My 2nd Year Project, which is to construct a real time audio descrambler.
Build a digital real time audio descrambler, with a goal to decode a secret message. This project involved using MSP432 and building our own circuitry using the knowledge learnt throughout the study and provide a great exposure to the basics of signal processing, both analog, and Digital Signal Processing (DSP).
Firstly, a MATLAB program was provided to enable us identify how the scrambling is done in the first place. The scrambler program works by recording audio signal for 5 seconds and will output the scrambled version of the signal. By utilising this program and doing some analysis, the way the scrambler works is by flipping the frequency spectrum of audio signal at 3.5kHz and adding a 8kHz sine wave.
Hence, to decode the scrambled message, the signal need to be flip the frequency spectrum of audio signal back at 3.5kHz and deleting the 8kHz sine wave. The way to accomplish this is by:
- Build a digital filter to remove 8kHz signal
- Multiply the signal by 7kHz wave. From trigonometry rule, this will produce a sinusoidal signal with an upper and lower frequency. The original signal is the lower frequency
- Build an analog low-pass filter to filter the unwanted upper sideband of the signal. This filter will also works as an anti-aliasing filter for DAC
All the operation (sample the audio input, perform digital filtering and produce an audio output) need to be done synchronously at the same sampling frequency of 50 kHz.
The block diagram of elements involved in this project is shown below:
Each of the element in the block diagram (excluding the audio signal and speaker) is explained further in its separate file. The full schematic of the audio descrambler is shown below:
- Implement the audio amplifier using another types of amplifier such as Class AB and Class D amplifier
- Implement the digital electronics section of the circuit using a DE0-Nano FPGA board