SignalsEverywhere is back this week and in her latest video Sarah talks about using a combination of Audacity, Minimodem and Multimon-ng to decode digital data that could be obtained from an SDR or other signal source.
Sarah was interested in the 2020 Hackasat space security challenge and specifically in completing the 56k Flex Magic challenge which consists of an emulated signal from an old 56k modem. Within the 56k modem signal is secret information required to complete the challenge.
Sarah first shows how to use Multimon-ng to decode the DTMF tone section of the signal. These are the tones heard when dialling on a landline phone. She then goes on to show how to use Audacity in spectrogram mode to take a closer look and analyze the next chunk of the signal. Then by using the information gained about the signal from the spectrogram analysis she is able to decode the data via minimodem.
Audacity Decoding Data?! Using Audacity Multimon-ng and Minimodem to Decode Digital Audio Data!
Over on his YouTube channel Kris Occhipinti has uploaded some videos where he shows how he is able to send text data over FM radio frequencies by using an MP3 audio file that encodes the text data, an FM transmitter connected to an Android phone or MP3 player to transmit the file and an RTL-SDR on the receiving side to receive the FM signal from the FM transmitter. The software used to encode the text into an MP3 is Minimodem, and on the receiving side Minimodem is also used which can easily decode the received audio. Minimodem is a command line program which can generate FSK modem tones from data.
These two videos are part of a series that Kris has been working on that includes many videos about using Minimodem to transfer data like text, files and images between computers via radio.
12 Minimodem an FM Transmitter and a USB SDR Dongle