Hi Nathan,
Just to clarify, I don't think that this
challenge has audio frequencies only.
I'm 100% certain it only has audio frequency content -- since the
the sampling rate of a stream of samples defines its maximum
frequency, this file definitely can't contain anything above 24kHz,
which is what I'd say is audio frequency content.
I suppose it's possible, and I hadn't
considered that as I assumed that the 2400 bauds hint meant that
it was an audio capture of a 2400 baud modem transferring a file.
Well, "baud" has, in a pure computer user context, probably most
often been used with telephone line modems, but that's because it
describes a property typical to modems. Read wikipedia on baud, and
things will clear up.
In viewing the Waterfall or Spectrum
analysis, the "hint" data is at a frequency outside of the main
frequency.
I don't fully understand what you mean with "main frequency". But I
guess you mean the energy on the lower frequencies.
I would guess this needs to be removed by a
low pass filter that would remove any signal above a certain
frequency?
Good guess, in my opinion ;)
Cheers,
Marcus
On 28.07.2015 00:48, Nathan Coppersmith
wrote:
Marcus,
Thank you for your input! Just to clarify, I don't
think that this challenge has audio frequencies only. I
suppose it's possible, and I hadn't considered that as I
assumed that the 2400 bauds hint meant that it was an
audio capture of a 2400 baud modem transferring a file.
Considering that a modem transferring a file has to
convert the digital signal to analog in order to
traverse a phone line, if you were spying on someone and
only had an audio clip of the transfer, you would need
to demodulate the analog signal into the digital
representation to extract the information contained
within. There's another clue located in the EXIF data
of the file pointing to the wikipedia article on
modulation. (which I read)
In viewing the Waterfall or Spectrum analysis, the "hint"
data is at a frequency outside of the main frequency. You
can hear it as a sort of high pitched whine or squeal when
listening to the audio. I would guess this needs to be
removed by a low pass filter that would remove any signal
above a certain frequency?
If I'm reading my spectrum analysis correctly, the "data" is
in the frequency of 45Hz to 5kHz, with the embedded hint
appearing at the 11kHz to 12kHz range.
I believe this to be an FSK modulated stream, the waveform
data is consistent with what's shown on the wiki page https://en.wikipedia.org/wiki/Frequency-shift_keying
as a modulated signal.
I'm reasonably (99%) certain this is a data stream as the way to
solve it is to submit a string that starts with flag- I'm
going to experiment with it some more. Thanks for the pointers!
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