Thread: How is information transmitted from satelites?

I'm trying to get to grips with the relations between mind (intelligence/code/information) and energy (force/radiation/vibrational frequencies)... for want of better words to describe these things.

So i'm thinking about satelites used for sending signals to and from T.V's and T.V stations.

How is such complex information such as a movie, or several movies/programs etc, transmitted in the microwaves sent to and from satelites?
digital and analogue is all interesting... it doesn't even need to revolve around T.V satelites or microwaves.

Any knowledge on how information is held or transmitted in energy such as microwave or electromagnetic forces would be most enlightening for me.

There must be something about the energy waves which tells the T.V which pixels and sounds to display on the screen and which sounds to play from the speakers.

It might be easiest to start with the basic principles of analogue radio transmission. The easiest (and oldest) is Amplitude Modulation (AM).

In this case, the radio signal starts of as a single frequency. The amplitude (energy level) of the signal is then continuously varied to correspond to the sounds to be transmitted. For various reasons, AM is not as good as Frequency Modulation (FM). In this case, the frequency of the radio signal is modified by the sound being transmitted.



That is all simple enough for analogue signals. We can then transmit digital data. The simplest way is to just have two frequencies that are switched between (corresponding to 0 and 1 in the raw data). This limits the rate at which data can be transmitted so more complex encoding schemes are used such as Quadrature Phase Shift Keying (QPSK). But in the end, the same principle applies: vary the transmitetd signal based on the data yiou want to send. Decode it at the receiver to recover the original signal.

Ok so your saying the information itself is not in the radiation.

Rather the information is translated into a frequency which can be decoded back into information at the recieving end?

Can you give more details of the process when applied to a movie?

A movie is stored on a disk... Using a large satelite dish the information is converted into a variety of frequencies of energy and sent to a satelite... the satelite redirects those frequencies to a recieving satelite dish... which is then sent to a digibox... the digibox translates the various frequencies into pictures and sounds and transmits the original movie on a T.V screen?

All of these frequencies are sent at the same time? each wave corresponding to the particular part of the movie shown at that time? so it takes as long to transmit the frequencies as it does to play the movie? presumably this could be accelorated?

So one wave of energy could contain various frequencies at the same time? one frequency for the sound, one for the image?

It still seems mind boggling to me that 1080 pixels and all those sounds can be dictated by the frequency of a wave of energy... please explain some more to me strange.

Ok so your saying the information itself is not in the radiation.

Yes, it is modulated on the transmitted radio signal.

I'm not sure I can explain much more, given the level if understanding you seem to be starting from.

Let's put it this way. When you play a DVD, say, the laser scans the dots on the disk and generates a signal that contains the encoded video and audio. That signal gets decoded by the electronics in your DVD player and TV to generate the images and sound (the images are encoded as a series of scan lines of the image, and then a series of frames made up of those scan lines).

Now, take that encoded signal and modulate it onto a radio signal so it can be sent through the air instead of down wire. Simples.

Right, Thanks.

I think I have understood all you have said so far.

I'm more interested in the radiation itself rather than how it is used to encode information.

A satelite uses microwaves?

I wonder how these waves are made or where they come from?
The wave itself will keep it's frequency from the point it leaves the satelite to the point it reaches a digibox/TV? Obviously the radio wave or microwave has to fight through all sorts of other waves on its journey, and these don't affect the waves frequency? how can that be?

So by this 'modulation' it is possible to change the volumn, pitch and timing of a wave? or is that purely for music modulation?

I apreciate it must be difficult explaining this to a layman... but please feel free to try as much as you like... I obviously am not familiar with any of the terminology but every little helps me to gain an understanding.

Those images you posted were intriging... I never realised the difference between AM and FM. Are these images exactly how waves perform? or are they the best representation? It's easy for me to see how the frequency of a wave is portrayed by the pictures... but my understanding it that a radio wave also travels much like a wave in water, and a sound wave... much like a ripple. Those images do not relate a ripple like motion to me, may the frequency of ripples, but not the waves themselves. Do you know what i'm getting at?

Originally Posted by question for you

A satelite uses microwaves?

I wonder how these waves are made or where they come from?

Microwaves are generated by specialised devices such as magnetrons. Basically they have a number of cavities that allow electrons to oscillate at the appropriate frequency and generate the radiation. That is about all I know about it.

The wave itself will keep it's frequency from the point it leaves the satelite to the point it reaches a digibox/TV?

Yes.

Obviously the radio wave or microwave has to fight through all sorts of other waves on its journey, and these don't affect the waves frequency? how can that be?

Electromagnetic waves (or photons, if you prefer to think of it that way) don't affect one another. The sort of problems that have to be dealt with are absorption by trees, reflections from other buildings, etc.

So by this 'modulation' it is possible to change the volumn, pitch and timing of a wave? or is that purely for music modulation?

Yes, modulation can be used to change the amplitude (volume), frequency (pitch) and phase (timing) of the radio signal. These changes don't necessarily correspond directly to the volume, pitch or phase of the original signal. In fact, in digital transmission, they don't have any direct relation. The encoding shemes used are very complex to squeeze the maximum amount of data into the bandwidth.

Those images you posted were intriging... I never realised the difference between AM and FM. Are these images exactly how waves perform?

Pretty much. The main difference between sound waves and electromagnetic waves is that sound is transmitted as a series of changes in air pressure. While radio (and light and all the other forms of electromagnetic radiation) are just variations in electric and magnetic fields.

or are they the best representation? It's easy for me to see how the frequency of a wave is portrayed by the pictures... but my understanding it that a radio wave also travels much like a wave in water, and a sound wave... much like a ripple. Those images do not relate a ripple like motion to me, may the frequency of ripples, but not the waves themselves. Do you know what i'm getting at?

Not really. The radio signal (the carrier) is like a ripple - i.e. a continuous sine-wave change in electric/magnetic strength (like the changing height of a ripple in water or the changing pressure of a sound wave).

Nice job Strange !!

Nice job indeed, this has been a most contstructive and informative thread, thanks you strange.

I still sense I have a lot to understand, so if anybody including strange wants to continue elaborating or going into details then please do.

Originally Posted by question for you

Those images you posted were intriging... I never realised the difference between AM and FM. Are these images exactly how waves perform? or are they the best representation? It's easy for me to see how the frequency of a wave is portrayed by the pictures... but my understanding it that a radio wave also travels much like a wave in water, and a sound wave... much like a ripple. Those images do not relate a ripple like motion to me, may the frequency of ripples, but not the waves themselves. Do you know what i'm getting at?

How do you mean they don't look like ripples? A ripple in water would be sinusoidal (sine wave) and so are the the images. Actually, the images represent one sine wave (the signal) superimposed on another sine wave (the carrier). Actually the signal is not usually a single frequency as shown. It is a complex waveform, but any complex waveform can be represented as a summation of a number of sine waves of various frequencies and amplitudes. This is done by a mathematical trick called Fourier analysis.

A ripple in the water looks like a sine function. A sound wave doesn't look like anything, but if you plot the pressure as a function of time at a certain point, the graph takes the shape of a sine wave. Same with the radio wave. The field strength plotted as a function of time is a sine function.

Originally Posted by Harold14370

Originally Posted by question for you

Those images you posted were intriging... I never realised the difference between AM and FM. Are these images exactly how waves perform? or are they the best representation? It's easy for me to see how the frequency of a wave is portrayed by the pictures... but my understanding it that a radio wave also travels much like a wave in water, and a sound wave... much like a ripple. Those images do not relate a ripple like motion to me, may the frequency of ripples, but not the waves themselves. Do you know what i'm getting at?

How do you mean they don't look like ripples? A ripple in water would be sinusoidal (sine wave) and so are the the images. Actually, the images represent one sine wave (the signal) superimposed on another sine wave (the carrier). Actually the signal is not usually a single frequency as shown. It is a complex waveform, but any complex waveform can be represented as a summation of a number of sine waves of various frequencies and amplitudes. This is done by a mathematical trick called Fourier analysis.

A ripple in the water looks like a sine function. A sound wave doesn't look like anything, but if you plot the pressure as a function of time at a certain point, the graph takes the shape of a sine wave. Same with the radio wave. The field strength plotted as a function of time is a sine function.

Ok I will try to explain what i meant...

The 'signal'... that looks like a sine wave to me, or a wave that we would see in water (and also how I presume sound waves to travel). A pretty much consistence series of waves. Does that make it clearer what I was trying to get at?

Take the AM... over all it looks to me like a sine wave, though within each 'oscilation' of the (over all) wave pattern, we see many more smaller waves

But the AM and FM waves are different. The both contain inconsistence wave lengths... some bigger and stretching further, some smaller and more closely compacted together.
What I am calling an osilation or an 'over all wave, is comprised of aproximately 12 smaller waves which gradually increase to the 'centre' of the over all wave, then decrease between each 'over all' wave, or oscilation. So to me, it looks like a consistent sine wave patern as we would see in the tides of the ocean, containing a lot of smaller 'inconsistent' waves which looks to me like some kind of oscilation of frequency withing an oscilating sine wave. Does that make any sense to you?

With the FM diagram, this 'over all' sine wave is not so apparent, though clearly there is still a sine wave created by aprox. 20 smaller (sine?) waves. These twenty smaller sine waves give me the impression of an insonsistently oscilating frequency with a larger consistent oscilating sine wave (frequency).

Thats the best I can do to explain what I meant at the moment, hopefully you can make some sense of this. Any further questions please don't hesitate, i'm very keen to get my head around this.

Originally Posted by Harold14370

How do you mean they don't look like ripples? A ripple in water would be sinusoidal (sine wave) and so are the the images. Actually, the images represent one sine wave (the signal) superimposed on another sine wave (the carrier). Actually the signal is not usually a single frequency as shown. It is a complex waveform, but any complex waveform can be represented as a summation of a number of sine waves of various frequencies and amplitudes. This is done by a mathematical trick called Fourier analysis.

I'm very tired. I think you answered the question with the highlighted text above.

What I described as the frequency is the signal (the 'inconsistent', smaller waves in the image), super imposed onto the carrier (the consistence larger waves in the image). Is that correct?

man I really am tired.

I beleive the 'signal' as illustrated, is the 'over all' and consistent sine wave I mentioned. The 'carrier' is illustrated by the smaller 'inconsistent' waves that make up the larger wave. Did I get it right this time?

It seemed counter intuitive for me that the carrier would be the smaller waves and the signal the bigger waves that we see.

Well, the am is fairly easy to explain. The amplitude of the carrier wave is modulated (hence the name, amplitude modulation) by a sine wave which is a much lower frequency than the carrier. So the resulting waveform looks like a sine wave whose amplitude is enveloped by a lower frequency sine wave.

If you will notice the fm wave is higher in frequency (the peaks are closer together) at the points where the signal is a maximum and spread out more where the signal is a minimum. The frequency is modulated (fm) by the signal.

Originally Posted by question for you

man I really am tired.

I beleive the 'signal' as illustrated, is the 'over all' and consistent sine wave I mentioned. The 'carrier' is illustrated by the smaller 'inconsistent' waves that make up the larger wave. Did I get it right this time?

It seemed counter intuitive for me that the carrier would be the smaller waves and the signal the bigger waves that we see.

The carrier is just a waveform that is a high frequency but constant amplitude and frequency and is not shown in the animation. The waveform labeled "am" or "fm" is the combination of the signal and the carrier.

Hmmm, but in each AM and FM it appears that the signal is at the same frequency.
In the FM the carrier is at a higher frequency than in the AM.

My next question is...(though you've probably already told me!) why does the length (not frequency) of the carrier waves vary according to the signal in the AM wave form, don't tell me, it's because of 'amplitude modulation'? So the amplitued, is the amount of variation in the carrier waves 'length' (rather than frequency)?

Whereas in the FM... the amplitude stays the same, it is not modulated by the signal wave. Yet the frequency does vary, the frequency is modulated by the signal wave.

Is that correct? Hopefully it is, I feel like i'm getting somewhere now. We have now covered amplitude and frequecy in the diagrams, what about phase or timing? the other thing which strange told me can be modulated.

If I have this all correct so far... I need to move on to understanding how this modulation can occur... how these carrier waves can be contained within a signal wave. Strange touch upon oscilating electrons... or something. What we have is a series of oscilating electrons/photons(?) (carrier), within a series of oscilating electrons/photons(?) (signal)?
And the signal somehow contains the carrier waves within itself through... electromagnetic forces?

Thanks for this fellas, I feel like I'm making some real progress here... You guys must be good!

Originally Posted by Harold14370

The carrier is just a waveform that is a high frequency but constant amplitude and frequency and is not shown in the animation. The waveform labeled "am" or "fm" is the combination of the signal and the carrier.

Thats thrown a spanner in the works... the carrier is not represented in the image? But surely it is? I see two seperate waves in those diagrams... the signal, which is illustrated by the top wave labbelled signal, and the carrier(?) which is represented by the higher frequency waves, whose amplitude is modulated in the AM but not in the FM... and whose frequency is modulated in the FM but not in the AM.

Is this wrong?

Originally Posted by question for you

Hmmm, but in each AM and FM it appears that the signal is at the same frequency.

Correct. As Harold says, that is just a simplified version of a real signal that might be transmitted. It would have been nice if they had included the unmodulated carrier in the diagram. It would have looked like the AM signal but with a constant height (amplitude).

In the FM the carrier is at a higher frequency than in the AM.

On average it is the same. Where the signal level goes up, the frequency is higher. Where the signal level goes down the frequency is lower.

My next question is...(though you've probably already told me!) why does the length (not frequency) of the carrier waves vary according to the signal in the AM wave form, don't tell me, it's because of 'amplitude modulation'? So the amplitued, is the amount of variation in the carrier waves 'length' (rather than frequency)?

Whereas in the FM... the amplitude stays the same, it is not modulated by the signal wave. Yet the frequency does vary, the frequency is modulated by the signal wave.

Maybe we need to make sure we are clear on the terms here:

Amplitude: the "height" of the signal. The FM signal has a constant amplitude (all the waves are the same height). The AM signal has a varying amplitude (it increases and decreases as the signal modulates it.

Wavelength: the "width" of each cycle, i.e. the distance between points on the wave. The distance between peaks on the AM signal is constant. The distance between peaks on the FM signal varies with the modulation by the carrier. This is because....

Frequency: is the inverse of the wavelength; the number of peaks in a given time. You can see that the number of peaks in a given time (left to right in the diagram) increases and decreases in the FM but is constant in the AM. The longer the wavelength, the lower the frequency. The higher the frequency, the shorter the wavelength. Radio stations may give their frequency as, well, frequency (e.g. KHz, MHz) or as wavelength (metres).

We have now covered amplitude and frequecy in the diagrams, what about phase or timing? the other thing which strange told me can be modulated.

I don't think phase modulation is used directly in radio/microwave transmission. It is used to encode signals before that stage so you can get more data in a given bandwidth.

Phase modulation is probably best thought of as a variant of frequency modulation:

If I have this all correct so far... I need to move on to understanding how this modulation can occur... how these carrier waves can be contained within a signal wave.

You know how to whistle? (Put your lips together and blow). Imagine you wanted to encode some information in your whistle; you can do it two ways. Either make the whistle get louder and quieter (AM) or change the pitch (FM).

Or, a more visible (and relevant) example. Imagine you have a light you want to use to transmit information. The easy way is to use a dimmer and make the light get brighter and dimmer to encode (modulate) the information (signal) you want to transmit. That is why AM came first; it is really simple to control the amplitude (volume, brightness, level) of a carrier.

Now imagine your light also has a knob that allows you to change the colour (i.e. frequency or wavelength) through the spectrum from blue to red and back. In that case you can encode (modulate) the signal onto the carrier (the light) by changing the colour (FM).

Radio transmitters do the same sort of thing using vales, transistors and other components to change either the amplitude or the frequency of the transmitted signal.

There is no real benefit to trying to think of this in terms of photons; the "classical" wave model works much better. But, if you insist, AM changes the number of photons (the amplitude of the signal). FM changes the energy of each photon (equivalent to the frequency).

Originally Posted by question for you

Originally Posted by Harold14370

The carrier is just a waveform that is a high frequency but constant amplitude and frequency and is not shown in the animation. The waveform labeled "am" or "fm" is the combination of the signal and the carrier.

Thats thrown a spanner in the works... the carrier is not represented in the image? But surely it is? I see two seperate waves in those diagrams... the signal, which is illustrated by the top wave labbelled signal, and the carrier(?) which is represented by the higher frequency waves, whose amplitude is modulated in the AM but not in the FM... and whose frequency is modulated in the FM but not in the AM.

Is this wrong?

I think Harold just meant the "pure" unmodulated carrier is not shown. The pink and blue lines are the modulated carriers.

Originally Posted by Strange

On average it is the same. Where the signal level goes up, the frequency is higher. Where the signal level goes down the frequency is lower.

Yes I noticed this in the diagram... but in reality, is it 'up' and 'down'? or is it left to right? or is it north to south? I mean since this is electro magnetic waves, then is there some kind of polarity involved? Is it polarity which 'pulls' the photons from the peaks to the troths of the waves? Or is it polarity which attracts the energetic photon to the centre?
Is it a spining of a polarized photon 'partical' which dictactes that it is either going 'up' or 'down' to or from the centre of the wave...?

Originally Posted by Strange

Maybe we need to make sure we are clear on the terms here:

Amplitude: the "height" of the signal. The FM signal has a constant amplitude (all the waves are the same height). The AM signal has a varying amplitude (it increases and decreases as the signal modulates it.

Wavelength: the "width" of each cycle, i.e. the distance between points on the wave. The distance between peaks on the AM signal is constant. The distance between peaks on the FM signal varies with the modulation by the carrier. This is because....

Frequency: is the inverse of the wavelength; the number of peaks in a given time. You can see that the number of peaks in a given time (left to right in the diagram) increases and decreases in the FM but is constant in the AM. The longer the wavelength, the lower the frequency. The higher the frequency, the shorter the wavelength. Radio stations may give their frequency as, well, frequency (e.g. KHz, MHz) or as wavelength (metres).

Yes thats all understood. In the case of a radio wave... what would the the respective wavelengths be at 'max' and 'min' frequencies? how does the compare to say a microwave? (which intuition would tell me is much smaller, therefore higher frequency). Whats the respective 'ranges' of frequency and wavelength?

Originally Posted by Strange

I don't think phase modulation is used directly in radio/microwave transmission. It is used to encode signals before that stage so you can get more data in a given bandwidth.

Bandwidth? a familiar term which has never been explained... is this the band or range between two frequencies? I'e max and min frequency of a waveform?

Originally Posted by Strange

If I have this all correct so far... I need to move on to understanding how this modulation can occur... how these carrier waves can be contained within a signal wave.

You know how to whistle? (Put your lips together and blow). Imagine you wanted to encode some information in your whistle; you can do it two ways. Either make the whistle get louder and quieter (AM) or change the pitch (FM).

Or, a more visible (and relevant) example. Imagine you have a light you want to use to transmit information. The easy way is to use a dimmer and make the light get brighter and dimmer to encode (modulate) the information (signal) you want to transmit. That is why AM came first; it is really simple to control the amplitude (volume, brightness, level) of a carrier.

Now imagine your light also has a knob that allows you to change the colour (i.e. frequency or wavelength) through the spectrum from blue to red and back. In that case you can encode (modulate) the signal onto the carrier (the light) by changing the colour (FM).

Ok, I like this. So the way information is 'given' by electromagnetic waves is bassically the same as morse code, but there are more variables so more information can be encoded in a waveform? it's bassically the same principle?

Originally Posted by Strange

Radio transmitters do the same sort of thing using vales, transistors and other components to change either the amplitude or the frequency of the transmitted signal.

Did you mean valves or vales? Any info on how exactly transistors and 'vales' and the other components actually work?

Originally Posted by Strange

There is no real benefit to trying to think of this in terms of photons; the "classical" wave model works much better. But, if you insist, AM changes the number of photons (the amplitude of the signal). FM changes the energy of each photon (equivalent to the frequency).

Now I want to know how these modulations actually take place... what is really happening? charging of photons? quantity of photons?... How are the photons charged? how are they 'created'? How is the quantity of them 'dictated'?

Also I need to get a grasp on how the signal wave is 'dictated' in relation to how the carrier wave is dictated... Are theses different types of wave form within the over all wave packet? or are these different aspects of the same waveform/photon activities?

Thanks again for all this help, I really apreciate it. I'm really hoping this post will make sense but i'm not sure as this is the first time i've tried to multiquoute! I'm going to make my comments bold, just incase the multi quoting goes horrible wrong.

Phew... the multiquotes worked a treat! thanks JG for that.

Originally Posted by question for you

Originally Posted by Strange

On average it is the same. Where the signal level goes up, the frequency is higher. Where the signal level goes down the frequency is lower.

Yes I noticed this in the diagram... but in reality, is it 'up' and 'down'? or is it left to right? or is it north to south? I mean since this is electro magnetic waves, then is there some kind of polarity involved? Is it polarity which 'pulls' the photons from the peaks to the troths of the waves? Or is it polarity which attracts the energetic photon to the centre?
Is it a spining of a polarized photon 'partical' which dictactes that it is either going 'up' or 'down' to or from the centre of the wave...?

I'm afraid none of that makes much sense.

Yes thats all understood. In the case of a radio wave... what would the the respective wavelengths be at 'max' and 'min' frequencies? how does the compare to say a microwave? (which intuition would tell me is much smaller, therefore higher frequency). Whats the respective 'ranges' of frequency and wavelength?

Radio is about 3KHz to 300GHz (or wavelengths of 1mm to 100,000km). Microwaves are at the top end of this: 300MHz to 300GHz (1mm to 1 meter).

(Then there is infra-red, visible, ultraviolet, x-ray, gamma-ray at ever higher frequencies / shorter wavelengths.)

Bandwidth? a familiar term which has never been explained... is this the band or range between two frequencies? I'e max and min frequency of a waveform?

Basically, yes: the difference between the minimum and maximum frequency that can be transmitted. This is important because it determines the rate at which information can be transmitted (more bandwidth = higher data rate).

Ok, I like this. So the way information is 'given' by electromagnetic waves is bassically the same as morse code, but there are more variables so more information can be encoded in a waveform? it's bassically the same principle?

Effectively, yes. Morse code (or Aldis lamps) are basically digital: you turn the signal on or off. AM and FM modulation are analogue, providing continuous control of amplitude or frequency.

Did you mean valves or vales? Any info on how exactly transistors and 'vales' and the other components actually work?

I did mean valves, thanks. I really don't want to get into a lot of detail. Think of them as electric taps: a small input signal on one terminal can control a larger current flowing through the device. This allows them to act as amplifiers (small signal in -> large signal out) or to drive the changing current in an antenna.

Now I want to know how these modulations actually take place... what is really happening? charging of photons? quantity of photons?... How are the photons charged? how are they 'created'? How is the quantity of them 'dictated'?

Forget photons. They are just a quantum level description of the electromagnetic radiation which is not useful in this context. Just think of it as a wave. Either the amplitude (strength/level) or the frequency of the wave is modulated.

Also I need to get a grasp on how the signal wave is 'dictated' in relation to how the carrier wave is dictated... Are theses different types of wave form within the over all wave packet? or are these different aspects of the same waveform/photon activities?

That just takes us back to the original diagram. Either amplitude or the frequency of the carrier waveform is changed by the signal.

Think of the signal being an audio signal. The sound pressure is converted by the microphone into an electrical voltage or current signal. This goes through an electrical amplifier which in turn is used to control and adjust the output of a radio transmitter. If the voltage signal from the microphone is zero, then the radio transmitter is sending out a pure carrier wave of a certain amplitude and frequency. The amplitude is a measure of the strength of the signal. Now when the voltage wave from the microphone adjusts the amplitude or frequency of the carrier wave, then you get a signal that looks like the above modulated am or fm signal. At the other end, you have an antenna which picks up the radio wave and changes it to a voltage again. The receiver filters out the higher frequency carrier and the signal amplified and used to drive a speaker, which in turn duplicates the original sound being reproduced.

QFY, if you'd like to see what real signals look like, you can check them out using Windows media player. Play a song, and choose the "now playing" window. Right click on the screen and under "Visualizations" choose "Bars and Waves" Then "Scope".