main.cpp

00001 #include "mbed.h"
00002 #include "USBAudio.h"
00003 
00004 // frequency: 8 kHz
00005 #define FREQ 8000
00006 
00007 // 1 channel: mono
00008 #define NB_CHA 1
00009 
00010 // length of an audio packet: each ms, we receive 48 * 16bits ->48 * 2 bytes. as there is one channel, the length will be 48 * 2 * 1
00011 #define AUDIO_LENGTH_PACKET (FREQ/500) * NB_CHA
00012 
00013 // as we don't use audio in, 48000 and 1 are dummy
00014 USBAudio audio(8000, 1, FREQ, NB_CHA, 0x0103, 0x4521);
00015 
00016 AnalogIn mic(p20);
00017 
00018 int16_t buf[AUDIO_LENGTH_PACKET/2];
00019 
00020 int main() {
00021     double mic_mean = 0.0;
00022     double mic_value;
00023 
00024     // compute average value of the microphone. We can then center the audio signal sent to the computer
00025     for (int j = 0; j < 1000; j++) {
00026         mic_value = (mic.read_u16() >> 3);
00027         mic_mean = (mic_mean*j + mic_value)/(j+1);
00028     }
00029 
00030     while (1) {
00031         for (int i = 0; i < AUDIO_LENGTH_PACKET/2; i++) {
00032             buf[i] = (mic.read_u16() >> 3) - mic_mean;
00033             if (i != AUDIO_LENGTH_PACKET/2) {
00034                 wait_us(80);
00035             }
00036         }
00037         audio.write((uint8_t *)buf);
00038     }
00039 }