Naked Science Forum
Non Life Sciences => Technology => Topic started by: PAOLO137 on 17/06/2014 17:08:04
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I understand that in digital broadcasting ( radio, TV, ect.) the carrier wave must be as before a sine wave since this is the way electromagnetic
radiation travels. What I don't find described anywhere is : what is the modulation that allows bits to be transmitted? Is it a matter of jumping
up and down between two amplitudes? Thanks for the attention, Paolo de Magistris, Rome.
[mod edit- please phrase subjects as questions in line with AUP]
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I understand that in digital broadcasting ( radio, TV, ect.) tha carrier wave must be as before a sine wave since this is the way electromagnetic radiation travels.
The modulated signal need not be sinusoidal ...
(https://upload.wikimedia.org/wikipedia/commons/thumb/3/39/Fsk.svg/533px-Fsk.svg.png)
https://en.wikipedia.org/wiki/Frequency-shift_keying
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I understand that in digital broadcasting ( radio, TV, ect.) tha carrier wave must be as before a sine wave since this is the way electromagnetic
radiation travels. What I don't find described anywhere is : what is the modulation that allows bits to be transmitted? Is it a matter of jumping
up and down between two amplitudes? Thanks for the attention, Paolo de Magistris, Rome.
A sine wave is one particular way EM radiation can travel, but it is not the only way. We use it a lot because its easy to deal with mathematically. The shapes RD posted are certainly allowed, as are many (infinitely many) others.
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The problem with using a pure sine wave to transmit data is that a sine wave continues forever, and has a bandwidth of 0 Hz. Unfortunately, with this waveform, you can transmit zero data, and it takes forever to reach the destination.
So, in practice, we use waveforms that are not pure sine waves, but are modulated (or changed) to add information to the original sine wave. The process of modulation means that the signal is no longer a pure sine wave, and it no longer has zero bandwidth, so it can carry information at a realistic and useful rate.
So we can support many different services on our shared radio spectrum, we also try to get the most data in the minimum bandwidth, a figure of merit called bps/Hz, or "Bits per Second (data speed)" per "Hertz (a measure of the radio spectrum used)".
There are many ways to modulate a signal, but one of the most common for digital transmission today is called Quadrature Amplitude Modulation, or QAM. This changes both the amplitude and the phase of the sine wave. US Digital TV uses 64-QAM, with 64 different combinations of phase and amplitude, carrying 6 bits per symbol.
See: http://en.wikipedia.org/wiki/QAM_(television) (http://en.wikipedia.org/wiki/QAM_%28television%29)
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The problem with using a pure sine wave to transmit data is that a sine wave continues forever, and has a bandwidth of 0 Hz. Unfortunately, with this waveform, you can transmit zero data, and it takes forever to reach the destination.
Not quite true. You the sine wave has an amplitude and frequency, so you can transmit two pieces of data. If it is an EM sine wave it also has polarization and direction, so you can transmit two other pieces of information. ;)
That's being pedantic, though--it is certainly not an efficient way to transmit data, even if it is a sinusoid with a start and stop and finite bandwidth!
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The theoretical throughput of a communication channel is described by the Shannon-Harley theorem (http://en.wikipedia.org/wiki/Shannon-Hartley_theorem):
C = B log2(1+S/N)
where:
- C is the channel capacity in bits per second;
- B is the bandwidth of the channel in hertz
- S is the average received signal power
- N is the average noise
- S/N is the signal-to-noise ratio (SNR)
With suitable error correction codes, it is possible to come fairly close to this theoretical limit.
One implication of this theorem is that a pure sine wave (Bandwidth= 0 Hz) can transmit no data.
If you want to transmit a message containing 1 bit of data (using amplitude modulation) or 2 bits (with polarisation modulation), you must turn on the sine wave. The action of turning on the sine wave increases its bandwidth to (slightly) more than 0 Hz, so you are able to transmit a small amount of information.
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You're correct. If you use Shannon information, you can transmit zero information on a single pure sinusoid. But this sinusoid had to be created at some point in time, and at that point, information is transmitted: polarization, frequency, amplitude, phase offset, direction. I suppose the fact that the Shannon of an infinite sinusoid is zero is a manifestation of the fact that this is an unreasonable approximation to a physical signal.
This is far too pedantic for the OP's question, however and I think we both agree that whether you send a handful of bits of data or zero with a single sinusoidal plane wave, it's not a useful way of sending data.
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The reference in Wikipedia did not seem particularly informative except to suggest that the industry has played games to thwart interoperabiltiy and access to signals when it suited them for business reasons.
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I tried searching out other links through Widipedia related to the ATSCstandard but was unable to find a working Web address for a source.
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The main ATSC standards seem to be here: http://www.atsc.org/cms/index.php/standards/standards?layout=default
The one I downloaded seemed to work: http://www.atsc.org/cms/standards/a53/A53-Part-1-2013.pdf
But don't expect it to be an easy read. A Digital TV standard draws on a wide variety of fields:
- Modulation changes the spectrum of the carrier(s) in mathematically complex ways
- Many stations need to fit into the available spectrum, so the bandwidth must be shaped
- Error-correcting codes rely on advanced number theory
- Signal processing often uses the Fourier Transform to analyse the data
- Reflections from mountains and buildings interfere with the signal, and these must be filtered out
- The characteristics of human visual and auditory perception.
- Reducing the bandwidth requires data compression algorithms
- The audio and video must be synchronised on playback.
- Transmitting supplementary information such as channel name, electronic program guide, subtitles and weather information requires their own standards.
- Standards are usually written to cover the interface between the transmitter and the receiver, which is the signal transmitted via radio. The standards often leave out many essential details which must be implemented in the transmitter and receiver to get the whole system to work reliably.