# In the movie Contact, how was the message from the aliens determined to be a video message?

How do signals at double the frequency indicate a video signal?

• This seems to be asking for a scientific explanation, which is off-topic here but may be on-topic elsewhere. Jan 5, 2018 at 15:47
• @NKCampbell scientifically, they don’t (either that or I should’ve failed my degree) this may be IU but it definitely sounds OOU Jan 5, 2018 at 16:19
• Actually it is explained in the novel: the message consists of two kinds of signals (say long and short burst), and the total number of bursts is the product of two prime numbers, say p and q: so the scientists draw a pxq grid, and paint black the long bursts, so an image appears, and so on...
– Luca
Jan 5, 2018 at 16:21
• In the movie, you're actually mingling two events in quick succession. Kent hears a minor disturbance in the rather full blast primes transmission that identifies the second signal (harmonics?) Ellie then gets the feed from that second signal on the spectrum analyzer, and looking at the shape of the second signal, identifies it as a video feed and calls for the tv. I don't know enough about radio signals to know why, but having to be familiar with every earth-source radio wave to filter out false positives probably helped (it WAS earth video, after all) Jan 5, 2018 at 16:37
• Both the script (scifiscripts.com/scripts/contact.txt) and the book have lots of info about how they knew. Honestly I think someone with a better grasp of the technology than me should answer this one, though Jan 5, 2018 at 22:18

I don't claim to be an expert, but I can try to explain this in my limited fashion.

Some old radio receivers used to have two modes of operation: AM and FM. AM stands for "amplitude modulation" and FM stands for "frequency modulation". We can emit radio waves of different amplitudes and wavelengths, and by changing them frequently we can encode signals. E.g. a region of higher amplitude or greater frequency would indicate `1`, and the reverse would indicate `0`. Using this we can encode anything, provided enough bandwidth.

There's also another type of modulation - polarisation modulation. The novel explains it well, in my opinion:

"When a wave of light comes at you--visible light, radio light, any kind of light--it's vibrating at right angles to your line of sight. If that vibration rotates, the wave is said to be elliptically polarized. If it rotates clockwise, the polarization is called right-handed; counterclockwise, it's left-handed. I know it's a dumb designation. Anyway, by varying between the two kinds of polarization, you could transmit information. A little right polarization and that's a zero; a little left and it's a one. Follow? It's perfectly possible. We have amplitude modulation and frequency modulation, but our civilization, by convention, ordinarily just doesn't do polarization modulation."
Contact by Carl Sagan

As you see, with three different types of polarisation one can have three signals in one. As far as I understand, the signal received from Vega was an AM signal containing the prime numbers. It's commonly believed that a natural phenomenon could not generate the prime sequence just like that, so it must be a sign of intelligent life (from the script, with a hat tip to Valorum for finding it):

```                    FISHER
Linearly polarized; a set of moving
pulses restricted to two different
amplitudes --
```

So the two amplitudes corresponded to binary numbers, and when converted to the decimal system they yielded the prime numbers. Later, David Drumlin suggests they also check the polarisation modulation:

```                    ELLIE
... could it be a nested code of
some sort?

DRUMLIN
You must have checked the signal for

Ellie hesitates.  Drumlin looks at her.  Whoops.

ELLIE
Dr. Lunacharsky...?

LUNACHARSKY
(takes a drag)
Analyzing signal polarization
shifts.

He type in a series of commands.  Something begins to
happen on the main display --

LUNACHARSKY
Bingo.

POV - THROUGH WINDOW

Kitz, on the phone, sees the activity around the console.

BACK TO SCENE

Lunacharsky types in commands as he talks.

LUNACHARSKY
A second layer, nested within the
main signal; possibly... a picture?
Product of three primes...

KITZ
(joining them)
What.

LUNACHARSKY
... definitely three dimensions,
either a hologram or a two-
dimensional picture that moves in
time; a movie.

DRUMLIN
(dryly)
Hope there's a cartoon.

KITZ
How is that possible?  How could all
that information be encoded in --

KENT
Well, sir, some bits of the signal
are bits that tell us how to
interpret the other bits.
Technically speaking.

JURYRIGGED 35-INCH MONITOR

Strings of zeros and ones fill the screen as the group
assembles around it.

ELLIE
Enlarge.

Willie types.  Ellie moves closer, studying it.
Intuitively:

ELLIE
Try plotting values in a three
dimensional coordinate system.

A pattern begins to emerge.

DRUMLIN
Throw a gray scale on it; standard
interpolation.

ELLIE
Rotate 90 degrees counterclock wise.

Willie enters commands.  All are mesmerized by the shadows
taking form on the screen.

ELLIE
It has to be an image.  Stack it up,
string-breaks every 60th character.

On the screen a distinct black and white moving image
forms; grays define it even further.  The group is
transfixed.  Kitz whispers to an aide who makes a call in