If aliens receive digital broadcasts, will they be able to decode them?

[Mangetout]

Analogue broacasts of TV and radio are fairly easy to decode intuitively - For radio, it's an oscillating signal that represents the sound wave, in the case of TV, it's raster data with repeating components - seems like it's not a massive leap of intuition to interpret that as images.

But what about digital broadcasts? Those are just going to look like noise, unless you already know the codec, aren't they? How complex a task would it be to reverse-engineer, say, MPEG2, based only on a collection of examples of the encoded signal, and without knowledge of the end product?

[friedo]

I would challenge your premise. Analog TV signals are not exactly simple. A lot of prior knowledge is required to decode NTSC or PAL into a useable image.

Even audio radio signals aren't plainly obvious. Assuming you figure out AM and FM, you still need to use a lot of trial-and-error to find the frequency boundaries.

[Ximenean]

But there are discernable patterns in analogue TV signals, both spatial and temporal, that should allow intelligent aliens to eventually reconstruct the picture. I know, I saw them do it in Contact ;).
The thing about compression schemes is that they are trying to throw away redundant, repeated information, to maximise the entropy in other words. A perfect compression scheme, in this sense, would yield data that looks random. MPEG2 doesn't try to achieve that kind of perfection, of course.

[gazpacho]

It is more complicated that you think Mangetout. You don't just send the bits for the MPEG2 stream in order they are used. Before you get to the point of reverse engineering MPEG2. You have to reverse engineer the error coding and interleaving. You can look an analog TV signal and see repeating patterns. I am not familiar with digital TV but digital cellular is very complicated and the repeating things tend to be long sequences of tones.

[constanze]

But Earth isn't starting out with digital signals at point 0. We've broadcast analog signals for decades, and since digital signals are limited also by the speed of light, they can't overtake the older signals. So the aliens and their equivalent to SETI would first notice the analog signals, decode them, learn our languages, watch the TV and listen to radio to learn our history, ...

and once they understand English and get all TV (since all signals are broadcast spherically), they can learn most of the technical stuff from there. There are geek and technical shows on TV, after all, and news shows cover the switchover to digital.

[Chronos]

You have to reverse engineer the error coding and interleaving.That part, at least, shouldn't be too hard. They probably use some sort of error-correcting encoding in their own communications, and the math behind that is the same everywhere. Once you get the idea to look for them, those patterns in the bits should be fairly easy to spot. Of course, that won't tell you anything about what the actual data stream is, though.

[Bytegeist]

I would challenge your premise. Analog TV signals are not exactly simple. A lot of prior knowledge is required to decode NTSC or PAL into a useable image.

I think if intelligent aliens sampled a black-and-white NTSC or PAL analog signal for a good long time (say a few days), and if they had a sense of vision as we do, they'd probably figure out the encoding without much trouble. They'd also know the correct vertical orientation of the picture by watching for falling objects, and the correct aspect ratio by looking for things that are presumably circles (like planets, or bubbles).

Figuring out the correct horizontal orientation would be trickier. The signal would have to include at some point an image of a well known object that looks the same to us as it does to them. Perhaps an image of the Andromeda Galaxy, for example.

For a color analog signal, I expect they'd overlook or dismiss the IQ/UV color information, chalking it up to noise — especially if they didn't have color vision themselves, or if they had designed their own color TVs "sensibly", and not in the backward-compatible way we did. They could still extract a coherent B&W picture though.

Obviously this is all speculative. (I don't know any aliens I can ask.)

[gazpacho]

That part, at least, shouldn't be too hard. They probably use some sort of error-correcting encoding in their own communications, and the math behind that is the same everywhere. Once you get the idea to look for them, those patterns in the bits should be fairly easy to spot. Of course, that won't tell you anything about what the actual data stream is, though.There are many error coding ideas that are currently in use. Turbo codes, convolutional codes elliptical codes block codes.

Lets take turbo codes. Turbo codes have three sorts of parameters to them. The convolutional code you use for the taps. The interleaving method and the code rate you send. The interleaving is basically changing the order of how you send the bits through the convolutional coder. For a well performing code you want the bit order to be random and as such there are billions and billions of different ways of doing it with each being just as good as the next. Without knowing the interleaving pattern you basically have random bits.

I think you are vastly underestimating the issue with reverse engineering the error coding. The thing about error coding is there are so many ways to do more or less the same thing it is a stretch to say they do the similar math on planet X.

[mlees]

Morse code is pretty analog/binary.

Hypothetical aliens can detect shorts & longs, grouped into blocks anywhere from one to five "characters" long.

How do they know that each of these block/groups is a letter?

How do they know what "sosir hfulkds" even means?

[Derleth]

Morse code is pretty analog/binary.Those are inherently contrastive ideas, so it's impossible to be both; Morse code is discrete, which is essentially the same as being binary.

Hypothetical aliens can detect shorts & longs, grouped into blocks anywhere from one to five "characters" long.True.

How do they know that each of these block/groups is a letter?Guessing is the only way they could possibly figure this out, the same as when we stumble on an undeciphered script in an archaeological dig.

How do they know what "sosir hfulkds" even means?They don't. They wouldn't have any cultural context, so they wouldn't be able to translate our language at all.

[Duckster]

and once they understand English and get all TV (since all signals are broadcast spherically), they can learn most of the technical stuff from there. There are geek and technical shows on TV, after all, and news shows cover the switchover to digital.

It's not only English-speaking broadcasts the ETs are receiving.

[mlees]

Those are inherently contrastive ideas, so it's impossible to be both; Morse code is discrete, which is essentially the same as being binary.

Oops. Thanks for the correction. You're right. For some reason I was using the sloppy definition where analog = pre-computerized. :smack:

[Mangetout]

I would challenge your premise. Analog TV signals are not exactly simple. A lot of prior knowledge is required to decode NTSC or PAL into a useable image.

Even audio radio signals aren't plainly obvious. Assuming you figure out AM and FM, you still need to use a lot of trial-and-error to find the frequency boundaries.

I'm under no illusions regarding the difficulty of decoding a contextless signal - it's hard enough even when the signal is purpose-designed to be self-unpacking.

But an analogue tv broadcast that contains some rectilinear graphics that stay on screen for a few frames - or better still, a test card- should contain enough clues to offer a fighting chance of being decoded by an intelligent agent seeing it completely afresh.

The point is that (as I understand it) the task of decoding compressed video devoid of context is orders of magnitude more difficult than analogue.

[cmyk]

Those are inherently contrastive ideas, so it's impossible to be both; Morse code is discrete, which is essentially the same as being binary.

Well, the signal could be analog (radio), while the language is binary (Morse). Perhaps a moot point, though.

[Derleth]

Well, the signal could be analog (radio), while the language is binary (Morse).Radio is not inherently analog. Radio is whatever has been modulated onto it, and Morse is not analog.

[Ximenean]

They wouldn't have any cultural context, so they wouldn't be able to translate our language at all.
Surely they could translate some nouns, at least? Example: they repeatedly hear sounds similar to "Obama" simultaneously with images of some kind of organism, which close study reveals to be the same organism. Conclusion: "Obama" identifies this individual.
Although I'm probably glossing over the difficulties of isolating words from continuous speech, especially for beings who have never encountered human speech.

[Mangetout]

Pretty site they'd get verbs too - if they had enough cookery shows.to sample

[Chronos]

Of course, the shows themselves would be cultural context. I mean, ultimately, each and every one of us has managed to decipher a language without any prior context.

[The Hamster King]

The better a data compression scheme is, the more its output resembles random noise.

The electromagnetic spectrum is a finite resource. So any civilization using it as a communications medium will, over time, employ better and better data compression algorithms to make the best use of the limited bandwidth.

This means that the probability of any technological civilization decoding a signal from another technological civilization is very low. There will be a brief period after radio is discovered where a civilization's transmissions will have some discernible structure, but within a few centuries the imperative of data compression will make every signal sound like static.

A signal from an alien race will either be a deliberate attempt at contact, or an incomprehensible flood of white noise from an unexpected source.

[Chronos]

The better a data compression scheme is, the more its output resembles random noise.Well, the more efficient the compression is, maybe, but "efficient" is not necessarily synonymous with "better". With most practical compression schemes in actual use, after they take out all of the redundancy they can find, they go and put some redundancy back in, to allow for error correction or at least detection (depending on the application). A signal which was genuinely indistinguishable from noise wouldn't be of much practical value.

[marshmallow]

AFAIK our broadcasts don't go that far. We're trying to entertain each other, not aliens. We'd need a huge source to make them go very far. So if they're close enough to receive our broadcasts they could just come a little closer and ask us how to do it.

[Sage Rat]

Of course, the shows themselves would be cultural context. I mean, ultimately, each and every one of us has managed to decipher a language without any prior context.
Assuming that they could determine that different broadcasts were in different languages.

[gazpacho]

I went and finally looked up how digital over the air TV is being done in the US. 8-vsb is the standard used in North America. There are several steps.

1) The mpeg2 data is fed into a randomizer. This is exclusive oring the incoming data with a pseudo random sequence of bits. The idea is to ensure that you don't have a long string of 0s or 1s. For this purpose there are tons and tons of sequences that will work just as well as any other so long as both the transmitter and receiver agree on the sequence.

2) The the data is fed through a Reed Solomon encoder. This is a block code that adds 20 bytes to each block of 187 bytes. The 20 bytes are basically a checksum. This allows correcting of multiple errors in the 187 bytes. I am not sure from my reading if the 187 data bytes are transformed or not. Reed Solomon codes are a general set of block codes so there are multiple way to do this sort of error correcting.

3) Data interleaver. The output of many blocks data from the Reed Solomon encoder are stored in a memory. This memory is then read out in a psuedo random order such that the bits from one block are spread out over a 4.5 ms period. This is done to protect one packet from being wiped out by a short burst of noise. The error correction from step 2 will allow you to correct a few wrong bits but not a large number of bits.

4) Trellis coder. This is another form of error coding. It adds one bit for every 2 bits sent.

5) Sync and pilot insertion. Sync is a sequence transmitted at the beginning of each frame. Pilot is a slight dc offset for the data before it is transmitting resulting in a carrier tone.

Steps 1 and 3 seem to me to be very difficult to reverse engineer.
Step 5 should be pretty easy to reverse engineer.
Step 4 is a little harder to reverse engineer
Step 2 I really don't have any idea about how hard this is to figure out.

One other observation about aliens listening in on our TV and cell phones. The amount of redundant information added to the signal (extra bits in step 2 and 4) and the power that the TV stations transmit is meant to get TV to you reliably over a distance of a few hundred miles. If you are more than a few hundred miles away the signal to noise ratio is so high that there are two many errors and the error correction algorithms don't work. You can fix this with bigger directional antennas. But TV stations that are far from each other on earth transmit on the same frequencies so this really increases the noise if you are trying to decode one station of the other. It seems really infeasible to monitor our current communications very far off earth.

[Mangetout]

But even if you reverse engineer all the broadcasting protocols, you still only have the mpeg2 stream. I think that's going to be really hard to unpack.

[Chronos]

How long is the "random" bitstring used in the randomizing step? I imagine that they use the same standard sequence repeatedly, since it's not intended for cryptographic security. But if it's of a significant length, that would be a huge problem for anyone trying to decode the sequence from scratch.

As for the mpeg encoding, it'd probably help if the aliens have had some jpegs to practice on first, since the algorithm is similar. The ideal would be if they had some sort of Rosetta stone image (which would probably end up being Lena) that was encoded in several different formats.

[gazpacho]

How long is the "random" bitstring used in the randomizing step? I imagine that they use the same standard sequence repeatedly, since it's not intended for cryptographic security. But if it's of a significant length, that would be a huge problem for anyone trying to decode the sequence from scratch.

As for the mpeg encoding, it'd probably help if the aliens have had some jpegs to practice on first, since the algorithm is similar. The ideal would be if they had some sort of Rosetta stone image (which would probably end up being Lena) that was encoded in several different formats.The sequence generator does not repeat for 2^16 -1 bytes. But as you guess they use the same sequence for each of the about 200 bytes segments. So the sequence is really only about 200 bytes.

[cmyk]

Even if a digital signal of ours were to be picked up by aliens, would it be obvious enough that it is of intelligent design?

If the situation were reversed, and we picked up a signal from a relatively local star system, chances are we wouldn't be able to decode it, be we sure as hell would be pointing all our instruments toward it, and even attempt some sort of long-term communication. It would be the most important discovery ever.

[Stranger On A Train]

Of course, the shows themselves would be cultural context. I mean, ultimately, each and every one of us has managed to decipher a language without any prior context.This is, of course, true, but all of us are also evolved in a common overarching cultural context of spoken language; that is, we are evolved to speak and interpret the spoken word into complex ideas and concepts. While the Chomskian argument of a "universal grammar" lacks rigorous evidence, it is certainly true that a significant portion of our brains is dedicated to spoken language processing, and it is demonstrable that it develops in response to exposure to specific grammars. Once formed, it is more difficult for most people to develop true fluency in an other language in the same cognitive workload. Reading, a more recent innovation, is more complex and abstract, and utilizes more primitive (but extensively developed) parts of the brain, which then links to the language centers in a way that is not really well understood.

An alien species may not utilize spoken language or other auditory communication at all, and the whistling noises made by blowing wind between moist slabs of fatty protein may mean little to them. (See Terry Bisson's "They're Made Out Of Meat?" for the perspective an alien species may have.) Certainly the spoken word is a very inefficient means of transmitting information or relaying concepts. Our own colloquialisms, such as "A picture is work a thousand words," acknowledge this fact. Images and the written word (especially in the form of lists or data) convey information with more efficacy and density.

On the other hand, mathematics is a nearly universal grammar; there are certain physical and mathematical constants that are essentially inevitable in any system of counting with integers. Frequency-based communications--both analog waveforms and idealized discrete "digital" signals--are all mathematics, and deciphering this code, while complex, is really just an exercise in signal processing, and is a natural evolution of the development of electrodynamics, which is all waves and interacting "signals". Any species capable of receiving electromagnetic signals is likely capable of discerning content patterns, regardless of whether those signals are basic analog frequency modulation or coded in multiplexing code access like CDMA, TDMA, or FDMA.

However, whether they could convert such coherent signals into video depends not so much on the format of the code, but whether they use the transmission of visual signals, or indeed are physically capable of vision. (It seems unlikely to many that an intelligent species would not have a visual sense, but this prejudice is largely based upon science fiction precepts that assuming human-like aliens; a species that evolved in on a world perpetually enshrouded by cloud may have as little use for vision as cave fish, and may instead investigate its environment via chemical signals, acoustic signals, electromagnetic fields, or other senses.

But this is all hypothetical; despite the fact that the Earth radiates in the radio band like a super-Jovian or brown dwarf, the ability to discern these signals from one another and against the background sources at interstellar distances is grossly unlikely. At such ranges, even the most powerful of these signals is just noise. Communication at such distances will require directed, coherent radiation in the multi-gigawatt range; masers, lasers, or highly energetic spectral absorption and reradiating bands, like that which would be discernible from the exhaust of a large nuclear-fusion-based rocket.

Stranger

[gazpacho]

TV signals are only intended to be received a few hundred kilometers away from the transmitter. The nearest star is 39,900,000,000,000 kilometers away. The power drops of with the square of the distance but the noise power from the rest of the universe remains constant. I don't think that there is any existing antenna or antenna array on earth that could detect TV signals at the power we send them from 39,900 trillion kilometers away. If we start picking up alien signals it is because they are sending highly directional signals at immense power to us or they are much closer to us than the nearest star.

[AnalogSignal]

TV signals are only intended to be received a few hundred kilometers away from the transmitter. The nearest star is 39,900,000,000,000 kilometers away. The power drops of with the square of the distance but the noise power from the rest of the universe remains constant.

An alien SETI program might overcome this by sending out vast numbers of nuclear-powered repeater satellites that would capture local signals. Signals of interest would be boosted and beamed back to other repeaters and eventually to the alien planet.

Once signals of Earth origin are received by the aliens, they would have to decide which ones to decode and analyze first. They might give highest priority to the most frequently repeated signals assuming they have the most importance. You can imagine their disappointment after spending years analyzing episodes of Gilligan's Island.

[Stranger On A Train]

An alien SETI program might overcome this by sending out vast numbers of nuclear-powered repeater satellites that would capture local signals. Signals of interest would be boosted and beamed back to other repeaters and eventually to the alien planet.

Once signals of Earth origin are received by the aliens, they would have to decide which ones to decode and analyze first. They might give highest priority to the most frequently repeated signals assuming they have the most importance. You can imagine their disappointment after spending years analyzing episodes of Gilligan's Island."Those poor people."

The problem with your proposal is that it would require these "repeater satellites" to actually enter the Solar system to be able to distinguish such signals from the background noise. (Using the most modern gain-amplifying receivers with signal processing capability developed for radar and passive sonar detection, we probably wouldn't be able to discern standard radio and television broadcast signals at the Moon's orbit, and certainly not as far as Mars.) If an alien species wanted to look for other life at stellar distances, the easiest thing to do is to look for signs of unnatural oxidizing atmospheres. This, of course, wouldn't tell you if there was intelligent life, but it would provide evidence of the precursors to develop aggressive and potentially multi-cellular carbon-based life.

Of course, if an alien species can zip from star to star on a weekly basis as depicted in a multitude of science fiction programs, it is trivial to create a fleet of self-reporting probes to inspect and return, or send signals. You could also look for messages deliberately transmitted from Earth, although such a search would be far, far worse in any quantitative sense than just a needle in a haystack. (Not only would you have to examine millions of potential stars, but they would have to be transmitting specifically to use using a massive amount of energy resources.) It seems unlikely, given the apparently commonality of organic precursors and the tendency of systems to achieve dynamic regulation of thermodynamic systems even in absence of any deliberate or cognitive plan, that life doesn't exist somewhere else, and not a massive stretch to assert that the steps that led to the development of multicellular life, specialized function, and ultimately intelligence on Earth are, while perhaps rare bottlenecks in any particular path, are among the uncountable number of potentially life-supporting habitats not only possible but even likely or perhaps inevitable; but that same mass of distributed probability may also preclude contact between intelligent species in different star systems. Not only the vast difference, but also the distribution of time between which a species may develop the ability to examine and explore other worlds, to the time that it becomes extinct (whether by natural causes or by competitive or unfortunate self-destruction) or transcends to a point that other species cannot relate, may make contact between species statistically unlikely. We'd like to think that we're special and unique, worthy of being contacted by another species, but in fact we may be less to a more advanced alien species than microbes are to us, and equally as incapable of sharing knowledge or insight.

Stranger

[Chronos]

However, whether they could convert such coherent signals into video depends not so much on the format of the code, but whether they use the transmission of visual signals, or indeed are physically capable of vision.They don't necessarily need vision; any method of communication which has varying values as a function of 2D position and time would work just as well. Maybe their equivalent of a "TV" is a flat plate with "pixels" that move in or out of the plane and they "watch" it by holding it up against a large area of sensitive skin, or something. But you could still "display" a video feed on such a device.

[RaftPeople]

They don't necessarily need vision; any method of communication which has varying values as a function of 2D position and time would work just as well. Maybe their equivalent of a "TV" is a flat plate with "pixels" that move in or out of the plane and they "watch" it by holding it up against a large area of sensitive skin, or something. But you could still "display" a video feed on such a device.

I'm not so sure this is correct. If they have some sense that results in 2D time varying input, but that is based on some entirely different physical attribute (maybe intensity of electrical field, or magnetic field, or various chemical receptors), then they will try to interpret what has been transmitted within the context of their sense, environment and experience.

They would have input, but no context other than their own.

[AnalogSignal]

The problem with your proposal is that it would require these "repeater satellites" to actually enter the Solar system to be able to distinguish such signals from the background noise.


I see. Would a nuclear explosion like Hiroshima or Nagasaki be detectable from beyond the solar system?


If an alien species wanted to look for other life at stellar distances, the easiest thing to do is to look for signs of unnatural oxidizing atmospheres.


Good point.


Of course, if an alien species can zip from star to star on a weekly basis as depicted in a multitude of science fiction programs, it is trivial to create a fleet of self-reporting probes to inspect and return, or send signals


Yes, warp drive is science fiction. Assuming wormholes exists, perhaps aliens could send probes through them as a space travel shortcut? This also sounds like science fiction but perhaps it is remotely plausible?



Not only the vast difference, but also the distribution of time between which a species may develop the ability to examine and explore other worlds, to the time that it becomes extinct (whether by natural causes or by competitive or unfortunate self-destruction) or transcends to a point that other species cannot relate, may make contact between species statistically unlikely.

Agreed. Contact would be extraordinarily unlikely.

[Measure for Measure]

If we start picking up alien signals it is because they are sending highly directional signals at immense power to us or they are much closer to us than the nearest star. Seth Shostak of the SETI institute discussed the OP's topic in 2009. Some of our radar installations send signals that we could pick up several light years away. "The most powerful radars, such as the one mounted on the Arecibo telescope (used to study the ionosphere and map asteroids) could be detected with a similarly sized antenna at a distance of nearly 1,000 light-years."

For an analog broadcast, "At 50 light-years distance, a nosy alien could find that signal peak in a hundred-hour search using an antenna the size of Texas. Note that the antenna needn’t be a giant, expensive parabolic dish like the one at Arecibo, Puerto Rico, or even thousands of small backyard dishes. It can just be acres and acres of cheap rabbit ears or rooftop yagis." http://www.seti.org/page.aspx?pid=225

Also: "It’s not impossible to pick up our DTV broadcasts from your favorite M-class planet, but I reckon it would require antennas at least five times larger than demanded for good, old analog TV. ET may balk at the additional cost. "

[Measure for Measure]

Sorry, Shostak was discussing detection, not decoding above. Another link (http://www.setileague.org/askdr/earthtv.htm) suggests that aliens probably could not decode TV transmissions from Earth.

[Chronos]

Yes, warp drive is science fiction. Assuming wormholes exists, perhaps aliens could send probes through them as a space travel shortcut? This also sounds like science fiction but perhaps it is remotely plausible?Usable wormholes are exactly as much science fiction as warp drives. Either one would require the same exotic material properties to exist.