digital signal transmission

Replies

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  Jeffrey Arulraj

  proffy

  hi friends, I got confused reading one article on LANs.
  
  Can anyone tell me if we can transmit digital signals(not digital data) via any physical media such as wired cable/coaxial cable, or a wireless media without any modifications to it.
  
  thanks

  To a small distance I think through a coax cable we can transmit
  
  Digital signal is just the presence and absence of voltage 0 or 1 so if the line losses are absent (Theoretically) then we can use digital signal directly
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  Anand Tamariya

  Digital data (0 and 1) directly translates to digital signal(5V and 0V). Digital signal can't travel long distances (low voltage) - hence they need some sort of modulation. Consequently you need slightly modified physical media for optimum performance.
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  YepSailOn

  Actually, digital signals have better noise immunity compared to analog ones because you only have to worry about two levels. So, they can be transmitted farther before signal degradation reaches unacceptable levels.
  
  I'm not certain if modulation is used in case of physical media like lan cables, afaik it isn't. Modulation is used with telephone cables though (ppoE) but in case of wireless technologies, modulation is obviously necessary. Also, digital signals aren't always 5/0 Volts
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  Anand Tamariya

  YepSailOn

  Actually, digital signals have better noise immunity compared to analog ones because you only have to worry about two levels. So, they can be transmitted farther before signal degradation reaches unacceptable levels.

  Signal degradation in this case is not because of noise, but simple resistance in the conducting wires.
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  Jeffrey Arulraj

  YepSailOn

  Actually, digital signals have better noise immunity compared to analog ones because you only have to worry about two levels. So, they can be transmitted farther before signal degradation reaches unacceptable levels.
  
  I'm not certain if modulation is used in case of physical media like lan cables, afaik it isn't. Modulation is used with telephone cables though (ppoE) but in case of wireless technologies, modulation is obviously necessary. Also, digital signals aren't always 5/0 Volts

  Modulation in LAN cables are not necessary as the net distance to transfer the data is very less and can provide lesser resistive path
  
  But in the case of telephone cables or those that are to be transmitted through longer distance then the need of modulation is a must as the signal noises become really a matter of concern
  
  And Who said Digital transmission is Noiseless analyse the fourier spectrum of the signal in digital form along with white noise the noise component is significant But it is limited due to Noise figure of the system
• 

  YepSailOn

  Anand Tamariya

  Signal degradation in this case is not because of noise, but simple resistance in the conducting wires.

  In any communication system, noise is what determines how far you can transmit something. Any signal be it continuous or discrete will degrade, the amount of noise in the channel (the wire in this case) determines the level beyond which the signal can be accurately received.
  
  Noise is always present and as you've said the wire impedance would cause it to degrade over distance. At some point in the cable, the noise level would exceed the signal and it would therefore be incomprehensible.
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  YepSailOn

  Conqueror

  Modulation in LAN cables are not necessary as the net distance to transfer the data is very less and can provide lesser resistive path
  
  But in the case of telephone cables or those that are to be transmitted through longer distance then the need of modulation is a must as the signal noises become really a matter of concern
  
  And Who said Digital transmission is Noiseless analyse the fourier spectrum of the signal in digital form along with white noise the noise component is significant But it is limited due to Noise figure of the system

  That's what I was wondering too, who said digital transmission is noiseless? 😛 What I said specifically was that compared to a continuous signal, the digital one has better noise immunity because we are only concerned with two levels. A logic 0 or a logic 1.
  
  Also, could you explain further what you were saying with respect to the Fourier analyses? From what I know, the Fourier spectra of a digital signal has several components, ie there are several frequencies for a given digital wave, but how does that relate to noise?
  
  You are right about telephone cables using modulated signals but that is for other reasons. Modulation allows better utility of the cable because you can transmit several modulated signals along with the voice signal simultaneously. Also, when it comes to transmission lines the line impedance is what we would consider, not resistance, at typical transmission frequencies, capacitance and inductances in the cable really mess up the signal over a distance.
• 

  Anand Tamariya

  YepSailOn

  In any communication system, noise is what determines how far you can transmit something. Any signal be it continuous or discrete will degrade, the amount of noise in the channel (the wire in this case) determines the level beyond which the signal can be accurately received.
  
  Noise is always present and as you've said the wire impedance would cause it to degrade over distance. At some point in the cable, the noise level would exceed the signal and it would therefore be incomprehensible.

  An ounce of practice is better than a pound of theory. Why don't you test out your hypotheses and let me know your results about how far can you transfer digital signal?
• 

  Jeffrey Arulraj

  YepSailOn

  Also, could you explain further what you were saying with respect to the Fourier analyses? From what I know, the Fourier spectra of a digital signal has several components, ie there are several frequencies for a given digital wave, but how does that relate to noise?
  
  .

  I think it is going to be a long explanation But you should now that when a Digital transmitted wave is received the noise is only present in the transition points from 0 to 1 and from 1 to 0
  
  This can be effectively studied using the frequency spectra than in time frame That is wat i meant need clarifications go through Allan V Oppenheim's Signals and system
• 

  YepSailOn

  Anand Tamariya

  An ounce of practice is better than a pound of theory. Why don't you test out your hypotheses and let me know your results about how far can you transfer digital signal?

  I thought it over again. My understanding of digital signals was that they were any signal with two discrete levels, +5/-5 or a signal with two discrete frequencies (FSK) but I think that's considered to be modulation too, so you're right, purely digital signal wouldn't travel too far.
• 

  YepSailOn

  Conqueror

  I think it is going to be a long explanation But you should now that when a Digital transmitted wave is received the noise is only present in the transition points from 0 to 1 and from 1 to 0
  
  This can be effectively studied using the frequency spectra than in time frame That is wat i meant need clarifications go through Allan V Oppenheim's Signals and system

  Just tell me what you understand of it. From what I remember, the Fourier transform of a square wave would have infinite frequency components. Were you talking about these components?
  
  As for the noise, did you mean that every transition point of any pure digital signal would CAUSE noise rather?
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  Jeffrey Arulraj

  YepSailOn

  As for the noise, did you mean that every transition point of any pure digital signal would CAUSE noise rather?

  Not really In digital transmission there is a accumulation of errors only in the transition point due to serial filters that attenuates the errors in other constant part of the signal.