4 ms·
I don't really get why the range is this low. I can see flashes of light from a signal mirror from a place more than 10km away (and there were signal mirror lin
by robryk 10y ago
I don't really get why the range is this low. I can see flashes of light from a signal mirror from a place more than 10km away (and there were signal mirror links of ~100km in very good weather). In these systems, the noise should be way lower (there are many sources of relatively-low-frequency visual noise that I know of and I know of none that would generate at least hundreds of MHz frequency noise) and attenuation should be similar. Signal mirrors are eyesafe, so similar signal levels should be possible.
What am I wrong about or missing?
- zw123456 10y agoGood question, you can see a light flash on and off but to encode large amounts of information onto a channel is determined by the signal to noise ratio per Shannon's law https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore... the information rate is proportional to the SNR. So you may be able to see a light flash on and one once per second but to put 1Gbps on it you need a pretty high SINR. Things like dispersion come into play.
- robryk 10y agoDo you mean dispersion as in "a pulse gets longer in time due to different length of paths taken by different photons due to scattering"? If not, what do you mean? My point about seeing a flash was precisely that SNR is high, because I can distinguish the flash from noise. EDIT: SNR depends on frequency only inasmuch noise level depends on frequency.