Acoustic Hacking: Pure Code DTMF Decoder

Most "Tone Decoders" cheat by using a dedicated hardware chip (like the MT8870) to perform the math. The Pure Code DTMF Decoder forces the Arduino's 16MHz ATmega328P chip to aggressively sample an analog input and run mathematical spectrum analysis in real-time, executing the Goertzel Algorithm natively.

Bypassing the analogRead() Speed Limit

To catch audio frequencies up to 1500Hz, the Arduino must take hundreds of samples per millisecond. The standard analogRead() function takes over 100 microseconds to run, which is way too slow for audio processing!

1. Direct ADC Manipulation: You must use advanced C++ to rewrite the Assembly-level Prescaler bits inside the ATmega chip.
2. By setting the ADCSRA register, you brutally overclock the Analog-to-Digital Converter, increasing the sampling speed from 9kHz to nearly 38kHz!

The Matrix Detection (Goertzel)

A DTMF tone is a mixture of two exact frequencies (e.g., pressing "3" outputs 697Hz + 1477Hz).

• The Arduino fills an array with the high-speed audio samples.
• It rapidly runs seven parallel Goertzel mathematics equations simultaneously, looking for the four specific high-band frequencies and three low-band frequencies.
• If the magnitudes for 697Hz and 1477Hz both cross the hard-coded threshold simultaneously, it prints Key Pressed: 3! to the Serial Monitor.

The Absolute Minimum Hardware

• Arduino Uno (or Mega for more RAM buffer).
• Electret/MEMS Microphone Breakout (MAX4466): Highly recommended to have onboard amplification because a bare microphone produces signals too small for the Arduino to read accurately.
• (Warning: This project requires heavy optimization. Erroneous Serial.print statements anywhere in the loop will instantly destroy the precise timing required to sample the audio).