Naked Science Forum
Non Life Sciences => Technology => Topic started by: Abigail on 09/11/2008 22:47:17
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Abigail asked the Naked Scientists:
Just does a voice travel along telephone lines and why do landlines still work when there is a power failure? How has this changed with cellular phones and what were analogue and digital signals? Did Alexander Graham Bell really steal a patent to make the telephone?
Thank You.
What do you think?
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The microphone in the telephone converts the sound of your voice into electrical waves instead of sound waves, it is these that travel through the line, and at the other end are converted back into sound through a speaker. Well that is basically an analogue signal anyway, a digital signal on the other hand is converted into 1's and 0's instead of waves of varying amplitude (heights). The 1's and 0's form a code which instructs the phone on the other end what sound to produce. Digital generally gives better clarity because even if there is a low signal strength, the reciever can usually still gather all the information, (better reception) because it either has a signal or it doesn't, because it either receives the 1 or 0 or it doesn't. When analogue signals are weak the signals themselves get confused with static noise on the line and you lose alot of information. Short sharp 1's or 0's are easier to pick out of static.
Landlines usually still work when there is a power failure because they have their own power supply on the line, independent from main power lines.
I don't know about Alexander Bell's escapades.
That's my understanding of it anyway, I trust I will be corrected on any inaccuracies by the guru's :)
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What travels along a phone line during a conversation?
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Ffarm2.static.flickr.com%2F1434%2F1330661388_666a3e406c.jpg%3Fv%3D0&hash=3368c8a59ad79dcf44581c59bb614db7)
I can't get a bunch of rope running squirrels out of my head......
[::)] [::)] [;D]
Madidus' answer is - of course - much more to the point
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Filters require reliable voltage and amperage according Ohms law to reproduce sound.
That can be dodgey at either end.
So. for ratio calibration sake the 1 and 0 zero system gives better clarrity because it can only have a finite reproduction calibration level.
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Digital systems do have a lot of advantages - once your technology can handle the complexity.
It is interesting to note that very few systems only use binary coding during transmission. There are much more bandwidth-efficient alternatives to binary coding.
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The analogue problem is much the same as AM is to FM in that the modulated band width of frequencies takes up too much space on the frequency spectrum for the efficiency of carrier information accuracy of reproduction of the origin signals, apart being outdated technology.
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Until you include the factors of noise and interference, there is no inherent diffference in quality or bandwidth between AM and FM( with the appropriate deviation).
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AM is considered cruder than FM voltage modulation.
As it is thats simply technical advancement finally, and narrowing of the filtered channel band
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Is 'cruder' a technical term?
Actually, it is far easier to produce FM signals with high power than AM.
Which channel band? They can both be the same.
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I was told when i did my public broadcast license in the 1980's as part of the course, that AM(in terms of business quality standard processes) is cruder because of both the filtering it does not use and the technology system/process used to apply the modulation by the input becuase it requires belting it into the top end power of the oscillator unlike the simple interfering with the carrier modulation area from an input to the modulator at a lower power level with something like a darlington transistor(alike getting pre-ampl.) in the PLL area.
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Seems to be some confusion over AM and FM.
Both have their good and bad points.
As used in broadcasting AM takes up far less frequency space.
Both are used in communications. Most two way radio systems (Police etc - although there is a move towards digital systems) use Narrow Band FM (NBFM) where the carrier is only deviated by a small amount compared to Wide Band FM as used in broadcasting. Air Traffic Control uses AM on VHF for approach, take-off and landing and a very efficient form of AM called (SSB) Single Sideband on short wave when in mid ocean.
Analogue TV uses AM for the picture in a form called Vestigial Sideband (VSB) and FM for the sound. The older 405 line TV used AM for the sound.
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As used in broadcasting AM takes up far less frequency space
That's because they choose 75kHz deviation. Narrow band FM can take up the same spectrum as AM and produce pretty similar quality and noise performance. The move to FM was by no means obvious when the VHF bands came available. Many engineers favoured wide band AM with dynamic limiting to get rid of impulsive noise. FM was the right move, though - particularly when they started cramming dozens and dozens of stations into the original frequency plan. (Capture effect is brilliant and a real bit of luck.)
The laughably simple AM detector made it a 'must' for receivers in the old days. FM transmitters are a lot less steamy and difficult, though. Hence- FM comms, these days rather than AM.
SSB is fine but the transmitter is not really what you'd call efficient - it needs a linear amp on the output. (Oh yes, there is a fancy PM/AM system which gets round this to produce SSB but I can't remember its name; wsa it AEG Telefunken?) You wouldn't like to listen to music through a drainpipe normally, though, so it's not much cop for Broadcasting.
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And finally that is all dependent the government spectrum allocations allowed.
Lucky not much was near the giga Hz region or there would have been real trouble reclaiming the frequencies.
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SSB as a system is much more efficient than AM or NBFM. There is no wasteful carrier. It takes up half the bandwidth. There is no power being transmitted when there is a gap in the speech and the average RF level is much less. The power requirement for that same received s/n is much less than AM or NBFM. AM is either generated at low level and then linear amplifiers are required or the DC supply to the final class C amplifier is modulated with audio so a lot of audio power is required.
http://www.qsl.net/vk5br/TransMods.htm
The telephone network used to use SSB through coax lines.. groups and supergroups.
"""Single Sideband Suppressed Carrier (SSB-SC) modulation was the basis for all long distance telephone communications up until the last decade. It was called "L carrier." It consisted of groups of telephone conversations modulated on upper and/or lower sidebands of contiguous suppressed carriers. The groupings and sideband orientations (USB, LSB) supported hundreds and thousands of individual telephone conversations."""
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You're right about the carrier not being there but, as far as the transmitter is concerned, you can't use a class C output stage so, for high power, you still have energy / capital cost problems, despite the 6dB lower radiated power. That ws the efficiency aspect I was referring to.
You can shoe horn in twice as many channels too. Interference from SSB is a bit ugly to listen to.
It is / was a good systems for comms but digital systems are more robust now, I believe, because you can use spread spectrum techniques to get over the vagaries of the HF propagation medium.
We didn't mention vsb modulation - as in the colour subcarrier on PAL and NTSC TV. That's pretty smart too.