Hello Everyone,
I have this following lay query and would request experts to help me understand it...

How is information packed and retrieved in a Radio Wave...

Lets say a transmitter wants to transmit a "face" of a person to a remote receiver (a Television)... So the photograph of the "face" from a camera would be sent to the transmitter, 'packed' in a carrier wave and transmitted...

Now the wave travels in space and the wavefront keeps expanding...

So let's say we began with a 1 Sq MM of wave packed with the information about the "face" right at the transmitter... As this wave with information travels in space, the wavefront, containing the information about "face", keeps expanding and gets bigger and bigger... (Which I am 'assuming' spreads the information contained in the wavefront)

Now let's assume when this wavefront passes by a receiver antenna, its expanded to a size of 1 Sq. meter... So the information "face" is spread out equally in this entire wavefront of 1 Sq. Meter... (again my assumption here)...

The receiving antenna intercepts only a tiny section of this wavefront, which perhaps means that the receiving antenna is only intercepting a small portion of the information (face)...

So how does the other part of the information reach the antenna, which is was unable to intercept...

Please excuse my very limited understanding and built in assumptions....

Regards,
Rahul

 

There are many ways that information is encoded onto an RF carrier wave. First you need to get your head around how an RF signal is modulated to carry information. One way is described here.

Once you can transmit a series of logic ones and zeroes then you can transmit the contents of a jpeg file (which could contain a picture of a face) bit by bit and reassemble those bits back into a jpeg file.

 

You have totally the wrong idea about how information is embedded into a radio wave. (The process is called modulation.) The picture is first broken down into pixels. (Think of them as squares on graph paper.) So if we use an example of 100 pixels horizontally and a hundred pixels vertically the total image would contain 10000 pixels. In the simplest form of modulation each pixel would have only two states. On or off. On would be a white pixel. Off would be a black pixel. The RF carrier would be switched on for a white pixel and off for a black pixel. These pixels would be sent sequentially. A complete image could be sent every 1/25 of a second (40 mS) In practice it is more complicated as synchronisation has to be sent as well as the pixels. Also you would want each pixel to be a shade of grey or a colour so more than one bit of information would have to be sent for every pixel. This could be achieved by changing the strength of the RF carrier (For grey scale.) rather than just switching it on or off. This is a very simplified description. This is how the very early television worked. These days the data is processed so that only pixels that have changed are transmitted. This reduces the required bandwidth of the transmission.

Les.

Les.

 

Here is a simpler description.

The transmitter is a light bulb. What you receive is what you see, the instantaneous intensity of light from the bulb.
The receiver circuitry converts the light intensity to a voltage.
Hence the circuit receives a voltage that is changing with time. All the information is encoded in this voltage.

Even though the light is spreading over physical space, all of the information is still captured where ever you are located. Everybody sees the same information.

The trick is the information is encoded in the signal. The image of a person's face is not presented as a physical picture frame. It is encoded over a time frame. It might take any length of time to send this frame, for example, 20ms or 10 minutes, depending on how the information is encoded. Images from a satellite in deep space might take a very long time to be transmitted, not because the satellite is far away but because the method of encoding is chosen for maximum reliability for data transmission and reception of a very weak signal.

 

Think of a radio wave as a symmetrically changing electrical pattern. A pattern rate. A frequency. This would be called the carrier.

If we vary that pattern, with another electrical pattern.........we call that modulation.

So it turns out that we are impressing a changing pattern on a constant pattern.

The changing pattern......can not vary faster than the constant pattern.......only slower. So far.

A radio wave is also fractal. This is why even with long distance and very weak signals......we still have the same pattern. Or same changing pattern.

 

The important thing to understand is that all the information, whether audio or visual, is transmitted in a serial fashion, with only a very tiny piece of the information sent at any instant.
For example, in the USA. the ATSC HDTV picture with sound is sent digitally one bit at a time at a rate of 19.4 Mbit/s which is digitally modulated (8VSB) into the old 6MHz analog TV channel bandwidth.

 

Thank you very much for the quick description and pointing me in the right direction...

From the foregoing, I can now imagine that "face" is broken down into small segments and each segment is transmitted serially....

So each segment/bit would be transmitted, one at a time, in one wavefront which would keep expanding as it propagates further in space...

The expansion of the wavefront, due to propagation in space, would spread out the information (making it less intense) throughout the wavefront, making it less intense but not 'different' across the surface (as was my initial assumption)...

The receiver, at any distance, will intercept the intensity of this wavefront and have in them amplifiers of varying gains, to reconstruct this information coming in serially one after another...!!

Hope this understanding, reconstituted from above posts, is more structured than my initial post... ...

Thank you very much once again

Regards
Rahul