Diff: main.cpp

Revision:

0:bbf6cf0eab95

Child:

2:9429f84ea165

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/main.cpp	Thu Mar 27 21:27:04 2014 +0000
@@ -0,0 +1,88 @@
+/*******************************************************************************
+ * Project: USB Sound Effects.
+ *
+ * Uses the mbed LPC1768 and mbed-application-board to create a USB audio device
+ * that streams audio from a host computer to headphones or powered speakers. 
+ * A couple different sound effects can be applied to the stream in real-time,
+ * tilting the mbed alters the effects.
+ *
+ *                                               ECHO
+ *       The joystick selects )                   |
+ *       one of three effect  )       STRAIGHT -  o  - STRAIGHT
+ *       modes.               )                   |
+ *                                              REVERB   
+ * 
+ *
+ *
+ *                                               \\           ||    
+ *       Tilting the mbed     )      ======       \\          ||
+ *       determines intensity )                    \\         ||
+ *       of the effect.       )
+ *                                     0%         50%         100%  
+ *
+ * The LCD display shows the current effect mode, intesity and buffer level.
+ *
+ * For better audio quality, packets are buffered before playbeck. Playback is
+ * halted when the buffer drops below two packets and resumes when it is greater
+ * than four packets. This creates a lag of a few milliseconds, which is not 
+ * detectable by a listener using straight playback mode.
+ * 
+ * The echo effect produces echos with a delay of about 250 milliseconds. Tilting
+ * the mbed alters the persistence of echo.
+ *
+ * The reverb effect utilizes multiple delay lines of length varying from  
+ * about 25 to 200 milliseconds. Again, tilting the mbed alters the 
+ * persistence. 
+ *
+ * Implemenation Notes:
+ * Audio playback is implement with an mbed ticker, and it is critical that this interrupt
+ * is not delayed to acheive decent audio quality. USB audio packet handling also uses
+ * interrupts, but the priority is set lower to ensure proper playback. The USBAudio class
+ * was modified to use a callback to transfer the received packet from the packet buffer to 
+ * the larger playback buffer. The user interface is handled entirely in the user-context
+ * since it has only soft timing requirements.
+ *
+ * Future work:
+ * 1. Clocks within the host computer and mbed will differ slightly, so the 
+ * average sample rate from the computer will not exactly match the average mbed
+ * playback rate. Over time the playback buffer level with creep until it becomes 
+ * totaly empty or full. The current implementation addresses this by simply 
+ * increasing the buffer size and selecting a sample rate that the mbed 
+ * can replicate most accurately (mulitple of 1 us to use mbed Ticker). A better 
+ * solution may be to slightly alter the playback rate in response to the buffer level.
+ * For example use 20us rate when the buffer level is high and 19us when the buffer 
+ * level is low. Of course this would distort the playback frequecy by 5%, so it would
+ * be even better to make a custom timer interrupt with better resolution than the 
+ * mbed ticker.
+ *
+ * 2. It would be interesting to add stereo audio, but the mbed-application-board 
+ * audio jack is mono so this would require hardware customization.
+ *
+ * Bryan Wade
+ * 27 MAR 2014
+ ******************************************************************************/
+
+#include "mbed.h"
+#include "audio.h"
+#include "effects.h"
+#include "user_interface.h"
+
+int main() 
+{
+    UI_Initialize();
+    Audio_Initialize(); // Start audio playback.
+    
+    while (true) 
+    {   
+        //       
+        int32_t level = Audio_CheckPlaybackBufferLevel();
+        UI_Update(level);          
+
+        Effects_SetGain(UI_GetEffectGain());
+        Effects_SetMode(UI_GetEffectMode());
+
+        // 50 milliseconds provides good UI responsivness
+        wait(.05);
+    }
+}
+