main.cpp

00001 // USBAudio speaker example
00002 
00003 #include "mbed.h"
00004 #include "USBAudio.h"
00005 
00006 // frequency: 48 kHz
00007 #define FREQ 48000
00008 
00009 // 1 channel: mono
00010 #define NB_CHA 1
00011 
00012 // 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
00013 #define AUDIO_LENGTH_PACKET 48 * 2 * 1
00014 
00015 // USBAudio (we just use audio packets received, we don't send audio packets to the computer in this example)
00016 USBAudio audio(FREQ, NB_CHA, 8000, 1, 0x7180, 0x7500);
00017 
00018 // speaker connected to the AnalogOut output. The audio stream received over USb will be sent to the speaker
00019 AnalogOut speaker(p18);
00020 
00021 // ticker to send data to the speaker at the good frequency
00022 Ticker tic;
00023 
00024 // buffer where will be store one audio packet (LENGTH_AUDIO_PACKET/2 because we are storing int16 and not uint8)
00025 int16_t buf[AUDIO_LENGTH_PACKET/2];
00026 
00027 // show if an audio packet is available
00028 volatile bool available = false;
00029 
00030 // index of the value which will be send to the speaker
00031 int index_buf = 0;
00032 
00033 // previous value sent to the speaker
00034 uint16_t p_val = 0;
00035 
00036 // function executed each 1/FREQ s
00037 void tic_handler() {
00038     float speaker_value;
00039 
00040     if (available) {
00041         //convert 2 bytes in float
00042         speaker_value = (float)(buf[index_buf]);
00043         
00044         // speaker_value between 0 and 65535
00045         speaker_value += 32768.0;
00046 
00047         // adjust according to current volume
00048         speaker_value *= audio.getVolume();
00049         
00050         // as two bytes has been read, we move the index of two bytes
00051         index_buf++;
00052         
00053         // if we have read all the buffer, no more data available
00054         if (index_buf == AUDIO_LENGTH_PACKET/2) {
00055             index_buf = 0;
00056             available = false;
00057         }
00058     } else {
00059         speaker_value = p_val;
00060     }
00061     
00062     p_val = speaker_value;
00063 
00064     // send value to the speaker
00065     speaker.write_u16((uint16_t)speaker_value);
00066 }
00067 
00068 int main() {
00069 
00070     // attach a function executed each 1/FREQ s
00071     tic.attach_us(tic_handler, 1000000.0/(float)FREQ);
00072 
00073     while (1) {
00074         // read an audio packet
00075         audio.read((uint8_t *)buf);
00076         available = true;
00077     }
00078 }