A MP3 and AAC Decoder using Helix

I am providing the Helix MP3 decoder from RealNetworks as a simple Arduino Library. The Helix decoders are based on 16bits integers, so they are a perfect fit to be used in Microcontrollers.

MP3 is a compressed audio file formats based on PCM. A 2.6 MB wav file can be compressed down to 476 kB MP3.

This project can be used stand alone or together with the arduino-audio_tools library. It can also be used from non Arduino based systems with the help of cmake.

The Helix MP3 decoder provides Layer 3 support for MPEG-1, MPEG-2, and MPEG-2.5. It supports variable bit rates, constant bit rates, and stereo and mono audio formats.

API Example

The API provides the decoded data to a Arduino Stream or alternatively to a callback function. The platform dependent I2S implementations usually implement I2S as subclass of Stream, so you can just provide the corresponding I2S object to the constructor.

Here is a MP3 example using the callback:

#include "MP3DecoderHelix.h"
#include "music_mp3.h"

using namespace libhelix;

void dataCallback(MP3FrameInfo &info, int16_t *pcm_buffer, size_t len, void*) {
    for (int i=0; i<len; i+=info.nChans){
        for (int j=0;j<info.nChans;j++){
            Serial.print(pcm_buffer[i+j]);
            Serial.print(" ");
        }
        Serial.println();
    }
}

MP3DecoderHelix mp3(dataCallback);

void setup() {
    Serial.begin(115200);
    mp3.begin();
}

void loop() {
    Serial.println("writing...");
    mp3.write(music_data, music_len);    

    // restart from the beginning
    delay(2000);
    mp3.begin();
}

Installation

For Arduino, you can download the library as zip and call include Library -> zip library. Or you can git clone this project into the Arduino libraries folder e.g. with

cd  ~/Documents/Arduino/libraries
git clone pschatzmann/arduino-libhelix.git

This project can also be built and executed on your desktop with cmake:

cd arduino-libhelix
mkdir build
cd build
cmake ..
make

Parameters

The decoder needs to allocate a buffer to store a frame and the decoded data. The following default values have been defined:

#define MP3_MAX_OUTPUT_SIZE 1024 * 5
#define MP3_MAX_FRAME_SIZE 1600 
#define AAC_MAX_OUTPUT_SIZE 1024 * 3 
#define AAC_MAX_FRAME_SIZE 2100 

These values are working with standard bitrates, but in some exceptional cases (with extreme (220kbs) or insane (320kbs) bitrates) you might need to increase these values e.g. to

#define MP3_MAX_OUTPUT_SIZE 2048 * 5
#define MP3_MAX_FRAME_SIZE 3200

Alternatively you can set the values in your sketch by calling the following methods:

setMaxPCMSize(int size)
setMaxFrameSize(int size)

Memory Management

On the ESP32 we support PSRAM: just activate it in the Arduino Tools menu and all the memory will be allocated in PSRAM.

Logging

You can define the log level as Debug, Info, Warning, Error

LOGLEVEL_HELIX = LogLevelHelix::Info;

Documentation

• The Class Documentation can be found here
• I also suggest that you have a look at my related blogs

I recommend to use this library together with my Arduino Audio Tools. This is just one of many codecs that I have collected so far: Further details can be found in the Encoding and Decoding Wiki of the Audio Tools.

Final Comments

This library is based on a adapted version of libhelix from Earle Phil Hower's ESP8288Audio library
The decoder code is from the Helix project and licensed under RealNetwork's RPSL license. For commercial use you might still going to need the usual AAC licensing.