I know that it’s electrowaves but how does it become sound again ? How are there several channels ?
In: Technology
Sound is just fast vibrations in air that happen at around 20-20000 times a second
Radio waves are fast vibrations in the electromagnetic field that happen around several millions to hundreds of millions of time a second
AM radios essentially used the fact that sound is much slower than radio waves and they just change the strength of the radio signal as the audio signal changes. A radio would essentially just filter out anything higher than 20 khz and you’d just be left with the original sound signal to send to the amplifier and speaker
As the frequency of the radio signal doesn’t really affect the audio sent through you can send multiple radio signals at the same time so long the frequency is different, and another filter inside the radio would take care of isolating only one as selected by a knob
FM uses exactly the same idea but instead of changing the power to the transmitter it just transmits a slightly higher frequency for louder sounds, a different circuit is still able to decide it back to sound, and this change to how the wound is encoded results in less distortion
OK, so let’s shift this up a bit in the spectrum. Imagine trying to send information using a torch at night. I can point a torch at someone, and turn it of and off (or put my hand in front of it, shift between aiming directly at the receiver and the sky, etc) and thus send a message via some agreed code (like morse code, or binary or “one if by land, two if by sea”, etc). The changes in the amplitude(“strength”) of the incoming signal is how we encode, the message. This is AM radio.
We can make it any colour we want, and just tell the person watching to look out for, say, red lights, or green lights. Maybe we give them coloured glasses so they can only see red lights(ie we add a filter). This way, we can have multiple signals being sent to multiple recipients at the same time without confusion
Another thing we could do it have a constant light, but if shifts between red and green to to encode the message. Now. Instead if changing the amplitude. We are changing the frequency of the signal for encoding. We might need to change our filter set up, but the basic principle of the recipients knowing to just look at the pre arranged pair of frequencies/colours and ignoring other ones stays the same.
Using this method of encoding information onto the light, we can send the information about the sounds over the link we have created, same as, say, electrical pulses down a wire
Does that make sense?
Think of it like two people holding a rope.
Person A is holding one end of the rope, and Person B is holding the other end. Person A is moving their end up and down, causing a wave in the rope. Person B can feel the rope moving and can tell if Person A is making big waves or making small waves.
They have previously agreed that making a small wave means no, making a medium wave means maybe, and making a large wave means yes
This is like the electromagnetic waves going from antenna to antenna. It’s by no means a perfect example, but it shows the general idea.
If you’ve ever tuned a guitar, I can explain channels easily. When you get two strings tuned to the exact same frequency, playing one string will make the other vibrate. Radios have tuneable electronic vibrators, and tuning in a station means tuning the vibrator until it matches the station exactly. The vibrator will resonate strongly with the carrier frequency from the station, but barely move at all from the cacophony of other channels coming in through the same antenna. The strong vibrations are then amplified, filtered, and transformed back into sound. The radio spectrum has an enormous frequency span, so you can fit many more channels inside it than you could tones on a guitar.
I see several useful answers addressing various bits of your questions. I will take a shot at the “how does it become sound again” bit.
As others mentioned, sound is simply a vibration that our ears are perceiving – mostly in air, but sound can actually travel in liquids and solids. The last bit is not very relevant right now, but handy to know.
As a vibration, it can make other things vibrate, and vibrating things (thing something like a guitar string, or a metal strip stuck on one end) will make the air vibrate, creating sound.
One important technical bit here involves how magnets interact with electrical currents: if you move a magnet through looped wire, it will induce current in the wire, and vice-versa – a current in a wire looped around a magnet will make the magnet move.
This movement and the current will correspond to each other – so if a magnet inside such wire will vibrate (move back and forth very fast), so will the current, and current moving back and forth very fast will make the magnet vibrate.
If you attach such a magnet to something that is easily vibrated by sound in air, you can make the current that you get out match the vibration of the air.
You can do the opposite as well – send such a current that corresponds to a vibration of the air from elsewhere into a wire around a magnet that is attached to something that can make air vibrate around it, and you will receive sound that corresponds to that current. That is how speakers work – and the opposite is a microphone, of course.
So now you can convert sound to electrical “vibration” using the microphone, then send it somewhere else or even store it (on a tape, or something more advanced), and then play the sound elsewhere with the speaker.
The others have mostly explained the radio aspect of this – the vibrating signal is simply sent over the radio and received and played back elsewhere.
The actual sending and receiving works by sending a signal of a specific frequency, and the receiver then is made to only respond to that frequency – it is not 100% exact as nothing truly is, therefore it’s more like that the receiver will see the signal as the strongest when at that or near that frequency. This specific signal is called the “carrier wave”.
The vibrating signal is relatively low frequency compared to radio frequency, so it can be added up to the carrier wave and the receiver will still see mostly the same frequency it is set up for – and afterwards the actual information (the signal, the sound “vibration” we were sending) will be easily extracted by doing the same thing in reverse – it is a bit more complicated in case of FM than AM but it still works the same. Once the original signal is reconstructed, it is exactly the same as receiving it over the wire or playing it from a tape – send it to a speaker and you get the sound back.
Latest Answers