3 ms·

I'm not sure I'd be so quick to dismiss the feasability, at least not based on your reasoning so far. FM Radio stations have managed to accomplish getting the

I'm not sure I'd be so quick to dismiss the feasability, at least not based on your reasoning so far. FM Radio stations have managed to accomplish getting the speaker in your car radio to "blink synchronously" according to a complex signal pattern in the exact same way as millions of other radios simultaneously.... for about a century now. As long as all streetlights are in locations with good radio reception (which, since they're all on streets and are tall, is likely), it would not be that hard to implement in principle a central FM or other radio signal that oscillates according to the expected blinking. Lights that are engaging in dimming could multiply another signal by the central signal to diminish it even more.

coryrc 3y ago
You must not be familiar with multipath interference. Radio isn't quite as synchronous as you believe it to be.
- saltcured 3y ago
  And of course the light itself is also undergoing multipath distortions like the radio signal. And for astronomy, the changing atmosphere might be the most important secondary reflection. And the multipath arrangement between emitter and observer is different for each pair. Could you make a dark period long enough to mask the multipath variance among emitters and observers? Or would this dark period be so long as to cause annoying flicker?
Evidlo 3y ago
Wavelength of FM radio is about 3m, so you only have to be a few centimeters shifted to be seeing an entirely different phase and be out of sync.
- p1mrx 3y ago
  If the LEDs blink at 150 Hz, then a single on-off cycle has a "wavelength" of 2000 km. The wavelength of the control signal doesn't seem relevant.
  - quickthrower2 3y ago
    Plus you just add a position based delay. This is quite the bikeshed!
    - p1mrx 3y ago
      How does the LED know its own position? At that point you might as well abandon FM and use GPS.
      - quickthrower2 3y ago
        Send a few bytes of data onto the SD card on lamp installation. If GPS can do it all in one then that is probably better given how cheap the units are these day.
calamari4065 3y ago
Okay, let's do some math. Very rough napkin numbers, but it'll get us close. A 150Hz blink rate means a period of 1/150s, or ~6.7ms. The speed of radio signals through air near the surface is a bit lower than c, let's call it 299,250 km/s. [0] When you multiply those numbers you get ~1500km. That means each 'on' and 'off' signal is 1500km long, moving near c. More specifically, the signals are concentric rings around the transmitter 1500km in width. Let's look at London. I picked it at random, but it just happens to have a land area around 1500km^2. Assume the transmitter is in the center of the perfectly circular city, that means it takes 3.3ms for each 'on' and 'off' signal to cover the entire city. Each on and off pulse is just about exactly long enough to cover the entire city. All that together means that at any given moment 50% of the lights in the city are on, and the other 50% are off. That's about as far from synchronized as you can get, really. Also, light is slightly faster than radio in these conditions[1]. The light reaching the center of the city won't be pulsing in the same phase as the signal you're sending out. It will also be blurred and muddied due to the difference in signal speed. You probably wouldn't see much flicker in the light at all, as 50% of the lights are always on. On average, you receive 50% of the light you would otherwise, but that value will never reach 0. There won't be any periods of total darkness where you can make an observation, as the article suggests. Moral of the story: light (and radio) is much, much slower than most people realize. [0] https://ieeexplore.ieee.org/document/170108 https://ieeexplore.ieee.org/document/170108 [1] I couldn't find a confident number in my 16 seconds of searching. Seems like just a few percent faster, but the real number doesn't matter here. Bonus: you can synchronize the streetlights exactly, but it's much more expensive and complex than a single radio transmitter. You need to have every single light contain a receiver with an accurate clock that synchronizes with a geosynchronous satellite with an atomic reference clock. Search for "GPS disciplined oscillator" if you're curious.
- anon_cow1111 3y ago
  >Let's look at London. I picked it at random, but it just happens to have a land area around 1500km^2 Um am I really really tired and not reading correctly, or is this confusing linear area with square area? London doesn't stretch from New york to Florida. 186000 miles/second * pulse of .003 = 558 miles ~= 900 km

coryrc 3y ago

You must not be familiar with multipath interference. Radio isn't quite as synchronous as you believe it to be.

saltcured 3y ago
And of course the light itself is also undergoing multipath distortions like the radio signal. And for astronomy, the changing atmosphere might be the most important secondary reflection. And the multipath arrangement between emitter and observer is different for each pair. Could you make a dark period long enough to mask the multipath variance among emitters and observers? Or would this dark period be so long as to cause annoying flicker?

Evidlo 3y ago

Wavelength of FM radio is about 3m, so you only have to be a few centimeters shifted to be seeing an entirely different phase and be out of sync.

p1mrx 3y ago
If the LEDs blink at 150 Hz, then a single on-off cycle has a "wavelength" of 2000 km. The wavelength of the control signal doesn't seem relevant.
- quickthrower2 3y ago
  Plus you just add a position based delay. This is quite the bikeshed!
  - p1mrx 3y ago
    How does the LED know its own position? At that point you might as well abandon FM and use GPS.
    - quickthrower2 3y ago
      Send a few bytes of data onto the SD card on lamp installation. If GPS can do it all in one then that is probably better given how cheap the units are these day.

calamari4065 3y ago

Okay, let's do some math. Very rough napkin numbers, but it'll get us close. A 150Hz blink rate means a period of 1/150s, or ~6.7ms. The speed of radio signals through air near the surface is a bit lower than c, let's call it 299,250 km/s. [0] When you multiply those numbers you get ~1500km. That means each 'on' and 'off' signal is 1500km long, moving near c. More specifically, the signals are concentric rings around the transmitter 1500km in width. Let's look at London. I picked it at random, but it just happens to have a land area around 1500km^2. Assume the transmitter is in the center of the perfectly circular city, that means it takes 3.3ms for each 'on' and 'off' signal to cover the entire city. Each on and off pulse is just about exactly long enough to cover the entire city. All that together means that at any given moment 50% of the lights in the city are on, and the other 50% are off. That's about as far from synchronized as you can get, really. Also, light is slightly faster than radio in these conditions[1]. The light reaching the center of the city won't be pulsing in the same phase as the signal you're sending out. It will also be blurred and muddied due to the difference in signal speed. You probably wouldn't see much flicker in the light at all, as 50% of the lights are always on. On average, you receive 50% of the light you would otherwise, but that value will never reach 0. There won't be any periods of total darkness where you can make an observation, as the article suggests. Moral of the story: light (and radio) is much, much slower than most people realize. [0] https://ieeexplore.ieee.org/document/170108 https://ieeexplore.ieee.org/document/170108 [1] I couldn't find a confident number in my 16 seconds of searching. Seems like just a few percent faster, but the real number doesn't matter here. Bonus: you can synchronize the streetlights exactly, but it's much more expensive and complex than a single radio transmitter. You need to have every single light contain a receiver with an accurate clock that synchronizes with a geosynchronous satellite with an atomic reference clock. Search for "GPS disciplined oscillator" if you're curious.

anon_cow1111 3y ago
>Let's look at London. I picked it at random, but it just happens to have a land area around 1500km^2 Um am I really really tired and not reading correctly, or is this confusing linear area with square area? London doesn't stretch from New york to Florida. 186000 miles/second * pulse of .003 = 558 miles ~= 900 km