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communication

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why dont we transmit digital signal in the air?

Because any transmitted signal needs a carrier which does as its name implies. You just can't start transmitting 1s and 0s. :)

Ron
 
Because any transmitted signal needs a carrier which does as its name implies. You just can't start transmitting 1s and 0s. :)

Ron

Well, actually we do all the time. It's called "spread spectrum communications". Cell Phones, WiFi,Wireless home telephones, and the GPS satellite system all use it, for example...
 
Well, actually we do all the time. It's called "spread spectrum communications". Cell Phones, WiFi,Wireless home telephones, and the GPS satellite system all use it, for example...
But all these schemes still modulate a carrier with the 1s and 0s.
 
I think the OP means "why dont we just connect a digital data source to an antenna?"

Digital signals have a very wide bandwidth, inthe frequency domain they contain many separate frequencies.
If this were connected to an antenna, it would just create a lot of noise in a radio receiver.
Hold a radio next to you computer, you will get the idea.

JimB
 
Well, actually we do all the time. It's called "spread spectrum communications". Cell Phones, WiFi,Wireless home telephones, and the GPS satellite system all use it, for example...

Yes Mike but I took the original post to mean just send 1s and 0s without an RF carrier. Or actually any carrier.

communication
why dont we transmit digital signal in the air?

Maybe I just don't know how to interpret the question? We transmit digital information all the time, everyday. I transmit digital information when I change channels on my TV.

Ron
 
But all these schemes still modulate a carrier with the 1s and 0s.

Look at the GPS signal, for example, with a spectrum analyzer. I challenge you to find anything that looks like a "carrier". Ditto WiFI.
 
Look at the GPS signal, for example, with a spectrum analyzer. I challenge you to find anything that looks like a "carrier". Ditto WiFI.
Perhaps we talking semantics here, but if the signal is not at baseband (the original signal frequency components), then it's a signal modulated unto a higher frequency carrier. You may not be find anything that looks like a "carrier" signal in the spectrum since modern digital modulation methods use a suppressed carrier (to conserve power), but that doesn't mean there is not a carrier. With a suppressed carrier, the signal will appear as some band of modulation frequencies centered at, or on one side of the carrier (SSB-SC for example). If you don't think there's a carrier, try demodulating these signals without some method of regenerating the carrier signal in the receiver.
 
Look at the GPS signal, for example, with a spectrum analyzer. I challenge you to find anything that looks like a "carrier". Ditto WiFI.
True.

But use the spectrum analyser to look at a signal which is frequency modulated with a 1 khz tone and turn up the deviation to say 400khz.
Any semblace of something with a carrier is long gone.

JimB
 
But still we need to modulate it....! another thing is that can air supports digital data??? is'nt it analog medium?? i still did'nt understand what mike say?
 
Let's get simple and think on-off keying. Carrier present =1, no carrier = 0 and through a chain of of ons and offs with some delineation we can pass information. I really do not understand this question. RF communication can be a form of serial data communication much like RS-232. So I don't get the question I guess, but I am in agreement with Mike at this point. Not the me Mike, the other Mike:)
 
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Let's get simple and think on-off keying. Carrier present =1, no carrier = 0 and through a chain of of ons and offs with some delineation we can pass information. I really do not understand this question. RF communication can be a form of serial data communication much like RS-232. So I don't get the question I guess, but I am in agreement with Mike at this point. Not the me Mike, the other Mike:)
Turning the carrier on and off with a digital signal is a form of modulation. If you look at the RF spectrum you will see the carrier plus harmonics of the on/off frequency.
 
so, that means we can transmit Digital signal means 1s and 0s in the air...! according to the ans of ajitkumar...!
if it require more bandwidth, this is the only disadvantage in digital communication.... isnt it....!
 
so, that means we can transmit Digital signal means 1s and 0s in the air...! according to the ans of ajitkumar...!
if it require more bandwidth, this is the only disadvantage in digital communication.... isnt it....!
That's not the only disadvantage. The other disadvantage is that you would be interfering with other legitimate signals in the spectrum and would soon draw the close attention of the FCC. The purpose of modulating a signal is to keep it in an assigned frequency spectrum and bandwidth as designated by the FCC. Operating outside these limits is illegal.
 
oh! i didnt know that , you are saying that process of modulation is done to be in the Rules and assigned frequency and bandwidth designated by FCC? mean that we can still transmit data without modulation....! how it can happen for larger distances..? i read in my book that for larger distances and communication the process of modulation is done, as low frequency signals can not be transmitted because for that large antennas are required, for that we modulate the signal....! i am now confusing with answers,,,, i only asked that can we transmit Digital signal having 1s and 0s in the air ...? sir plz clear my point........!
another thing is that Japan is communicating at there own frequency ranges and bandwidth, i think it is from 79 to 90mhz i think so...! what about that????
 
I mean you can not transmit un-modulated data directly without violating FCC rules (unless it's a very low power transmitter over a very short distance, perhaps a few tens of meters). Other countries have similar rules. These rules usually cover the RF frequencies from the low kHz up to hundreds of GHz.
 
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