Points: 431 Solves: 60
I remember my father talking about those old age radios and how they work. Pretty cool if you ask me. Also he told me about a radio channel which had a frequency between 1500-1599khz from PHILIPPINES. Note: Sampling Rate for the file is 24kHz
Author: d4y0n3
Okay this was a pretty fun challenge. First things first, open up the given iq file in inspectrum and there is a clearly visible carrier frequency of 6 kHz. The low frequency of the carrier meant the signal was most likely amplitude modulated.
So I drew up a flow graph in gnuradio-companion to demodulate the signal, hoping that it would turn out to be AM only. We first multiply the input with the carrier frequency to downconvert the signal
Here is a pic showing the input signal (the one on top) and the downconverted signal
Next add an AM Demod block with the audio_stop parameter at around 10 kHz and we get a pitched up audio recording from the input. This is the final flow graph in GNU Radio gnuradio-companion
Pitch this audio down and slow it down, and you get the flag
Spent too much time wondering wth did the Philippines thing have to do with this puzzle. And I still have no idea.
BITSCTF{welcome_to_our_radio_enjoy_our_song_collection}