Challenge description: The aliens' capabilities have advanced and they've decided to improve the reliability of their transmissions. Can you crack their message?
To understand this writeup, it's important to grasp modulation concepts, so check out
this lesson :)
We have several segments; zoom in on one of them.
It clearly resembles
ASK. If you're not familiar with this modulation, I suggest reading the writeup for
that challenge which covers ASK in detail.
Select
ASK as the modulation type and manually determine the
Samples/Symbol by choosing the smallest segment, the value indicated in
red.
Switch the view to
ASCII to see what the decoded signal yields. Initially, you get three similar lines that don't make much sense.
I recall that during the
first challenge, I spent time understanding why it didn't decode until I disabled the
Pause Threshold (set it from
8 to
0). So, I disable it. Now the decoded signal appears as follows:
Alright, not perfect, but remember, this is the final challenge of the series. Given that the aliens have "improved" the reliability of their transmission, it suggests that the encoding method may be the issue.
Note: Do not confuse modulation with the encoding method.
To send a digital (binary) message, first you encode it, that is, you determine how the bits are organized (like Morse code, for example), and then you modulate that digital signal (using ASK, for instance) to transform it into a transmittable radio wave.
In Universal Radio Hacker, the default decoding method is
NRZ. You can see this in the
Analysis tab marked in red:
URH comes with presets for known encoding methods like Manchester coding. In Manchester coding, a 0 is represented by a transition from low to high (0 to 1) at the midpoint of the bit interval, while a 1 is represented by the inverse transition.
Note: The white lines in the diagram are only connecting lines and not part of the encoding.
URH offers two options: Manchester 1 and Manchester 2 (Differential Manchester is different). The only difference is that Manchester 2 inverts the bits (so 0s become 1s and vice versa).
When I select Manchester 2, the flag appears!
And that's it for this final alien transmission challenge.
