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Low bandwidth prevents audio signals
editThe article states that the low bandwidth prevents the transmission of audio signals, but it doesn't explain why (and neither does the bandwidth article.) Why is it not possible to use AM to transmit voice in this band? -- Malvineous (talk) 06:41, 28 April 2012 (UTC)
- Clarified that a bit. HTH.--Harumphy (talk) 07:45, 28 April 2012 (UTC)
- Thanks! It answered that question but raised another :-) I think I'll head over to a radio forum for that one though! -- Malvineous (talk) 11:17, 5 May 2012 (UTC)
- Just to add a little additional information to Malvineous's contribution: the primary reason is the narrowness of the band, compared with the bandwidth of sound. The VLF band is 27 kHz wide. Sound frequencies go up to about 16 kHz, so one high fidelity audio channel would occupy half the band. Even a telephone-quality voice channel requires 3 kHz (although compression can reduce that considerably). An additional 3 kHz must be allocated for channel separation to prevent interference, so a voice station would require 6 kHz. Since VLF transmitters are typically long range, covering most of a continent, only 4 or 5 voice stations could operate per continent, leaving no room for the other VLF services. Radio spectrum bandwidth is scarce and expensive, and regulators feel that audio is an inefficient use of this band.
- Thanks! It answered that question but raised another :-) I think I'll head over to a radio forum for that one though! -- Malvineous (talk) 11:17, 5 May 2012 (UTC)
- However, there are additional technical reasons why VLF transmitters can't transmit audio. Existing VLF transmitters have extremely narrow bandwidths. Not only can they not transmit voice, but even text messages are sent very slowly. The US Navy's VLF stations reportedly have a transmission rate of around 200 baud, around 28 characters per second. Although there may be other reasons for the narrow bandwidth, the reason I am aware of is the construction of the antenna. Due to the length of VLF waves, VLF antennas are extremely electrically short, meaning they are much shorter than a resonant antenna, which is one quarter of a wavelength long. This makes them very inefficient. In order to increase its efficiency, the top of the antenna is connected to a large spiderweb of suspended cables that acts as a capacitor plate to ground. The bottom of the antenna is connected to a gigantic loading coil, several stories tall. This makes the antenna act like a huge LC circuit of extremely high Q. Deviation of the frequency during modulation of more than a few hundred hertz causes high voltage arcs at the ends of the antenna. So information is added to the transmitter signal by minimum frequency shift keying (MFSK), shifting the frequency of the carrier back and forth by only about 100 hertz, to indicate binary ones and zeros. This is only capable of sending text at a very slow rate. --ChetvornoTALK 13:20, 5 May 2012 (UTC)
- The Chu-Harrington limit tells you what the bandwidth could be for an antenna size and frequency. Graeme Bartlett (talk) 07:11, 6 May 2012 (UTC)
- These statements are misleading. It must be noted that they refer to analogue transmission of signals. It is quite possible to transmit a HD movie across VLF, in digital transmission, it would just be very slow. Its a bit similar to your modem or router in that respect. One way to speed things up is to create hundreds or even thousands of carriers separated by microhertz or millihertz. This can turn a one hertz bandwidth into the equivalent of a broadband connection. With modern DSP and DSP filters, its certainly possible, but the infrastructure required to achieve this makes it prohibitive to anyone but the military. 84.13.194.135 (talk) 09:28, 25 May 2012 (UTC)
- Carriers separated by microhertz cannot transmit data quickly, instead they take weeks to send one bit each. Graeme Bartlett (talk) 13:22, 25 May 2012 (UTC)
- Yes, and the normal meaning of audio transmission is real time audio, not waiting half an hour for a few minutes of digital audio to come through. --ChetvornoTALK 15:45, 25 May 2012 (UTC)
- Carriers separated by microhertz can be modulated up to the frequency of the wave. The bandwidth is in the modulation, rather than the range of frequencies used. Of course, this is digital transmission, not analogue. It takes some serious equipment to transmit and receive the signals though. That said, a million channels between 19000Hz and 19001Hz provides some serious throughput at low frequencies. 78.146.89.66 (talk) 10:25, 26 May 2012 (UTC)
- Just to give you an idea, you can push about 17.7GB (not Gb) per second with this technique between 19000Hz and 19001Hz. 78.146.89.66 (talk) 10:36, 26 May 2012 (UTC)
- Got some sources to support this? --ChetvornoTALK 12:40, 26 May 2012 (UTC)
- I'm working on it at the minute. The idea itself is simple, the photons for 19000.00001Hz are different from 19000.00002Hz. It just takes an appropriate filter to separate them. When separated, in this example, you have two different carriers on the 19KHz band. Then you can modulate them on and off. So, whilst the carrier is only 1 microhertz wide, it is still at a frequency of 19000.00001Hz or 19000.00002Hz. The fastest rate of this modulation equals the frequency of the photon, not the bandwidth. The key is developing a bandpass filter that fits within the area of interest (i.e. within 1 microhertz at a given frequency). Obviously, given the amount of filters required per hertz we are looking at a specialized chip. Also, there is a finite amount energy that can be supplied per microhertz band, so some cooling at the receiver may be required to increase the minimum detectable signal. I'm looking for web references at the minute, but I can't see any. If I find some, I'll add them here for you. 78.146.89.66 (talk) 13:37, 26 May 2012 (UTC)
- Before someone mentions the Shannon–Hartley theorem, please be aware that this applies only to analogue signals and digital signals where bandwidth forms part of the modulation scheme. In the digital world, apply "Marx Theorem" where capacity equals frequency. There is also approximately 10^29 carriers available in any 1 Hz bandwidth. To calculate the approximate "total theoretical capacity" of any given Hertz multiply the frequency by 10^29. I say approximate because the channel capacity expands as the frequency rises and 10^29 is an approximate value. 78.146.89.66 (talk) 13:57, 26 May 2012 (UTC)
- Here is an example of its use in the field. This is an image from a waterfall of the German VLF station DHO38 earlier today. We can see the 5 minute high bandwidth broadcast in the center of the image. We can't tell if its microhertz comms from the picture, but if we assume at least hertz comms with 80 channels, they are pushing about 224MB (not Mbit) per second. The entire broadcast is about 65GB in 5 mins. http://i.imgur.com/RXwQH.jpg
- Before someone mentions the Shannon–Hartley theorem, please be aware that this applies only to analogue signals and digital signals where bandwidth forms part of the modulation scheme. In the digital world, apply "Marx Theorem" where capacity equals frequency. There is also approximately 10^29 carriers available in any 1 Hz bandwidth. To calculate the approximate "total theoretical capacity" of any given Hertz multiply the frequency by 10^29. I say approximate because the channel capacity expands as the frequency rises and 10^29 is an approximate value. 78.146.89.66 (talk) 13:57, 26 May 2012 (UTC)
- I'm working on it at the minute. The idea itself is simple, the photons for 19000.00001Hz are different from 19000.00002Hz. It just takes an appropriate filter to separate them. When separated, in this example, you have two different carriers on the 19KHz band. Then you can modulate them on and off. So, whilst the carrier is only 1 microhertz wide, it is still at a frequency of 19000.00001Hz or 19000.00002Hz. The fastest rate of this modulation equals the frequency of the photon, not the bandwidth. The key is developing a bandpass filter that fits within the area of interest (i.e. within 1 microhertz at a given frequency). Obviously, given the amount of filters required per hertz we are looking at a specialized chip. Also, there is a finite amount energy that can be supplied per microhertz band, so some cooling at the receiver may be required to increase the minimum detectable signal. I'm looking for web references at the minute, but I can't see any. If I find some, I'll add them here for you. 78.146.89.66 (talk) 13:37, 26 May 2012 (UTC)
- Got some sources to support this? --ChetvornoTALK 12:40, 26 May 2012 (UTC)
- Just to give you an idea, you can push about 17.7GB (not Gb) per second with this technique between 19000Hz and 19001Hz. 78.146.89.66 (talk) 10:36, 26 May 2012 (UTC)
- Carriers separated by microhertz can be modulated up to the frequency of the wave. The bandwidth is in the modulation, rather than the range of frequencies used. Of course, this is digital transmission, not analogue. It takes some serious equipment to transmit and receive the signals though. That said, a million channels between 19000Hz and 19001Hz provides some serious throughput at low frequencies. 78.146.89.66 (talk) 10:25, 26 May 2012 (UTC)
- Yes, and the normal meaning of audio transmission is real time audio, not waiting half an hour for a few minutes of digital audio to come through. --ChetvornoTALK 15:45, 25 May 2012 (UTC)
- Carriers separated by microhertz cannot transmit data quickly, instead they take weeks to send one bit each. Graeme Bartlett (talk) 13:22, 25 May 2012 (UTC)
- These statements are misleading. It must be noted that they refer to analogue transmission of signals. It is quite possible to transmit a HD movie across VLF, in digital transmission, it would just be very slow. Its a bit similar to your modem or router in that respect. One way to speed things up is to create hundreds or even thousands of carriers separated by microhertz or millihertz. This can turn a one hertz bandwidth into the equivalent of a broadband connection. With modern DSP and DSP filters, its certainly possible, but the infrastructure required to achieve this makes it prohibitive to anyone but the military. 84.13.194.135 (talk) 09:28, 25 May 2012 (UTC)
- The Chu-Harrington limit tells you what the bandwidth could be for an antenna size and frequency. Graeme Bartlett (talk) 07:11, 6 May 2012 (UTC)
- However, there are additional technical reasons why VLF transmitters can't transmit audio. Existing VLF transmitters have extremely narrow bandwidths. Not only can they not transmit voice, but even text messages are sent very slowly. The US Navy's VLF stations reportedly have a transmission rate of around 200 baud, around 28 characters per second. Although there may be other reasons for the narrow bandwidth, the reason I am aware of is the construction of the antenna. Due to the length of VLF waves, VLF antennas are extremely electrically short, meaning they are much shorter than a resonant antenna, which is one quarter of a wavelength long. This makes them very inefficient. In order to increase its efficiency, the top of the antenna is connected to a large spiderweb of suspended cables that acts as a capacitor plate to ground. The bottom of the antenna is connected to a gigantic loading coil, several stories tall. This makes the antenna act like a huge LC circuit of extremely high Q. Deviation of the frequency during modulation of more than a few hundred hertz causes high voltage arcs at the ends of the antenna. So information is added to the transmitter signal by minimum frequency shift keying (MFSK), shifting the frequency of the carrier back and forth by only about 100 hertz, to indicate binary ones and zeros. This is only capable of sending text at a very slow rate. --ChetvornoTALK 13:20, 5 May 2012 (UTC)
This is almost correct. There is a good post here that provides the math. http://www.physicsforums.com/showpost.php?p=3947771&postcount=86 — Preceding unsigned comment added by 2.100.85.153 (talk) 19:45, 12 June 2012 (UTC)
- Just to set the record straight, there are no reliable sources that information can be transmitted faster than the Shannon-Hartley theorem allows. --ChetvornoTALK 00:09, 27 December 2013 (UTC)
- And if you are invoking photons, then the Heisenberg's uncertainty principal comes into play. In order to be certain of a Minute frequency difference you need lots of time. So there is still a trade-off between time and bandwidth. Graeme Bartlett (talk) 04:52, 17 August 2018 (UTC)
Protective bubble
editOriginal paper at:
Power
editin: "Trideco" ... at a power of 1.8 megawatts, the most powerful radio station in the world.
It should be noted from UHF television broadcasting, "A UHF TV station could be licensed for up to five megawatts of carrier power" — Preceding unsigned comment added by 192.225.121.101 (talk) 01:49, 12 August 2022 (UTC)
- Okay, but firstly we're discussing VLF Radio (radio carrying audio modulation) vs UHF Television (radio carrying audio and video modulation), so it's technically not a fair comparison. Either way, these would require research, because I think both, "facts" are dubious. Read-write-services (talk) 06:40, 19 August 2022 (UTC)
- For the UHF TV they are talking about effective radiated power rather than total power. UHF can use directional antennas to concentrate the power to the horizon, boosting EIRP. Graeme Bartlett (talk) 08:42, 19 August 2022 (UTC)