dpethes / mp3dp

MP3 decoder for Pascal

mp3dp - MP3 decoder for Pascal

Decoder for MPEG layer 3 streams, usable for Freepascal projects. Compiles to about 35kB of code on x86.

Usage and compilation

Use recent Lazarus with Freepascal (version 3.0 and higher) to compile. Older versions may work, but are not recommended. Tested under Windows 32&64 bit, but has no OS specific code so it'll most likely work anywhere.

There are 2 Lazarus project files that show the usage of the decoder:

• mp3_decode - decodes sample file to raw 16 bit signed PCM file (2 channels)
• mp3player_demo - decodes sample file and plays audio through SDL2 library, uses ImGui for display

History

The project started by trying to fix decoding errors in the Delphi port of "maplay". There were some occasional audible errors in its output, so I started trimming down the code to narrow down the cause. I got more interested in whole mp3 format and its details in the process. Eventually, bugs got fixed, but I kept on fiddling with the code, rewriting chunks of it (a lot of unused/weird code, dating back to 90's C decoders), doing memory usage and code size optimizations, making the code work on 64bit and replacing the sample player. The result should be a small mp3 decoder that can be included in other projects easily.

Limitations

• assumes little-endian arch
• doesn't support MPEG2 LSF extension
• doesn't decode Layer 1&2 (nobody misses it)
• doesn't decode free format bitstream (very rare?)
• no intensity stereo mode decoding (super rare or non-existent in the wild)
• output of x87 vs. SSE FPU mode is not bitexact (should not bother you)

Todo

• decode by frames/chunks to reduce required memory size
• mp3 file analysis to handle tags, seeking, free format bitstream
• parse VBR, lame info tags
• table based vlc decoding
• reduce code size, both source and binary
• re-add MPEG2 LSF extension handling
• add intensity stereo mode decoding under $define, just for the sake of completeness?
• compliance check

Resources & Thanks

• https://sourceforge.net/projects/delphimpeg/

  Delphi port of "maplay" version 1.2

• https://github.com/ZaneDubya/MP3Sharp

  Decode MP3 files to PCM bitstreams entirely in .NET managed code

• http://blog.bjrn.se/2008/10/lets-build-mp3-decoder.html

  Good description of mp3 decoding process.

• http://keyj.emphy.de/minimp3/ , https://keyj.emphy.de/kjmp2/

  Small mp2/mp3 decoders with some interesting details in blogposts.

• https://github.com/lieff/minimp3

  Minimalistic MP3 decoder single header library

• https://www.bensound.com/royalty-free-music

  The sample MP3 file "Happy Rock" is from Bensound.com.