How exactly is info transported via radio waves? Are the waves 3D and depending on waves pointing out on the Z axis (Using the X,Y,Z, axis’s for a 3D graph) determines what info they carry? I know a 2D wave determines its Hz but i have never heard what role the 3rd dimention plays in radio waves.
Most commonly, information is encoded into the wave by either varying the amplitude (kinda like changing the volume control) of the wave or by varying the frequency of the wave. These are called Amplitude Modulation (AM) and Frequency Modulation (FM). The AM and FM radio bands are named after the modulation type used. In television, the voice signal is frequency modulated and the picture signal is amplitude modulated. Digital signals typically convert the data to an analog representation, then use either AM or FM.
astro’s link seems to cover it fairly well. What’s difficult for most people to grasp about it is that the radio waves have a much higher frequency (and thus lower wavelength) than the sounds waves that you’re actually going to here. Thus the peaks and troughs of the radio waves trace out a waveform from which the sound wave can then be recovered. The exact mechanism was explained in my sophomore year engineering class, but regretably I fell asleep during many of the lectures, so I can’t recall it exactly.
Now that AM and FM are covered, we’ll move on to OFDM…
To expound upon ITR champion’s point, the fact that radio frequency radiation is at such a higher frequency than audible sound is important if you want audio resolution.
Think of a newsprint image. The most important property in how good the image looks is who it’s of… er, how many dots per inch compose the image. The more dots, the better each dot fades into the background and the better the illusion of real shading is maintained.
The same thing works with sound encoded on a carrier wave. The faster the carrier wave cycles, the more cycles per second you have to encode a second of sound. If you think of each cycle as a single data point, it becomes obvious that the more data points you have, the better each one fades into the background and the better the illusion of real sound is maintained.
(As a little note, sound and radiation aren’t really comparable except that both exist as waves (well, in radio we see the radiation as waves) and, therefore, both have amplitude, frequency, modulation, and direction. Sound waves are a compression of matter, whereas radiation is a generated field.)
By the way, there’s one more way to encode information on a radio carrier: pulsing it. Turning it on and off in a pattern, in other words, like in Morse code (and Baudot and Radio Teletype (RTTY) and some others). But that doesn’t give you sound per se, although you can hear RTTY on shortwave.
It wouldn’t be hard to argue that pulsing the wave and amplitude modulation are the same thing. There’s also phase modulation, which is a bit difficult to describe without pictures. But, it wouldn’t be hard to argue that phase modulation and frequency modulation are the same thing, as well.
The value electric field hitting our receiver can be modeled as a sinusoid, like so:
E = A * cos(2pif* t + phi)
‘A’ is the amplitude (or volume, as said before) of the field, ‘f’ is the frequncy (in Hz), and phi is the phase of the field. Phase is a bit tricky to understand: the difference between a sine and cosine of the same frequency is that they are of different phase. Again, this is something that would be better explained with pictures. Oh, and ‘t’ is time and pi is around 3.141.
Putting information onto the wave can be done in two ways:
Changing what is on the outside of the cosine function (the value of ‘A,’ called amplitude modulation)
Changing what’s on the inside of the cosine function (the value of [2pif*t + phi]). Since we don’t have control of the value of pi or t, we’re left with changing the frequency or changing the phase.
All of this is done in the hopes that some electronic circuit somewhere will notice our subtle manipulations and then understand what they mean.
Pulsing and full amplitude modulation are only similar in very, very broad strokes. In pulsing, the signal is on or off, like a switch being flipped. (Exactly like, in fact.) In AM, the signal goes from softer to louder along with the sound that’s being encoded on the carrier.
Of course, the two techniques are used for very different things: Pulsing is used to encode text or other discrete symbols on a carrier, whereas AM is practically always used to encode sound. It’s obvious to see why pulsing would be horrible at encoding sound, and why AM would be horrible at encoding discrete symbols.
Why do you think pulsing would be horrible to encode sound? Digital audio, used in CDs is exactly that. Pulse coded sound with extremely good performance.
AM and FM are really old technologies - newer systems such as cell phones and wireless LAN don’t use them. But of the things that AM is used for, very prominent on that list is the video (luminance) signal on a TV station. And by the way, TV sound isn’t AM, it’s FM.
This analogy doesn’t work. It doesn’t really matter whether you have 1,000,000 cycles per second or 100,000,000 cycles per second, the sound can still be modulated onto that without problems associated with digital sample rates. AM radio is noisier than FM because there’s more environmental noise down at those frequencies, and that AM is inherently more susceptible to noise than FM.
Not strictly true but I know where you’re coming from. Some common types of detectors rely on a large frequency difference but this does not mean it is required or that any information is lost and cannot be recovered. The information is there and can always be recovered with a synchronous detector. If A is the modulating signal and B is the carrier then A*B is the modulated carrier and you just need to divide by B to recover A.
To expand on signal to noise ratios: with AM modulation you can improve the S/N ratio by increasing signal power while with FM you can improve it by increasing signal bandwidth.
The luminance (brightness) signal is only half the total video signal, at least for a color NTSC broadcast. The other half is the color signal, or chrominance. S-video separates these two signals to improve picture sharpness.
I believe the luminance signal carries quite a bit more than half of the total information and the chrominance quite a bit less than half (which is also true of JPG compression).
Well, you can say it “separates them differently” or “separates them more” but they are always “separate”. The luminance modulates the carrier and the two luminance signals modulate subcarriers of the same frequency but 90º difference in phase. BTW, because the luminance signals are carried on carriers of the same frequency but 90º difference in phase you need synchronous detectors to recover the separate signals. An envelope detector like Derleth was thinking will not work here.
There are people who’d disagree with your analysis of the performance of CDs, you know.
Really? I thought AM sound was flat and muddy because of the lower frequency. Could you get acceptable sound with, say, 50 cycles per second? 2 cycles per second? Assuming you’re in a lead-shielded laboratory, of course.
As for TV using AM for the image and FM for the sound: I think I knew that, but it’s really not relevant to my point. My point was that AM and FM are both analog encoding technologies, whereas pulsing is digital (like what is used physically on CDs). Analog is good for fidelity but it’s a bitch to record in a truly non-volatile way. A sequence of discrete symbols can never fully capture an analog signal, but you can reproduce them endlessly with no degredation.
Both luminance and chrominance are amplitude modulated
This is correct. AM radio stations are received with lower quality because
(a) they have a very low bandwidth (in other words, lower quality to begin with)
(b) AM modulation is more subject to interference by noise
(c) the AM broadcast band (800 - 1600 KHz) has more noise than the higher FM band
So, if you modulate the same carrier frequency with higher bandwidth and transmit though a channel with lower noise (say cable) or just increase broadcast power to bring the s/n ration lower, you can get as high quality as you want. There is nothing inherent to AM frequency band which makes it unsuitable. You can get as much quality as you like. I repeat: with AM modulation you can improve the S/N ratio by increasing signal power while with FM you can improve it by increasing signal bandwidth.
Since noise adds to the amplitude of the signal, AM is more susceptible to noise. FM has constant amplitude, so noise has little effect on it.
The bandwidth used to transmit a signal is at least equal to the highest frequency of the information. Generally it is the double of this frequency, since the modulating signal creates bands in both sides of the carrier. AM stations have a bandwidth of 20 kHz (double band), so the maximum frequency they can theoretically transmit is 10 kHz (in reality it is less than that), below the capacity of human earing of approximately 20 kHz, so high frequencies are not informed in AM signal. FM stations use a bandwidth of 75 kHz, so they can transmit more information in high frequencies, resulting in a better sound.
While it’s true that you can’t perfectly capture an analog signal in digital form, you can get as close to perfect as you can afford, by increasing the sample rate and resolution. There are limits to what human ears can discern, so an imperfect digital copy can still be indistinguishable from the original analog signal. For the average listener, any perceived difference between an analog sound and a 44.1 KHz, 16-bit (CD quality) digital reproduction of it is much more likely to be caused by his audio equipment than by the digital approximation of the sound wave.
I don’t think a 50 Hz signal would be acceptable, because human voice tends to be between 300 and 3400 Hz. IANAEE, but I don’t think you can reproduce a sound using a radio frequency that’s lower than the frequency of that sound.
I already answered this. You are mistaken. An envelope detector relies on the carrier being of much higher frequency than the modulating frequencies but a synchronous product detector can detect the modulating signal no matter what the carrier frequency.
Amplitude modulation is just a simple case of the many different forms of modulation and they are all studied mathematically in the time and frequency domains. Trying to understand modulation by looking at an envelope detector is like trying to understand how a car works by looking into the exhaust pipe.
To expand on my previous post. There are many kinds of modulation and they are represented by mathematical operations. Amplitude modulation is simply where you take a single carrier frequency and multiply its amplitude by a the modulating signal plus a constant so that the modulating signal plus the constant are always positive. If the carrier is of much greater frequency then you can easily detect the modulating signal with a simple envelope detector. But, in any case, knowing the mathematical operation involved all you have to do is design a circuit which will reverse it.
A modulator which multiplies carrier by modulating signal (without the constant) is a Single Side Band (SSB) modulator called a “balanced modulator” and an envelope detector will not work, now you need a coherent detector.
In FM you modulate the frequencyand you need to detect the frequency changes with a product detector.
Some circuits are easier to do than others but that does not change the fact that the information is there and can be detected with the right circuit.
Modulation methods and carrier frequencies are chosen taking into account a number of factors among which how the carrier behaves along the channel to be used is one of the most important. The reason for using a carrier is that it performs better in that channel than the signal we want to transmit. Using a carrier frequency similar to the frequency we want to transmit makes no sense because there would be no point.