Naked Science Forum
General Science => General Science => Topic started by: classic_phone on 05/02/2009 02:16:27
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Hi,
Im wondering whats the difference between carrier wave and single tone signal?
Both of them are Sinusoids with Single Frequency.
Is there a convention that above certain frequency it is call a carrier wave, specifically for Landline application?????
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Easy!
A single tone signal carries only one piece of information - its frequency. It goes on at the same level for ever.
To transfer any more information than that, you have to modulate the amplitude, phase or frequency of this 'tone'. As soon as you vary some aspect of the tone, you can call it a carrier wave - it is carrying information and will then occupy more spectrum space (bandwidth) and other frequencies will be introduced.
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Thank you very much for the info!
So meaning single tone + information = carrier wave ?
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The single tone becomes a carrier wave once it carries information. The other frequencies are known as sidebands.
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what does the other frequencies refers to?
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When a sinusoidal single tone signal is less than 100% amplitude modulated with information, say for example a lower frequency sinusoidal signal, This produces sidebands at the carrier frequenct plus and minus the single tone modulation. For a more complex signal say voice or music the sidebands contain the frequencies in the modulating signal. For small amplitude frequency modulation the sidebands are similar except tha the phase of the positive sidebands are in antiphase with the negative sidebands.
These methods of modulation are simple and rather inefficient because the energy devoted to transmitting the information is much less than the total transmisison energy also the bandwidth occupied is larger than the highest frequency it is wished to use ie only about half a bit of information or less per cycle per second. Most modern communications systems use more efficient modulation methods like single sideband where only one of the sidebands is transmitted without any carrier or digital phase modulation where many bits of information can be transmitted for every cycle per second of bandwidth
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The point about communicating information is that it takes up bandwidth (spectrum space). For simple modulation systems, the bandwidth is at least twice that of the highest frequency in the information (audio / TV / data stream) you want to carry.
Even if you are only sending morse code by hand, there are sidebands generated by turning the carrier on and off. These could be only a few Hz on either side of the carrier but they are still there.
This is a sort of fundamental law of nature - a bit like the one which says that you can't have perpetual motion and that things always cool down, eventually.