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libraries/audio-tools/examples/examples-player/player-callback-i2s/README.md

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+# A Simple Callback Audio Player
+
+The example demonstrates how to implement an __MP3 Player__ which __provides the data via callbacks__ and sends the audio via I2S to an external DAC. 
+
+This demonstrates the minimum implementation. I recommend however to provide implementations for the other callback methods to enhance the functionality: e.g absolute positioning, file selection by name etc. 
+
+
+## SD Card
+
+Here is the information how to wire the SD card to the ESP32
+
+| SD    | ESP32 
+|-------|-----------------------
+| CS    | VSPI-CS0 (GPIO 05) 
+| SCK   | VSPI-CLK (GPIO 18) 
+| MOSI  | VSPI-MOSI (GPIO 23) 
+| MISO  | VSPI-MISO (GPIO 19) 
+| VCC   | VIN (5V) 
+| GND   | GND 
+
+
+### External DAC:
+
+For my tests I am using the 24-bit PCM5102 PCM5102A Stereo DAC Digital-to-analog Converter PLL Voice Module pHAT
+
+![DAC](https://pschatzmann.github.io/Resources/img/dac.jpeg)
+
+I am just using the default pins defined by the framework. However I could change them with the help of the config object. The mute pin can be defined in the constructor of the I2SStream - by not defining anything we use the default which is GPIO23
+
+ 
+DAC  |	ESP32
+-----|----------------
+VCC  |	5V
+GND  |	GND
+BCK  |	BCK (GPIO14)
+DIN  |	OUT (GPIO22)
+LCK  |	BCK (GPIO15)
+FMT  |	GND
+XMT  |	3V (or another GPIO PIN which is set to high)
+
+- DMP - De-emphasis control for 44.1kHz sampling rate(1): Off (Low) / On (High)
+- FLT - Filter select : Normal latency (Low) / Low latency (High)
+- SCL - System clock input (probably SCL on your board).
+- FMT - Audio format selection : I2S (Low) / Left justified (High)
+- XMT - Soft mute control(1): Soft mute (Low) / soft un-mute (High)
+
+
+
+## Dependencies
+
+- https://github.com/pschatzmann/arduino-audio-tools
+- https://github.com/pschatzmann/arduino-libhelix
+- https://github.com/greiman/SdFat

libraries/audio-tools/examples/examples-player/player-callback-i2s/player-callback-i2s.ino

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+/**
+ * @file player-sd-callback.ino
+ * @brief see https://github.com/pschatzmann/arduino-audio-tools/blob/main/examples/examples-player/player-callback-i2s/README.md
+ * 
+ * @author Phil Schatzmann
+ * @copyright GPLv3
+ */
+
+
+// Install https://github.com/pschatzmann/arduino-libhelix.git
+
+#include "Arduino.h"
+#include "AudioTools.h"
+#include "AudioTools/AudioCodecs/CodecMP3Helix.h"
+#include <SPI.h>
+#include <SD.h>
+
+
+// forward declarations
+void callbackInit();
+Stream* callbackNextStream(int offset);
+
+// data
+const int chipSelect=PIN_CS;
+AudioSourceCallback source(callbackNextStream, callbackInit);
+I2SStream i2s;
+MP3DecoderHelix decoder;
+AudioPlayer player(source, i2s, decoder);
+File audioFile;
+File dir;
+
+void callbackInit() {
+  // make sure that the directory contains only mp3 files
+  dir = SD.open("/TomWaits");
+}
+
+Stream* callbackNextStream(int offset) {
+  audioFile.close();
+  // the next files must be a mp3 file
+  for (int j=0;j<offset;j++)
+    audioFile = dir.openNextFile();
+  return &audioFile;
+}
+
+void callbackPrintMetaData(MetaDataType type, const char* str, int len){
+  Serial.print("==> ");
+  Serial.print(toStr(type));
+  Serial.print(": ");
+  Serial.println(str);
+}
+
+
+void setup() {
+  Serial.begin(115200);
+  AudioToolsLogger.begin(Serial, AudioToolsLogLevel::Info);
+
+  // setup SD
+  SD.begin(chipSelect);
+
+  // setup output
+  auto cfg = i2s.defaultConfig(TX_MODE);
+  i2s.begin(cfg);
+
+  // setup player
+  player.setMetadataCallback(callbackPrintMetaData);
+  player.begin();
+}
+
+void loop() {
+  player.copy();
+}