main.cpp

00001 #include <mbed.h>
00002  
00003 // Boolean types
00004 #define TRUE 1
00005 #define FALSE 0
00006 
00007 /* ADC for the microphone/input, DAC for the speaker/output */
00008 AnalogIn mic(p19);
00009 AnalogOut speaker(p18);
00010  
00011 // Allocate a buffer to be used for the audio recording
00012 static const size_t   BufferSize = 15 * 1024;
00013 static unsigned short Buffer[BufferSize];
00014  
00015  
00016 int main(void)
00017 {
00018     unsigned short        ReadSample = 0xFFFF;
00019     // Indices to track where the playback and recording should take place in the
00020     // audio buffer.  The recording can occur one sample behind the current playback
00021     // index since it is no longer required.
00022     int                  Index = 0;
00023     // Reverse the direction the buffer is walked between each iteration to save memory
00024     int                  Direction = 1;
00025     // Have audio to playback
00026     int                  Playback = FALSE;
00027     // The amount of data to be recorded before starting reverse playback
00028     // NOTE: Probably want this to be configured at runtime via a knob, etc.
00029     int                  ChunkSize = BufferSize;
00030     
00031     // Infinite loop of recording and reverse playback
00032     for (;;)
00033     {
00034         unsigned short PlaySample;
00035  
00036         // Slow down the sampling rate so that it doesn't exceed 20kHz
00037         wait_us(50);
00038         
00039         // Read out the sample from the buffer to be played back
00040         if (Playback)
00041         {
00042             PlaySample = Buffer[Index];
00043             speaker.write_u16(PlaySample);
00044         }
00045         
00046         // Obtain current audio sample from the A/D converter.
00047         ReadSample = mic.read_u16();    
00048         
00049         // Record the sample into the buffer right where a space was freed up from the PlaySample read above
00050         Buffer[Index] = ReadSample;
00051         
00052          // Increment the buffer pointer
00053         Index += Direction;
00054         
00055         // Check to see if the chunk has been filled
00056         if (Index < 0)
00057         {
00058             // Now have a chunk to be played back
00059             Playback = TRUE;
00060             // Reverse the direction of playback and recording
00061             Direction *= -1;
00062             Index = 0;
00063         }
00064         else if (Index >= ChunkSize)
00065         {
00066             // Now have a chunk to be played back
00067             Playback = TRUE;
00068             // Reverse the direction of playback and recording
00069             Direction *= -1;
00070             Index = ChunkSize - 1;
00071         }
00072     }
00073 }