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Ask HN: How to Decode Interstellar Signal with Unknown Bitrate

3 points by n0pe_p0pe 8y ago

I have a question about FSK modulation I'm going to frame in terms of a hypothetical: Imagine that you are transmitting a message to an ET in another solar system. The message is encoded in binary and embedded in the radio signal using FSK modulation. Lets say the signal is broadcast at 5 GHz and FSK +50kHz (i.e. 0 = 5,000,000 kHz and 1 = 5,000,050 kHz). Finally let's say you want to broadcast @ 1 bit per second. Now lets say the message you send is "1111111111" (10 ones). Would the ET recipient of this message be able to decode it without knowing the bit rate? My argument would be no, since to the ET it would just look like an unmodulated signal at 5000050 kHz that lasts for 10 seconds. If the ET didn't know the bit rate, it would be impossible for it to determine whether there was 100 ones or 10 ones encoded in the signal unless some zeroes were present as well--that is, unless the signal was modulated. Even in a modulated signal, however, it seems difficult to determine the number of bits in the transmission without knowing the bitrate. Are there any techniques for "reverse engineering" baud in a signal whose transmission parameters are unknown?

FatalLogic 8y ago
Maybe clock recovery strategies can help? But the initial problem is that the hearer doesn't know they're hearing a message (maybe?), which is like detecting steganography.
- n0pe_p0pe 8y ago
  Good point. I'm assuming this problem of letting the ET know a message is being sent can be overcome with certain design choices, such as using a really narrow bandwidth for transmission to mark the signal as artificial (most cosmic radio sources have really large bandwidths). What sort of clock recovery strategies are you thinking of?
bluGill 8y ago
First of all, you never send 11111111111. Even one earth when we know the encoding and expected bit rate: long sequences of ones (or zeros) are hard to decode. There are various schemes to encoding so that we never send more than a few. Second, we send much longer messages. 1111111111 doesn't mean anything, so decoding it as 11111 or 11 or even 1 is all the same. Instead we send very long messages with a lot of things that we believe they know. Maps of every star in the galaxy for instance - presumably they already have their own map. Thus once they think of this idea (which we hope is obvious) figuring out how we encoded it from the vast amount of data isn't very hard. Once they are sure of the encoding they start looking at other parts of the message that don't make sense, but they already have a map of the encoding so they know they have the data right, it is just a matter of understanding it. We start by adding things to the map - they already know it is a map and have a good idea where we sent the message from, so a "we are here" arrow is easy to figure out. Then we add more and more information, each building on the next. Then after sending all that data we repeat it because they probably didn't even notice it the first 100 times we sent it. The fact that a long sequence that seems random keeps repeating is a very clear sign that the sequence ins't random.

3 points by n0pe_p0pe 8y ago

FatalLogic 8y ago

Maybe clock recovery strategies can help? But the initial problem is that the hearer doesn't know they're hearing a message (maybe?), which is like detecting steganography.

n0pe_p0pe 8y ago
Good point. I'm assuming this problem of letting the ET know a message is being sent can be overcome with certain design choices, such as using a really narrow bandwidth for transmission to mark the signal as artificial (most cosmic radio sources have really large bandwidths). What sort of clock recovery strategies are you thinking of?

bluGill 8y ago

First of all, you never send 11111111111. Even one earth when we know the encoding and expected bit rate: long sequences of ones (or zeros) are hard to decode. There are various schemes to encoding so that we never send more than a few. Second, we send much longer messages. 1111111111 doesn't mean anything, so decoding it as 11111 or 11 or even 1 is all the same. Instead we send very long messages with a lot of things that we believe they know. Maps of every star in the galaxy for instance - presumably they already have their own map. Thus once they think of this idea (which we hope is obvious) figuring out how we encoded it from the vast amount of data isn't very hard. Once they are sure of the encoding they start looking at other parts of the message that don't make sense, but they already have a map of the encoding so they know they have the data right, it is just a matter of understanding it. We start by adding things to the map - they already know it is a map and have a good idea where we sent the message from, so a "we are here" arrow is easy to figure out. Then we add more and more information, each building on the next. Then after sending all that data we repeat it because they probably didn't even notice it the first 100 times we sent it. The fact that a long sequence that seems random keeps repeating is a very clear sign that the sequence ins't random.