Android, Arduino, and the Headphone Jack

“ Can you hear me now ? ”

Using a phone’s 3.5mm audio jack for data exchange

Tip Ring Ring Sleeve

build an embedded system that captures, encodes, and transmits sensor data

the theory behind audio encoding / decoding

build an Android app that receives, decode, and displays the sensor data

Android Ground

iPhone Ground

Transferring a Message

• Encode the Message (e.g. ASCII or Morse Code)

• Convert encoded message into audio signals [D➜A]

• Send (Play) the audio signals

• Receive (Listen/Record) audio signals

• Interpret (Filter/Transform) audio signals [A➜D]

Amplitude

Period (time required for one complete cycle)

FSK Frequency Shift Keying

ASK Amplitude Shift Keying

PSK Phase Shift Keying

The Sender

Arduino on the Web http://arduino.cc

Getting Started with Arduino (Make: Projects)

by Massimo Banzi, Co-founder of the Arduino project

const int ledPin = 13;const int outPin = 11;const int freq = 2000;const int duration = 2000;const int pause = 1000;

void setup() { pinMode(ledPin, OUTPUT); pinMode(outPin, OUTPUT);

void loop() { digitalWrite(ledPin, HIGH); tone(outPin, freq); delay(duration); noTone(outPin); digitalWrite(ledPin, LOW); delay(pause);

Arduino + external Speakerplays 2000 Hz sound for 2 seconds

Port 11

Transferring a Message

• Encode the Message (e.g. ASCII or Morse Code)

• Convert encoded message into audio signals [D➜A]

• Send (Play) the audio signals

• Receive (Listen/Record) audio signals

• Interpret (Filter/Transform) audio signals [A➜D]

• Decode digital signal into original message

ASCII - Encoding

H e l l o Wo r l d !

0x48 0x65 0x6C 0x6C 0x6F 0x20 0x57 0x6F 0x72 0x6C 0x64 0x21

frequency = ((value -k)*M + offset ) * frequency_resolution

E.g.: ‘ A’ = 0x41 = 65 = value1st char in range: 45 = k

frequency bins per Symbol: 5 = M0Hz .. 5200 Hz* unusable: 60 = offset

44100 Hz / 512 Samples = 86.13.. = frequency_resolution((65 - 45)*5 + 60)*86.13 = 13780.8 Hz

* related to specific hardware

Mapping ASCII to Frequency

Usable Frequency Range depends on the Hardware

const int ledPin = 13; // built-in LEDconst int outPin = 11; // PWM Port outconst int inPin = 0; // Light Sensor

const char STX = '?'; // Start Token const char ETX = '@'; // End Token

const unsigned int SAMPLE_RATE = 44100; // Hzconst int SAMPLES = 512 ; // Num of Samples (128,256,512,1024)const int REPEAT = 3; // prolong the signalconst int OFFSET = 60; // 60 un-usable bin on the spectrum's low end

const double FREQ_RES = (double) SAMPLE_RATE / SAMPLES; // 86.13 frq per binconst int DURATION = (int) REPEAT * (1000 / FREQ_RES); // about 35msconst int ENC_ETX = encodeAscii(ETX); // pre-calc for later use in loop

unsigned int frq[6]; // global frequency array, changes every run 1 of 3

Initialization

void setup() { pinMode( ledPin, OUTPUT ); pinMode( outPin, OUTPUT ); pinMode( inPin, INPUT ); Serial.begin( 9600 ); // debugging on

/** * Encode the message into an frequency array * Wrap the message into Start and End tags */void encodeInt(int m) {

char message[5]; itoa( m,message,10 );

frq[0] = encodeAscii( STX ); for ( int i=0; i<5; i++ ) {

if ('\0'==message[i]) { // replace '\0' marker w/ ETX frq[i+1] = encodeAscii( ETX ); break;

} frq[i+1] = encodeAscii( message[i] );

2 of 3

Setup and Encoding a message

unsigned int encodeAscii(char c) { return (unsigned int) ((5 * (c-45) + OFFSET) * FREQ_RES);

void loop() { const int brightness = analogRead(inPin); // reads value 0-1023 encodeInt(brightness); // into global frq[] digitalWrite(ledPin, HIGH); int i=0; do {

tone(outPin, frq[i]); delay(DURATION); noTone(outPin);

} while (frq[i++] != ENC_ETX); digitalWrite(ledPin, LOW); delay(500); Serial.println(brightness);

} 3 of 3

Encoding a single char and main loop

Arduino + external Speaker + TRRS-Plugplays LightSensor value Message every .5 seconds

The Receiver

• The total number of times the signal is sampled in one second is defined as the sampling frequency. Only 44.100 Hz is guaranteed on all Android Devices

• Nyquist–Shannon sampling theorem: The sampling frequency should be at least twice the highest frequency contained in the signal.

I.e. Signals from the Arduino board should be in the 0..22,050 Hz range.

At a 44,100 Hz sample rate, 512 samples are taken in 11.6 ms

We took 512 samples in 11.6 ms

But we need to know the frequency the Arduino played.

FFT (Fast Fourier Transform)

FFT is an effective algorithm to convert signals from time domain to the frequency domain.

We use the 512 samples in the audio buffer as input; and the FFT algorithm returns a complex array, allowing us to calculate the magnitude of 512 frequency ranges.

• Sampling Rate 44,100 Hz

• Effective Frequency Range 0..22,050 Hz (...Nyquist–Shannon)

• 512 Samples

• Frequency Range is split into 256 ranges

• The Frequency Resoultion 22,050/256 = 861.52 Hz

We input 512 samples and as a return we know which of the 256 frequency ranges had the strongest signal.

So we won’t know the exact frequency the Arduino played, but we know, what’s called the frequency bucket or frequency bin.

Text .. Light sensor value 0..1023

ProgressBar %

Arduino + Android + external Speaker + TRRS-PlugLightSensor value encoded into Audio SignalsAudio Signal decoded into Text and Progress-bar updated every .5 seconds

Summary

We built an embedded system that captured sensor dataEncoded the data into ASCII and then into frequenciesGenerated tones for a predefined duration

On an Android Phone, we received the Audio Signal via TRRS CableDecoded the signal back into ASCII and then NumbersDisplayed the Text and animated a progress bar every .5s

Thanks for coming

http://wolfpaulus.com

Android, Arduino, and the Headphone Jack

Technology

Transcript of Android, Arduino, and the Headphone Jack

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