Question regarding fibre optics.

Robert

Robert

Robert

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We had a "2-channel optical fibre system" demo'd to us at uni the other day. Now I'm really confused because I'm sure the guy said that the sound would be LOUDER over a 1km fibre cable than a 3m/1m. Also I seem to remember him saying the sound would be distorted over the 1m cable, but should be fine over the 3m cable - then sound much louder over the 1km cable. Now I can't find anything in my notes about it - nor on google. Well, saying that - I've found this tidbit:

"As mentioned above, some signal loss occurs when the light is transmitted through the fiber, especially over long distances (more than a half mile, or about 1 km) such as with undersea cables. Therefore, one or more optical regenerators is spliced along the cable to boost the degraded light signals. An optical regenerator consists of optical fibers with a special coating (doping). The doped portion is "pumped" with a laser. When the degraded signal comes into the doped coating, the energy from the laser allows the doped molecules to become lasers themselves. The doped molecules then emit a new, stronger light signal with the same characteristics as the incoming weak light signal. Basically, the regenerator is a laser amplifier for the incoming signal. See Photonics.com: Fiber Amplifiers for more details."


The thing is - is this built into ALL fibre cables over a certain lengths, or just specific ones? Also - the cables where raw cables - just both ends connected to the machinery (no in-between bits).
 
He used 1 kilometre line and a 1&3 metre size cables.

Doesn't really help mate. Why would it sound better and louder on the 1k line?

Edit: Shouldn't this be the other way round? The only thing I can think of is that the thing above which is spliced into the cable boosts the sound to a greater volume etc over a shorter cable - but why would the shorter cable produce a degraded sound?
 
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Robert said:
. . . the sound would be LOUDER over a 1km fibre cable than a 3m/1m . . .
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Since when was light measured in decibels?

Robert said:
. . . Also I seem to remember him saying the sound would be distorted over the 1m cable, but should be fine over the 3m cable . . .
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Surely digital signals don't distort in that respect?

I'm no scientist, but there's a faint whiff of shennanigans going on.
 
Basically the system produced music (connected to some cheap pc speakers). The music was transmitted over the optical cable. When the sound came out through the speakers over the 1km line - it was louder than the 3m/1m lines.

Picture.jpg


Here's a schematic of the circuit.
 
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If he's using the fact its louder as a selling point then hes talking garbage.

The signal will have very little attenuation over that distance of fibre but thats not really the point. The signal was converted to digital to be sent over the fibre which means that it needed to be converted back at the speaker end, sound can also be sent over coax in this way. The difference you will be hearing will be down to the electronics converting it back at the other end not how 'loud' it is over the fibre the negligable attenuation will have nothing to do with the loudness of the result, this is dictated purely by the information encoded into the digital signal.

Its just a daft comparison

MB
 
Right - so the length of the cable means jack all? Argh! This was a MSC student doing this demo for us in a lab :(.

So the sound will be EXACTLY the same over all lengths?
 
Robert said:
Right - so the length of the cable means jack all? Argh! This was a MSC student doing this demo for us in a lab :(.
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The length of the cable in this case is irrelivent.

The same result could be achieved by encoding the data to a compact flash card and then feeding that into a system with an amp at the other end, or if it comes to it me sending you a CD that I recorded at home.

Digital will either work or not. if you go far enough away without a repeater the signal will fail but it won't get quieter it will just fail to work.

MB
 
Why do we get fed so much BS at uni. Half of my mates are writing their reports based on the fact that it apparently got "quieter" when using shorter cable compared to longer. Now thats gone out the window. We can't even contact the student to get more info as his e-mail addy expired and he won't give us his personal one.
 
Robert said:
The thing is - is this built into ALL fibre cables over a certain lengths, or just specific ones? Also - the cables where raw cables - just both ends connected to the machinery (no in-between bits).
Click to expand...

Just specific ones, you can buy cable in massive lengths without repeaters

I know, I used to sell it.

You'll find attenuation is less with single mode fibre rather than multimode, this is because of how the light is propagated.

MB
 
The issue here revolves around what he means by quieter

A digital signal sent over fibre will not become quieter, it WILL get to a stage where it can no longer trigger a reciever and eventually peater out to nothing, but this is not quieter once translated back from digital, weaker but not quieter.

If by quieter he means less powerful then yes hes right and if the signal is analogue then depending on how the reciever translates it then it is feisable but fibre is used predominately for digital and LEDs which are what he is feeding with a digital in nature I believe.

MB
 
Are you sure that you are correct in saying louder and quieter ?? Or are you talking about signal to noise as the signal to noise ratio would change, given the length of cables used.
 
It seems as though it is combined into one signal - converted to digital then changed back to analogue. At one point he turned the volume nob and it had no affect? The sound stayed at the same volume?
 
Robert said:
It seems as though it is combined into one signal - converted to digital then changed back to analogue. At one point he turned the volume nob and it had no affect? The sound stayed at the same volume?
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Turned the volume knob at which end?

MB
 
By 'louder' I suspect he meant there was less attenuation of the signal (ie lower loss in dB).
Or the music may appear louder to your ears because there is less background static for your ears and brain to process out.

The only reason I can see that the signal will be less noisy after 1Km as opposed to 1m is that the frequencies of the two channels are very similar and only separate enough to be resolved accurately by the DAC after a decent length of time.
(This is more a signal processing problem than optics)

Did no-one think to ask this of the demonstrator after he had finished his presentation?
 
I can't see any reason whatsoever for a 1km digital fibre link to produce a louder or more powerful signal than a 1m or 3m link, the signal will just be more attenuated, but if the reciever can see it and it is digital it should be reproduced in exactly the same way.

MB
 
Yes. This is the point - but it's still confusing. When connected with the 1km cable, it was just louder - full stop. When connected via the 1m, it was noisey (interferance sound) and lower volume.

Hmm - the singal is only analogue at the start and end, weird. This shows why 35 people have complained about this lecturer.
 
Borris said:
Since when was light measured in decibels?
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Losses in optical fibres are generally measured in dB/km (decibels per kilometre)


To the original poster:

What type of fibre were the 1m and 3m? Short links are often plastic which do tend to lose a lot of the signal.


I don't really get the whole louder and quieter thing though, it shouldn't make any difference as long as the signal is getting through.
 
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