What's the lowest radio frequency one could usefully transmit phone quality speech/music upon? I am ashamed to admit I have entirely forgotten what little wireless theory I knew.
Note his point about longwave, then glance at the whopping antennas as you drive past Droitwich on the M5.
Anything lower than mediumwave and you'll need an antenna the size of... well, bigger than any land area you or any of your friends can afford (regardless of whether you mount horizontal or vertical - it still has to have guy ropes, and probably air traffic clearance at that height).
Mediumwave antennas will comfortably fit inside a field or on a ship. Hence why there are pirate mediumwave stations but not longwave stations. Sunshine Radio out of Ludlow operated an AM-MW TX from a field (and briefly from a raft in a lake, if anecdotes are to be believed), before they went legal and rented a proper TX off of Merlin like all the other ILR sell-outs.
Radio 4's wavelength is about one and a half kilometres, for example. It only gets longer as the frequency gets lower. The rough equation is "divide 300,000 by KHz to get metres", although I'm sure a physics student will be along in a moment to talk about the speed of light etc.
Well, I don't have any qualifications in physics but I do have a HNC in electronics, specialising in communications.
I can't actually remember anything as I've been doing software since I graduated, but I one thing I specifcally don't remember is any fudge factor for variance in the speed of light in wavelength calculations; the speed of propogation though air was assumed to be constant (at ground level).
(insert hazy memory may be talking bollocks disclaimer here)
The only problem I remember re variance in the speed of light was the fact that different frequencies propogate at different speeds[1]. Fourier shows us that a complex waveform is in fact made up of many sine waves of different frequencies. If all these frequencies are travelling at different speeds when it gets to the other end your waveform is going to be right mullered
( ... )
You can modulate phone audio onto DC. As a convenient proof of this I offer ADSL and its ability to transmit 8M of data over phone wires. The only _possible_ explanation for this is the use of Demonology, not Nyquist. If ADSL works, it must therefore be possible to get phones over damp string and a bit of twanging. Q-E-fnor-D.
Phone audio is defined as 300Hz to 3.3kHz and used to be limited to such, back in the days of analogue multiplexing onto trunks. This is why from the mid '70s or so a local line was often audibly better quality than a good trunk call. In the mid '80s, the increased use of digital multiplexing meant that actual quality of calls was far beyond this (about 8kHz tops, AFAIR) and the punters began to complain if they were getting just what they'd paid for and no more (a neat example of Machiavellian principles in marketing, where giving the customer a better deal simply turns into a shifted expectation and increased complaints
( ... )
Comments 17
IIRC: Long-wave went down to about 150kHz.
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Anything lower than mediumwave and you'll need an antenna the size of... well, bigger than any land area you or any of your friends can afford (regardless of whether you mount horizontal or vertical - it still has to have guy ropes, and probably air traffic clearance at that height).
Mediumwave antennas will comfortably fit inside a field or on a ship. Hence why there are pirate mediumwave stations but not longwave stations. Sunshine Radio out of Ludlow operated an AM-MW TX from a field (and briefly from a raft in a lake, if anecdotes are to be believed), before they went legal and rented a proper TX off of Merlin like all the other ILR sell-outs.
Radio 4's wavelength is about one and a half kilometres, for example. It only gets longer as the frequency gets lower. The rough equation is "divide 300,000 by KHz to get metres", although I'm sure a physics student will be along in a moment to talk about the speed of light etc.
Reply
I can't actually remember anything as I've been doing software since I graduated, but I one thing I specifcally don't remember is any fudge factor for variance in the speed of light in wavelength calculations; the speed of propogation though air was assumed to be constant (at ground level).
(insert hazy memory may be talking bollocks disclaimer here)
The only problem I remember re variance in the speed of light was the fact that different frequencies propogate at different speeds[1]. Fourier shows us that a complex waveform is in fact made up of many sine waves of different frequencies. If all these frequencies are travelling at different speeds when it gets to the other end your waveform is going to be right mullered ( ... )
Reply
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Phone audio is defined as 300Hz to 3.3kHz and used to be limited to such, back in the days of analogue multiplexing onto trunks. This is why from the mid '70s or so a local line was often audibly better quality than a good trunk call. In the mid '80s, the increased use of digital multiplexing meant that actual quality of calls was far beyond this (about 8kHz tops, AFAIR) and the punters began to complain if they were getting just what they'd paid for and no more (a neat example of Machiavellian principles in marketing, where giving the customer a better deal simply turns into a shifted expectation and increased complaints ( ... )
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