picomp3lib

mp3 C library for Raspberry Pico

Based on the helix mp3 library created by RealNetworks in 2003
Additions for ARM from AdaFruit https://github.com/adafruit/Adafruit_MP3

Further Changes

1. Size of frame returned by MP3FrameInfo
2. Improved MULSHIFT32 for ARM Cortex M0+
3. CMake files, so can be used as a library for Pico

Purpose

There are 4 main steps in decoding and playing an MP3 file on the RP Pico

1. Read the MP3 file from internal flash or SD Card
2. Extract MP3 frames from the file
3. Decode the MP3 frame into PCM data chunks
4. Generate sounds from the PCM data chunks through e.g. PWM or I2S

The original helix library (in the src directory) provides support for (3). Additions (in the interface directory) provides support for (1) and (2)

To initially experiment with the library, without having to be concerned with reading an SD Card, or configuring PWM or I2S, there is an off target example in the test directory. This example can be built for the Raspberry Pi (or other Linux based systems). The example reads an MP3 file and writes a wav file containing the decoded contents. Build instructions are contained in the Readme

Directory Structure

interface directory

Provides a high level interface to the MP3 decoder. Will also parse and decode wav files.
Uses FatFS to access files. See https://github.com/carlk3/no-OS-FatFS-SD-SPI-RPi-Pico for a Pico compatible FatFS library

The MP3 file to play is initially opened using

musicFileCreate

The format of the file (to e.g. configure the output mechanism) can then be found using

musicFileGetSampleRate

musicFileGetChannels

musicFileIsStereo

The file is decoded through calls to

musicFileRead

The progress through the file can be determined through calls to

musicFileBytesLeft

To terminate playback, the file is closed using

musicFileClose

A simple on target example can be found at: https://github.com/ikjordan/picomp3test

test directory

An off target test harness that can be run on a Raspberry Pi (or other Linux systems), uses a shim layer to emulate FatFS. Decodes an MP3 file and creates a wav file. See Readme for more details

src directory

Base source code for library. The public interface consists 6 functions, however typically the higher level public interface defined in music_file.h in the interface directory is used.

MP3InitDecoder

Initialises the decoder.

MP3FreeDecoder

Clears buffers

MP3Decode

Decodes one frame of MP3 data

MP3GetLastFrameInfo

Returns info about last MP3 frame decoded (number of samples decoded, sample rate, bitrate, etc). Typically called directly after a call to MP3Decode

MP3GetNextFrameInfo

Parses the next MP3 frame header

MP3FindSyncWord

Locates the next byte-aligned sync word in the raw mp3 stream

Usage

This library should be included in a project as a submodule.
For examples of how to use this library as a submodule see:

1. https://github.com/ikjordan/picosounds
2. https://github.com/ikjordan/picomp3test