main.cpp

00001 /*******************************************************************************
00002  * Project: USB Sound Effects.
00003  *
00004  * Uses the mbed LPC1768 and mbed-application-board to create a USB audio device
00005  * that streams audio from a host computer to headphones or powered speakers. 
00006  * A couple different sound effects can be applied to the stream in real-time,
00007  * tilting the mbed alters the effects.
00008  *
00009  *                                               ECHO
00010  *       The joystick selects )                   |
00011  *       one of three effect  )       STRAIGHT -  o  - STRAIGHT
00012  *       modes.               )                   |
00013  *                                              REVERB   
00014  * 
00015  *
00016  *
00017  *                                               \\           ||    
00018  *       Tilting the mbed     )      ======       \\          ||
00019  *       determines intensity )                    \\         ||
00020  *       of the effect.       )
00021  *                                     0%         50%         100%  
00022  *
00023  * The LCD display shows the current effect mode, intesity and buffer level.
00024  *
00025  * For better audio quality, packets are buffered before playbeck. Playback is
00026  * halted when the buffer drops below two packets and resumes when it is greater
00027  * than four packets. This creates a lag of a few milliseconds, which is not 
00028  * detectable by a listener using straight playback mode.
00029  * 
00030  * The echo effect produces echos with a delay of about 250 milliseconds. Tilting
00031  * the mbed alters the persistence of echo.
00032  *
00033  * The reverb effect utilizes multiple delay lines of length varying from  
00034  * about 25 to 200 milliseconds. Again, tilting the mbed alters the 
00035  * persistence. 
00036  *
00037  * Implemenation Notes:
00038  * Audio playback is implement with an mbed ticker, and it is critical that this interrupt
00039  * is not delayed to acheive decent audio quality. USB audio packet handling also uses
00040  * interrupts, but the priority is set lower to ensure proper playback. The USBAudio class
00041  * was modified to use a callback to transfer the received packet from the packet buffer to 
00042  * the larger playback buffer. The user interface is handled entirely in the user-context
00043  * since it has only soft timing requirements.
00044  *
00045  * Future work:
00046  * 1. Clocks within the host computer and mbed will differ slightly, so the 
00047  * average sample rate from the computer will not exactly match the average mbed
00048  * playback rate. Over time the playback buffer level with creep until it becomes 
00049  * totaly empty or full. The current implementation addresses this by simply 
00050  * increasing the buffer size and selecting a sample rate that the mbed 
00051  * can replicate most accurately (mulitple of 1 us to use mbed Ticker). A better 
00052  * solution may be to slightly alter the playback rate in response to the buffer level.
00053  * For example use 20us rate when the buffer level is high and 19us when the buffer 
00054  * level is low. Of course this would distort the playback frequecy by 5%, so it would
00055  * be even better to make a custom timer interrupt with better resolution than the 
00056  * mbed ticker.
00057  *
00058  * 2. It would be interesting to add stereo audio, but the mbed-application-board 
00059  * audio jack is mono so this would require hardware customization.
00060  *
00061  * Bryan Wade
00062  * 27 MAR 2014
00063  ******************************************************************************/
00064 
00065 #include "mbed.h"
00066 #include "audio.h"
00067 #include "effects.h"
00068 #include "user_interface.h"
00069 
00070 int main() 
00071 {
00072     UI_Initialize();    // Init the user interface.
00073     Audio_Initialize(); // Starts audio playback.
00074     
00075     while (true) 
00076     {   
00077         // Check the playback buffer level.      
00078         int32_t level = Audio_CheckPlaybackBufferLevel();
00079         
00080         // Update the user interface, passing in the current buffer level 
00081         // for display
00082         UI_Update(level);          
00083 
00084         // Set the effect mode and gain according to the user input.
00085         Effects_SetMode(UI_GetEffectMode());
00086         Effects_SetGain(UI_GetEffectGain());
00087 
00088         // 50 milliseconds provides good UI responsivness.
00089         wait(.05);
00090     }
00091 }
00092