{"id":6192,"date":"2024-03-27T15:34:01","date_gmt":"2024-03-27T14:34:01","guid":{"rendered":"https:\/\/www.pschatzmann.ch\/home\/?p=6192"},"modified":"2024-03-27T17:25:01","modified_gmt":"2024-03-27T16:25:01","slug":"libhelix-behind-the-scene","status":"publish","type":"post","link":"https:\/\/www.pschatzmann.ch\/home\/2024\/03\/27\/libhelix-behind-the-scene\/","title":{"rendered":"Libhelix – Behind the Scene"},"content":{"rendered":"

I am providing an Arduino MP3 and AAC decoder library for Arduino<\/a> which is based on libhelix<\/strong>.<\/p>\n

In this library I added a simple Arduino inspired C++ API<\/strong>. This was one of my first libraries and I found my implementation quite confusing. So it was time to do some refactoring<\/strong> to clean things up. After all, the C API should be quite easy to use and there must be some reason why I ended up with this complexity.<\/p>\n

I am taking the opportunity here to document the quirks<\/strong> of the C API of libhelix and how I managed around them. But let’s start with the API:<\/p>\n

Basic Libhelix C API<\/h2>\n
HMP3Decoder MP3InitDecoder(void);\nvoid MP3FreeDecoder(HMP3Decoder hMP3Decoder);\nint MP3Decode(HMP3Decoder hMP3Decoder, unsigned char **inbuf, int *bytesLeft, short *outbuf, int useSize);\nvoid MP3GetLastFrameInfo(HMP3Decoder hMP3Decoder, MP3FrameInfo *mp3FrameInfo);\nint MP3GetNextFrameInfo(HMP3Decoder hMP3Decoder, MP3FrameInfo *mp3FrameInfo, unsigned char *buf);\nint MP3FindSyncWord(unsigned char *buf, int nBytes);\n\n<\/code><\/pre>\n
So decoding is quite simple: you just MP3Decode() until it reports an underflow with -1 and then you continue to provide the next batch of data.<\/p>\n
I was testing this with a MP3 stream from the Internet and it was working like a charm. The AAC side looks the same:<\/p>\n
HAACDecoder AACInitDecoder(void);\nHAACDecoder AACInitDecoderPre(void *ptr, int sz);\nvoid AACFreeDecoder(HAACDecoder hAACDecoder);\nint AACDecode(HAACDecoder hAACDecoder, unsigned char **inbuf, int *bytesLeft, short *outbuf);\n\nint AACFindSyncWord(unsigned char *buf, int nBytes);\nvoid AACGetLastFrameInfo(HAACDecoder hAACDecoder, AACFrameInfo *aacFrameInfo);\nint AACSetRawBlockParams(HAACDecoder hAACDecoder, int copyLast, AACFrameInfo *aacFrameInfo);\nint AACFlushCodec(HAACDecoder hAACDecoder);\n<\/code><\/pre>\n
Except, this was not working as expected: Instead of getting a -1 the AACDecode() got stuck and never returned when it was running out of data.<\/p>\n
The solution was simple: Just make sure that the decoding buffer is keeping a minimum size<\/strong> and if this is used up, request for more data: I was initially using 400 bytes as limit, but later increased this.<\/p>\n
So internet music streaming<\/strong> was working now for MP3 and AAC. The next thing was to test with some files and here this simple approach falls down and needs some extension.<\/p>\n
Handling of Invalid Audio Data<\/h2>\n
Files might contain some ID3 Header<\/strong> or they might just be damaged and it turns out if this is fed to the decoder, it will not be able to generate any audio any more. So I extended the logic as follows:<\/p>\n