some effort on your part, any explanation we might offer would be pointless.

11. davebee Well-Known Member

Oct 22, 2008
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I'd say the defining factor would be that a digital signal has discrete states where an analog signal doesn't. Apart from that, they both are analog, from a physical point of view.

I think the digital signals are compressed before the signal modulate the carrier, which allows use of a narrower bandwidth since less information is being sent. But I don't know about resistance to interference, unless maybe error-correction codes are built in.