C/C++: Streaming MP3

Question

In a C++ program, I get multiple chunks of PCM data and I am currently using libmp3lame to encode this data into MP3 files. The PCM chunks are produced one after another. However, instead of waiting until the PCM data stream finished, I'd like to encode data early as possible into multiple MP3 chunks, so the client can either play or append the pieces together.

As far as I understand, MP3 files consist of frames, files can be split along frames and published in isolation. Moreover, there is no information on length needed in advance, so the format is suitable for streaming. However, when I use libmp3lame to generate MP3 files from partial data, the product cannot be interpreted by audio players after concatted together. I deactivated the bit reservoir, thus, I expect the frames to be independent.

Based on this article, I wrote a Python script that extracts and lists frames from MP3 files. I generated an MP3 file with libmp3lame by first collecting the whole PCM data and then applying libmp3lame. Then, I took the first n frames from this file and put them into another file. But the result would be unplayable as well.

How is it possible to encode only chunks of an audio, which library is suitable for this and what is the minimum size of a chunk?

Answer 1

I examined the source code of lame and the file lame_main.c helped me to come to a solution. This file implements the lame command-line utility, which also can encode multiple wav files, so they can be appended to a single mp3 file without gaps.

My mistake was to initialize lame every single time I call my encode function, thus, initialize lame for each new segment. This causes short interruptions in the output mp3. Instead, initializing lame once and re-using it for subsequent calls already solved the problem. Additionally, I call lame_init_bitstream at the start of encode and use lame_set_nocap_currentindex and lame_set_nogap_total appropriately. Now, the output fragments can be combined seamlessly.