revv
Just wondering if I understand "radio bands" correctly I am not sure I do.

Lets say you have a radio band of 10 GHZ to 30 GHZ and another from 10 HERTZ to 30 HERTZ, the information or a song transmitted will sound the same in those two ranges, the only thing that matters is the amplitude or the frequency of the waves within the bands?

## Answers and Replies

Gold Member
Just wondering if I understand "radio bands" correctly I am not sure I do.

Lets say you have a radio band of 10 GHZ to 30 GHZ and another from 10 HERTZ to 30 HERTZ, the information or a song transmitted will sound the same in those two ranges, the only thing that matters is the amplitude or the frequency of the waves within the bands?
Do you think an audio signal can fit within the 10 to 30 Hz bandwidth?

revv
Do you think an audio signal can fit within the 10 to 30 Hz bandwidth?

Sorry I guess that's impossible, I assume an audio signal wouldn't fit in a 10-30 Hz because we would have trouble hearing at that kind of frequency?

Mentor
Your example, as @256bits indicated, has a problem, but the idea is okay. Human hearing operates between 30 Hz and about 18000Hz depending on the age of the person. That upper number drops with age.
So there are high pitched sounds that teens hear that older adults do not.

This has been exploited as a crowd control device aimed at teens, called the Mosquito:
https://en.wikipedia.org/wiki/The_Mosquito

revv
Your example, as @256bits indicated, has a problem, but the idea is okay. Human hearing operates between 30 Hz and about 18000Hz depending on the age of the person. That upper number drops with age.
So there are high pitched sounds that teens hear that older adults do not.

This has been exploited as a crowd control device aimed at teens, called the Mosquito:
https://en.wikipedia.org/wiki/The_Mosquito

I see, I should've of used these for examples instead? 10 Ghz to 30 Ghz and 5 Khz to 10 Khz? Does this make more sense?

The amplitude and frequency of the wave WITHIN bands is what determines information or speech correct?

Thank you all for the help by the way.

revv
My brain hurts already, I don't know if I am making sense anymore.

I'm trying my best here, let's say you have a song or information you want to send out... this information is transformed into electrical signals that then radiates radio waves from the antenna... after this part I am a little confused and not sure. Do the radio waves emitted have a frequency based on the strength of the signal? So for example if you want a higher frequency signal you use more power?

Gold Member
My brain hurts already
No doubt. Trying to understand waves and modulation tends to do that.

There are three aspects of a wave that one can change - amplitude, frequency, and phase.
Your basic AM radio uses amplitude modulation ( AM ), an FM station would use frequency modulation . Phase modulation is similar to FM, but not quite either. Extended modulations exist also, but AM and FM are the most basic.

Taking AM, a carrier signal has the information signal added to it, resulting in amplitude changes of the carrier.
For FM, the carrier signal has its frequency altered by the information signal.

Adding a signal frequency to the carrier frequency creates sidebands. Carrier - Signal frequency and Carrier + Signal frequency. Bandwidth is how far the Sidebands can extend from the Carrier frequency.

some reading may be appropriate,
Say,

diogenesNY
Guineafowl
My brain hurts already, I don't know if I am making sense anymore.

I'm trying my best here, let's say you have a song or information you want to send out... this information is transformed into electrical signals that then radiates radio waves from the antenna... after this part I am a little confused and not sure. Do the radio waves emitted have a frequency based on the strength of the signal? So for example if you want a higher frequency signal you use more power?
You might start by defining AM and FM radio signals:

AM: Amplitude modulation. The carrier wave is fixed between about 500-1500 kHz (depending on what station you’re tuned into) and the sound signal is encoded as variations in amplitude (strength) of that carrier wave.

FM: Frequency modulation. Carrier wave between 88-108 Mhz and the sound is encoded as variations of frequency.

https://images.app.goo.gl/PezJ7as6fXXjapR99

Nik_2213
IIRC, there's a limit to how fast you can transmit data via a frequency band. Nyquist ??

"The theoretical formula for the maximum bit rate is: maximum bit rate = 2 × Bandwidth × log2V. Here, maximum bit rate is calculated in bps. Bandwidth is the bandwidth of the channel. V is the number of discrete levels in the signal."

And... https://en.wikipedia.org/wiki/Bandwidth_(signal_processing)

Classic example is old, carbon-mike land-line phones, which had a very narrow audio bandwidth. More recently, CDs are sampled such they miss 'low-ultrasonic' frequencies, but have a wider dynamic range for what they do grab.

Don't forget the real-slow data rate of ELF transmissions to 'Boomer' subs, with message content barely more than a validated 'door-bell'...

Mosquito sounders ??
"... high pitched sounds that teens hear that older adults do not. This has been exploited as a crowd control device aimed at teens, called the Mosquito:"

Ha ! Several years ago, I warned the 'Customer Service' desk of my favourite local supermarket that their big illuminated logo was 'squealing', may have a pending failure. They summoned a security guy, who bade me repeat my tale, then point out the source.

"That's nothing to do with the logo's lighting, Mr. Nik, that's to persuade nocturnal groups of adolescents & teens to, ahem, 'Buzz Off'."

As I'm clearly 'post-middle-aged', I should NOT have been able to hear such...

Irony is most kids and youths of that age and disposition have premature hearing loss due to discos, raves and over-driven head-phones, so only their dogs would notice...

berkeman and Asymptotic
Gold Member
I assume an audio signal wouldn't fit in a 10-30 Hz
I see, I should've of used these for examples instead? 10 Ghz to 30 Ghz and 5 Khz to 10 Khz? Does this make more sense?
5KHz to 10kHz could be possible 'in principle' but you would be limited to 5kHz maximum audio frequency and the modulation system would have to be inconveniently complicated.
The lowest frequency used in practical analogue radio communications is more than 100kHz for various reasons. The main reason is the 'fractional bandwidth' involved. Circuits are much easier to make when they operate at a frequency that's well in excess of the 'baseband' bandwidth. The channel frequency response needs to be tolerably flat and that gets progressively easier as the fractional bandwidth reduces.
You might start by defining AM and FM radio signals:

AM: Amplitude modulation. The carrier wave is fixed between about 500-1500 kHz (depending on what station you’re tuned into) and the sound signal is encoded as variations in amplitude (strength) of that carrier wave.

FM: Frequency modulation. Carrier wave between 88-108 Mhz and the sound is encoded as variations of frequency.
You need to be careful here. Things aren't helped by the broadcasters use of modulation systems and frequency bands in a sloppy way.

The modulation system that's used for analogue radio just happens to be within those bands. This is for historical reasons. AM was the earliest modulation used for sound broadcasting and the limit to available transmitter operating frequency was not far above 1Mhz. Receivers were also ridiculously simple to make (Crystal sets etc.) The channel spacing was around 9kHz which limited the top audio frequency to about 4.5kHz.
Higher frequencies were used when AM ('Ancient Modulation') equipment became available and HF bands - up to about 30MHz were used for broadcasting (short wave) over longer distances and also for comms. Many more 9kHz slots are available in nearly 30MHz. Some channels used Single Sideband (about half the occupied bandwidth), which sounds bad but it squeezes more channels in and has more range for comms.
TV signals (AM) used the next higher available (VHF) Band I which allowed several channels with a higher baseband bandwidth (natch, for video). The next available (Band II) covered 88-108 Mhz and there were tests of wideband AM and FM tried. FM was chosen (by only a narrow margin aamof) as FM transmitters were cheaper and receivers were available. BUT there are many comms channels in the next Band III that use AM and also FM.

And that's just a start. With digital coding, some very acceptable audio quality can be carried in a channel space of 1kHz.

Paul Colby, Asymptotic and Guineafowl
Guineafowl
5KHz to 10kHz could be possible 'in principle' but you would be limited to 5kHz maximum audio frequency and the modulation system would have to be inconveniently complicated.
The lowest frequency used in practical analogue radio communications is more than 100kHz for various reasons. The main reason is the 'fractional bandwidth' involved. Circuits are much easier to make when they operate at a frequency that's well in excess of the 'baseband' bandwidth. The channel frequency response needs to be tolerably flat and that gets progressively easier as the fractional bandwidth reduces.

You need to be careful here. Things aren't helped by the broadcasters use of modulation systems and frequency bands in a sloppy way.

The modulation system that's used for analogue radio just happens to be within those bands. This is for historical reasons. AM was the earliest modulation used for sound broadcasting and the limit to available transmitter operating frequency was not far above 1Mhz. Receivers were also ridiculously simple to make (Crystal sets etc.) The channel spacing was around 9kHz which limited the top audio frequency to about 4.5kHz.
Higher frequencies were used when AM ('Ancient Modulation') equipment became available and HF bands - up to about 30MHz were used for broadcasting (short wave) over longer distances and also for comms. Many more 9kHz slots are available in nearly 30MHz. Some channels used Single Sideband (about half the occupied bandwidth), which sounds bad but it squeezes more channels in and has more range for comms.
TV signals (AM) used the next higher available (VHF) Band I which allowed several channels with a higher baseband bandwidth (natch, for video). The next available (Band II) covered 88-108 Mhz and there were tests of wideband AM and FM tried. FM was chosen (by only a narrow margin aamof) as FM transmitters were cheaper and receivers were available. BUT there are many comms channels in the next Band III that use AM and also FM.

And that's just a start. With digital coding, some very acceptable audio quality can be carried in a channel space of 1kHz.
Yes, I was giving some ballpark figures as the OP seemed to be struggling in the attempt to understand radio transmission without the basis of AM/FM. Thanks for clarifying.

sophiecentaur
Gold Member
Yes, I was giving some ballpark figures as the OP seemed to be struggling in the attempt to understand radio transmission without the basis of AM/FM.
There's so much to take on board and the popular view doesn't help at all.
I remember struggling for ages about how the TV signal actually gets on the screen. Nothing I read (until I got technical) helped me at all.

Staff Emeritus
Just wondering if I understand "radio bands" correctly I am not sure I do.

Lets say you have a radio band of 10 GHZ to 30 GHZ and another from 10 HERTZ to 30 HERTZ, the information or a song transmitted will sound the same in those two ranges, the only thing that matters is the amplitude or the frequency of the waves within the bands?
You could generalise like so: Whatever station you tune your radio to, whether it's in the ordinary AM band ("medium wave"), or a shortwave band, or an FM station on the VHF band, the same song will sound the same (within broad technical limitations).

Now, work backwards and draw conclusions from this.

alan123hk
Lets say you have a radio band of 10 GHZ to 30 GHZ and another from 10 HERTZ to 30 HERTZ, the information or a song transmitted will sound the same in those two ranges, the only thing that matters is the amplitude or the frequency of the waves within the bands?

I think so, it's just the difference in frequency range and bandwidth, but the songs you send will sound the same.

Let us consider the digital communication system. In theory, the success of lossless transmission can be achieved at very low or very high carrier frequencies, and even within very narrow bandwidths, which may come at the cost of reduced data rates, and when the transmitted signal power are lower than certain values, real-time audio / video transmission may not be possible.

On the other hand, what the important is to choose the correct modulation scheme to achieve an appropriate trad-off between power and bandwidth to obtain system optimization.

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davenn
Gold Member
Let us consider the digital communication system.
I think this could be a gear change too high for the present thread.
It's a very difficult thing to 'equate' an analogue channel with the digital channel that will carry all the relevant information in the original analogue signal. The actual information on the analogue output of a microphone can be very high indeed. The precision with which the transducer produces Volts for instantaneous sound pressure will depend on linearity and noise etc.. Do you need to transmit all that information? MP3 tells you that you really don't.
AM wastes a lot of its bandwidth by trying to transmit the effects of non linearity and noise in the original baseband signal, whether you are after the highest Fi or the lowest quality telephony. Digital coding treats the analogue input signal in a sophisticated way that selects the 'really important' information only and it produces a signal that is almost impervious to channel noise until it reaches a threshold level - at which point the channel will just die.
So which system is 'better'? Ask the users of Digital TV and Sound systems. (Also ask people who drive through fringe areas where the signal keeps cutting out completely when they could have been listening to 'intelligible' but 'wasteful' rough AM audio.)

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Guineafowl
Ask the users of Digital TV and Sound systems. (Also ask people who drive through fringe areas where the signal keeps cutting out completely when they could have been listening to 'intelligible' but 'wasteful' rough AM audio.)

Yes! Digital TV was marketed as ‘all-or-nothing’, either a perfect picture or none at all. But there is a slight threshold between the two. Anyone who has sat down to watch an anticipated TV programme, only to experience the blocky screen refresh and maddening cutting audio with the high-pitched ‘sssippp’ sounds, will understand why I once nearly threw my TV out of the window.

Gold Member
why I once nearly threw my TV out of the window.
I think we've all been there at some stage. There is a way round that, though. If you delve inside the Settings on the TV and look at Tuning, you can get a semi-quantitative indication of the actual signal strength with a bar display or broad descriptive words. If you are not getting "Good" then that's the time to improve your signal with 1. Pointing the antenna better, 2. Put in a low noise head amplifier or 3. Buy a better antenna. A set top aerial is pushing ones's luck.

Many people are using the same old antenna that was there when they moved into their home and which was 'OK' for four (or five) analogue signals. The UHF Freeview digital multiplexes cover a bigger range of frequencies, especially if, for instance, you want the new and slightly optional low power HD multiplexes that sit right at the top of the UHF band. I put in a fancy wide band Yagi plus a distribution amplifier up near the antenna. All the digital channels come roaring in, including the HD ones and I'm in Essex, a long way and near the fringe of Crystal Palace, the main London Transmitter, service area.

sysprog
Guineafowl
I think we've all been there at some stage. There is a way round that, though. If you delve inside the Settings on the TV and look at Tuning, you can get a semi-quantitative indication of the actual signal strength with a bar display or broad descriptive words. If you are not getting "Good" then that's the time to improve your signal with 1. Pointing the antenna better, 2. Put in a low noise head amplifier or 3. Buy a better antenna. A set top aerial is pushing ones's luck.

Many people are using the same old antenna that was there when they moved into their home and which was 'OK' for four (or five) analogue signals. The UHF Freeview digital multiplexes cover a bigger range of frequencies, especially if, for instance, you want the new and slightly optional low power HD multiplexes that sit right at the top of the UHF band. I put in a fancy wide band Yagi plus a distribution amplifier up near the antenna. All the digital channels come roaring in, including the HD ones and I'm in Essex, a long way and near the fringe of Crystal Palace, the main London Transmitter, service area.
I have also seen signal strength meters for sale for quite cheap, but I’m not sure if they’re any good.

This problem was intermittent, the signal seeming to wax and wane according to weather, time of day, or at any rate to my inconvenience. After messing around with the aerial for a bit, I finally called in a TV man who determined that my signal was coming across the North Sea from Wick, and that the Tesco aerial booster I had fitted was in fact attenuating the signal.

Even he couldn’t find a decent signal, so he fitted a Freesat dish and I never had a problem again.

sysprog and sophiecentaur
Kenwstr
HI,
My understanding is that Audio is carried as a modulation of the radio wave Carrier frequency, either amplitude modulation (AM) or frequency modulation (FM). In the case of AM, it's easier to visualize the modulation as variations in peak to trough. To put it into the more familiar digital analogy, you could view the peak and the trough as equivalent to digital sample values. That is, the peak represents an audio DSD bite value. As for sample rate, human hearing is considered 20 Hz - 20 kHz and according to the Nyquist theorem, you need at least 2 samples per audio wavelength. Therefore, a peak and a trough represent 2 sample values per wavelength. Therefore mono audio could be transmitted above 20kHz and stereo above 40 kHz. You may note that the Nyquist theorem requires at least this bit-rate and lossless stereo bit-rate is 44.1 kHz. Therefore I'd suggest that an AM radio wave of at least 44.1 kHz would be needed to transmit audio in lossless stereo. Now, radio amplitude varies greatly with distance so radio receivers include an automatic level control (ALC) this amplifies the average radio amplitude up to a predefined level in the receiver which allows the audio modulation (AM in this example) to retain a stable volume reference regardless of the radio signal strength. That way, you don't get variations in volume on your car radio as you drive around at lease, not while you are within the ALCs operational parameters for signal strength. All things have their limitations after all. Hopefully that may be a useful analogy. Note though that AM transmission began well before Nyquist or digital audio so do not be surprised if analogue AM frequencies do not meet these frequencies in practice.

StandardsGuy
My brain hurts already, I don't know if I am making sense anymore.

I'm trying my best here, let's say you have a song or information you want to send out... this information is transformed into electrical signals that then radiates radio waves from the antenna... after this part I am a little confused and not sure. Do the radio waves emitted have a frequency based on the strength of the signal? So for example if you want a higher frequency signal you use more power?
No more power does not increase the frequency. The following link may help:
Difference between AM and FM

davenn
Staff Emeritus
Hmm, maybe this is a case where pictures are better than words. @revv , try this video

revv
Another thing, do carrier waves ever carry any "Useful information" or is the information only in the side bands?

When I tune my radio to a particular frequency, that frequency is called the carrier right?

So from what I understand right now is that the carrier is modulated by an input signal, the carrier wave is changed by amplitude or frequency according to the electrical voltage of the input signal? Does this make any sense?

Staff Emeritus
Another thing, do carrier waves ever carry any "Useful information" or is the information only in the side bands?
It tells you which frequency to tune. :-)

When I tune my radio to a particular frequency, that frequency is called the carrier right?
Right.

So from what I understand right now is that the carrier is modulated by an input signal, the carrier wave is changed by amplitude or frequency according to the electrical voltage of the input signal? Does this make any sense?

I think you got it now. I hope we helped.

Gold Member
Another thing, do carrier waves ever carry any "Useful information" or is the information only in the side bands?

When I tune my radio to a particular frequency, that frequency is called the carrier right?

So from what I understand right now is that the carrier is modulated by an input signal, the carrier wave is changed by amplitude or frequency according to the electrical voltage of the input signal? Does this make any sense?
You set the receiver to the 'nominal' carrier frequency - same as you point a camera at the centre of a scene, despite the fact that there may be no actual detail there.
The only information that an unchanging carrier carries is whether or not the transmitter is switched on - more or less zero information. In many forms of modulation there is, in fact, no actual carrier (it's sometimes wasted transmitter power) but it can be re-generated from the rest of the signal when the demodulator synchronises with the signal and can then determine the changing phase (and maybe amplitude) which carries the information.

The spectrum of a high deviation fm signal may have no significant carrier component. Take a look at this link. It is rather long but hang in there. If you look at 32m in, you will see the well known 'angel's wings' spectrum plot where the carrier sweeps (relatively slowly) over a big range - you can't see a 'carrier' at all. The wings are higher at the ends because the carrier spends longer (peaks of the sine wave) there so there's more average energy.

Unfortunately, fm is far from intuitive, compared with am which tends to make sense even when you are first introduced to it.

sysprog
alan123hk
Another thing, do carrier waves ever carry any "Useful information" or is the information only in the side bands?

Whether the carrier wave carries information depends on modulation method.

For example, in the DSB-C AM, the power in the carrier conveys no useful information. Therefore, DSB-SC AM is basically an amplitude modulation wave without the carrier for reducing power waste. In addition, since both sidebands in DSB-SC AM contain identical copies of the information, thus SSB-SC AM can further increase the efficiency by transmitting one sideband only.

DSB-C AM - Double Sideband Large Carrier AM
DSB-SC AM - Double Sideband Suppressed Carrier AM
SSB-SC AM - Single Sideband Suppressed Carrier AM

Gold Member
Whether the carrier wave carries information depends on modulation method.
Not really . To 'carry information' it has to be modulated, which means it is no longer a single frequency. Splitting hairs, perhaps but the information has to be spread over a finite bandwidth (the channel). In AM and some other systems, there is a carrier present at all times. No modern system would use unwanted carrier power 'cos it's such bad value. The carrier does the job of self - demodulation for simple AM and, of course, the crystal set and its descendants needed a carrier. To demodulate even basic FM needs a 'frequency discriminator' circuit which outputs a Voltage related to the instantaneous carrier frequency. But the expression "instantaneous carrier frequency" is very dodgy for any other than very slow frequency sweep and wide deviation. (FM is really hard to understand, even from the start).
An effective bodge for demodulating FM with an AM receiver is to offset the tuning so that the received signal sits on one of the skirts of the receiver filter. As the carrier frequency varies, the output of the AM detector varies and it 'becomes AM'. This Slope Detection system only works when the FM channel bandwidth is similar to the AM channel bandwidth. No good for your living room HiFi, though.

SSB is great value for spectrum use and noise performance BUT it does need a special receiver which makes its own carrier. The demodulated signal can frequently sound like Donald Duck so it's only of use for Comms.

alan123hk
Not really . To 'carry information' it has to be modulated, which means it is no longer a single frequency.

Of course, the carrier wave must be modulated in order to carry information, a constant carrier wave or anything unchanged can not provide any further information except the proof of existence of itself.

I just mean that the carrier frequency component in the spectrum of the modulated carrier of the traditional AM broadcasting does not carry information of the modulating signal.

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sophiecentaur
Gold Member
Of course, the carrier wave must be modulated in order to carry information, a constant carrier wave or anything unchanged can not provide any further information except the proof of existence of itself.

I just mean that the carrier frequency component in the spectrum of the modulated carrier of the traditional AM broadcasting does not carry information of the modulating signal.
We have got to the stage of definitions and classifications which is always a dead end. We both know what's what, here, I think. People tend to over estimate the importance of a 'carrier'. I would say that the only mod system that has an actual carrier is AM or its near derivatives. In that system, the carrier actually has a job to do in simple receivers but simplicity of receiver circuits is no longer important. Anyone can sync up a local oscillator to allow demodulation these days.
BTW, I just recalled "Exalted Carrier Demodulation" which is a quaint name for a system that helps performance in some variations go AM. It involves filtering and boosting the level of a carrier that's purposely transmitted at low level and then following by simple envelope detection. The term has a rather self-important air about it - out of the past, I feel.

alan123hk
Gold Member
Another thing, do carrier waves ever carry any "Useful information" or is the information only in the side bands?
I think the answer is ideally yes. But the carrier would have to be on and unchanging for infinite time to have the least possible information.

People use CW (just a carrier) for communication using Morse code. The carrier is cycled on and off to form dots and dashes. The act of turning the carrier on and off spreads the frequency spectrum a little. In fact such transmitters must be constructed to turn on and off at a slow enough rate so as not to spread too far into neighboring channels.

alan123hk
Gold Member
I think the answer is ideally yes. But the carrier would have to be on and unchanging for infinite time to have the least possible information.

People use CW (just a carrier) for communication using Morse code. The carrier is cycled on and off to form dots and dashes. The act of turning the carrier on and off spreads the frequency spectrum a little. In fact such transmitters must be constructed to turn on and off at a slow enough rate so as not to spread too far into neighboring channels.
CW is an oxymoron. The wave isn’t continuous when Morse keyed so where is the difference between that, AM and all other forms of mod?
Your post could easily Be taken to imply some hard boundary between modulation and non-modulation. Arbitrary exceptions can confuse.

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Averagesupernova
Gold Member
In fact such transmitters must be constructed to turn on and off at a slow enough rate so as not to spread too far into neighboring channels.
Imagine you made a receiver for 'CW comms' with a very narrow band receive filter, (Bandwidth of 0.01Hz). If it were tuned spot on the carrier frequency, you would just see the carrier with no visible envelope. That's despite the fact that the carrier "isn't there" for half the time; the filter would have stored Energy at the carrier frequency for 100s or more. The modulation has been wiped off.

One great advantage of 'CW' is that you can, in fact use a receive filter with just a few Hz bandwidth which can drag the signal up from what would seem to be unusable noise levels. Of course, the Tx and Rx tuning would need to match well within a Hz or so. No problem at all these days. Great for covert comms and not requiring any fancy codulation systems.

Paul Colby
Gold Member
Your post could easily Be taken to imply some hard boundary between modulation and non-modulation. Arbitrary exceptions can confuse.
Agreed, but only if you don't read it and think carefully about it. CW is what people call it. It is in fact an amplitude modulation.