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Minimalistic MP3 decoder single header library

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minimp3

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Minimalistic, single-header library for decoding MP3. minimp3 is designed to be small, fast (with SSE and NEON support), and accurate (ISO conformant). You can find a rough benchmark below, measured using perf on an i7-6700K, IO included, no CPU heat to address speedstep:

Vector Hz Samples Sec Clockticks Clockticks per second PSNR Max diff
compl.bit 48000 248832 5.184 14306684 2.759M 124.22 1
he_32khz.bit 32000 172800 5.4 8426158 1.560M 139.67 1
he_44khz.bit 44100 472320 10.710 21296300 1.988M 144.04 1
he_48khz.bit 48000 172800 3.6 8453846 2.348M 139.67 1
hecommon.bit 44100 69120 1.567 3169715 2.022M 133.93 1
he_free.bit 44100 156672 3.552 5798418 1.632M 137.48 1
he_mode.bit 44100 262656 5.955 9882314 1.659M 118.00 1
si.bit 44100 135936 3.082 7170520 2.326M 120.30 1
si_block.bit 44100 73728 1.671 4233136 2.533M 125.18 1
si_huff.bit 44100 86400 1.959 4785322 2.442M 107.98 1
sin1k0db.bit 44100 725760 16.457 24842977 1.509M 111.03 1

Conformance test passed on all vectors (PSNR > 96db).

Comparison with keyj's minimp3

Comparison by features:

Keyj minimp3 Current
Fixed point Floating point
source: 84kb 70kb
binary: 34kb (20kb compressed) 30kb (20kb)
no vector opts SSE/NEON intrinsics
no free format free format support

Below, you can find the benchmark and conformance test for keyj's minimp3:

Vector Hz Samples Sec Clockticks Clockticks per second PSNR Max diff
compl.bit 48000 248832 5.184 31849373 6.143M 71.50 41
he_32khz.bit 32000 172800 5.4 26302319 4.870M 71.63 24
he_44khz.bit 44100 472320 10.710 41628861 3.886M 71.63 24
he_48khz.bit 48000 172800 3.6 25899527 7.194M 71.63 24
hecommon.bit 44100 69120 1.567 20437779 13.039M 71.58 25
he_free.bit 44100 0 0 - - - -
he_mode.bit 44100 262656 5.955 30988984 5.203M 71.78 27
si.bit 44100 135936 3.082 24096223 7.817M 72.35 36
si_block.bit 44100 73728 1.671 20722017 12.394M 71.84 26
si_huff.bit 44100 86400 1.959 21121376 10.780M 27.80 65535
sin1k0db.bit 44100 730368 16.561 55569636 3.355M 0.15 58814

Keyj minimp3 conformance test fails on all vectors (PSNR < 96db), and free format is unsupported. This caused some problems when it was used here, and was the main motivation for this work.

Usage

First, we need to initialize the decoder structure:

//#define MINIMP3_ONLY_MP3
//#define MINIMP3_ONLY_SIMD
//#define MINIMP3_NO_SIMD
//#define MINIMP3_NONSTANDARD_BUT_LOGICAL
//#define MINIMP3_FLOAT_OUTPUT
#define MINIMP3_IMPLEMENTATION
#include "minimp3.h"
...
    static mp3dec_t mp3d;
    mp3dec_init(&mp3d);

Note that you must define MINIMP3_IMPLEMENTATION in exactly one source file. You can #include minimp3.h in as many files as you like. Also you can use MINIMP3_ONLY_MP3 define to strip MP1/MP2 decoding code. MINIMP3_ONLY_SIMD define controls generic (non SSE/NEON) code generation (always enabled on x64/arm64 targets). In case you do not want any platform-specific SIMD optimizations, you can define MINIMP3_NO_SIMD. MINIMP3_NONSTANDARD_BUT_LOGICAL define saves some code bytes, and enforces non-standard but logical behaviour of mono-stereo transition (rare case). MINIMP3_FLOAT_OUTPUT makes mp3dec_decode_frame() output to be float instead of short and additional function mp3dec_f32_to_s16 will be available for float->short conversion if needed.

Then. we decode the input stream frame-by-frame:

    /*typedef struct
    {
        int frame_bytes;
        int channels;
        int hz;
        int layer;
        int bitrate_kbps;
    } mp3dec_frame_info_t;*/
    mp3dec_frame_info_t info;
    short pcm[MINIMP3_MAX_SAMPLES_PER_FRAME];
    /*unsigned char *input_buf; - input byte stream*/
    samples = mp3dec_decode_frame(&mp3d, input_buf, buf_size, pcm, &info);

The mp3dec_decode_frame() function decodes one full MP3 frame from the input buffer, which must be large enough to hold one full frame.

The decoder will analyze the input buffer to properly sync with the MP3 stream, and will skip ID3 data, as well as any data which is not valid. Short buffers may cause false sync and can produce 'squealing' artefacts. The bigger the size of the input buffer, the more reliable the sync procedure. We recommend having as many as 10 consecutive MP3 frames (~16KB) in the input buffer at a time.

At end of stream just pass rest of the buffer, sync procedure should work even with just 1 frame in stream (except for free format and garbage at the end can mess things up, so id3v1 and ape tags must be removed first).

For free format there minimum 3 frames needed to do proper sync: 2 frames to detect frame length and 1 next frame to check detect is good.

The size of the consumed MP3 data is returned in the mp3dec_frame_info_t field of the frame_bytes struct; you must remove the data corresponding to the frame_bytes field from the input buffer before the next decoder invocation.

The decoding function returns the number of decoded samples. The following cases are possible:

  • 0: No MP3 data was found in the input buffer
  • 384: Layer 1
  • 576: MPEG 2 Layer 3
  • 1152: Otherwise

The following is a description of the possible combinations of the number of samples and frame_bytes field values:

  • More than 0 samples and frame_bytes > 0: Succesful decode
  • 0 samples and frame_bytes > 0: The decoder skipped ID3 or invalid data
  • 0 samples and frame_bytes == 0: Insufficient data

If frame_bytes == 0, the other fields may be uninitialized or unchanged; if frame_bytes != 0, the other fields are available. The application may call mp3dec_init() when changing decode position, but this is not necessary.

As a special case, the decoder supports already split MP3 streams (for example, after doing an MP4 demux). In this case, the input buffer must contain exactly one non-free-format frame.

Seeking

You can seek to any byte in the stream and call mp3dec_decode_frame; this will work in almost all cases, but is not completely guaranteed. Probablility of sync procedure failure lowers when MAX_FRAME_SYNC_MATCHES value grows. Default MAX_FRAME_SYNC_MATCHES=10 and probablility of sync failure should be very low. If granule data is accidentally detected as a valid MP3 header, short audio artefacting is possible.

High-level mp3dec_ex_seek function supports precise seek to sample (MP3D_SEEK_TO_SAMPLE) using index and binary search.

Track length detect

If the file is known to be cbr, then all frames have equal size and lack ID3 tags, which allows us to decode the first frame and calculate all frame positions as frame_bytes * N. However, because of padding, frames can differ in size even in this case.

In general case whole stream scan is needed to calculate it's length. Scan can be omitted if vbr tag is present (added by encoders like lame and ffmpeg), which contains length info. High-level functions automatically use the vbr tag if present.

High-level API

If you need only decode file/buffer or use precise seek, you can use optional high-level API. Just #include minimp3_ex.h instead and use following additional functions:

#define MP3D_SEEK_TO_BYTE   0
#define MP3D_SEEK_TO_SAMPLE 1

#define MINIMP3_PREDECODE_FRAMES 2 /* frames to pre-decode and skip after seek (to fill internal structures) */
/*#define MINIMP3_SEEK_IDX_LINEAR_SEARCH*/ /* define to use linear index search instead of binary search on seek */
#define MINIMP3_IO_SIZE (128*1024) /* io buffer size for streaming functions, must be greater than MINIMP3_BUF_SIZE */
#define MINIMP3_BUF_SIZE (16*1024) /* buffer which can hold minimum 10 consecutive mp3 frames (~16KB) worst case */
#define MINIMP3_ENABLE_RING 0      /* enable hardware magic ring buffer if available, to make less input buffer memmove(s) in callback IO mode */

#define MP3D_E_MEMORY  -1
#define MP3D_E_IOERROR -2

typedef struct
{
    mp3d_sample_t *buffer;
    size_t samples; /* channels included, byte size = samples*sizeof(mp3d_sample_t) */
    int channels, hz, layer, avg_bitrate_kbps;
} mp3dec_file_info_t;

typedef size_t (*MP3D_READ_CB)(void *buf, size_t size, void *user_data);
typedef int (*MP3D_SEEK_CB)(uint64_t position, void *user_data);

typedef struct
{
    MP3D_READ_CB read;
    void *read_data;
    MP3D_SEEK_CB seek;
    void *seek_data;
} mp3dec_io_t;

typedef struct
{
    uint64_t samples;
    mp3dec_frame_info_t info;
    int last_error;
    ...
} mp3dec_ex_t;

typedef int (*MP3D_ITERATE_CB)(void *user_data, const uint8_t *frame, int frame_size, int free_format_bytes, size_t buf_size, uint64_t offset, mp3dec_frame_info_t *info);
typedef int (*MP3D_PROGRESS_CB)(void *user_data, size_t file_size, uint64_t offset, mp3dec_frame_info_t *info);

/* decode whole buffer block */
int mp3dec_load_buf(mp3dec_t *dec, const uint8_t *buf, size_t buf_size, mp3dec_file_info_t *info, MP3D_PROGRESS_CB progress_cb, void *user_data);
int mp3dec_load_cb(mp3dec_t *dec, mp3dec_io_t *io, uint8_t *buf, size_t buf_size, mp3dec_file_info_t *info, MP3D_PROGRESS_CB progress_cb, void *user_data);
/* iterate through frames */
int mp3dec_iterate_buf(const uint8_t *buf, size_t buf_size, MP3D_ITERATE_CB callback, void *user_data);
int mp3dec_iterate_cb(mp3dec_io_t *io, uint8_t *buf, size_t buf_size, MP3D_ITERATE_CB callback, void *user_data);
/* streaming decoder with seeking capability */
int mp3dec_ex_open_buf(mp3dec_ex_t *dec, const uint8_t *buf, size_t buf_size, int seek_method);
int mp3dec_ex_open_cb(mp3dec_ex_t *dec, mp3dec_io_t *io, int seek_method);
void mp3dec_ex_close(mp3dec_ex_t *dec);
int mp3dec_ex_seek(mp3dec_ex_t *dec, uint64_t position);
size_t mp3dec_ex_read(mp3dec_ex_t *dec, mp3d_sample_t *buf, size_t samples);
#ifndef MINIMP3_NO_STDIO
/* stdio versions of file load, iterate and stream */
int mp3dec_load(mp3dec_t *dec, const char *file_name, mp3dec_file_info_t *info, MP3D_PROGRESS_CB progress_cb, void *user_data);
int mp3dec_iterate(const char *file_name, MP3D_ITERATE_CB callback, void *user_data);
int mp3dec_ex_open(mp3dec_ex_t *dec, const char *file_name, int seek_method);
#ifdef _WIN32
int mp3dec_load_w(mp3dec_t *dec, const wchar_t *file_name, mp3dec_file_info_t *info, MP3D_PROGRESS_CB progress_cb, void *user_data);
int mp3dec_iterate_w(const wchar_t *file_name, MP3D_ITERATE_CB callback, void *user_data);
int mp3dec_ex_open_w(mp3dec_ex_t *dec, const wchar_t *file_name, int seek_method);
#endif
#endif

Use MINIMP3_NO_STDIO define to exclude STDIO functions. MINIMP3_ALLOW_MONO_STEREO_TRANSITION allows mixing mono and stereo in same file. In that case mp3dec_frame_info_t->channels = 0 is reported on such files and correct channels number passed to progress_cb callback for each frame in mp3dec_frame_info_t structure. MP3D_PROGRESS_CB is optional and can be NULL, example of file decoding:

    mp3dec_t mp3d;
    mp3dec_file_info_t info;
    if (mp3dec_load(&mp3d, input_file_name, &info, NULL, NULL))
    {
        /* error */
    }
    /* mp3dec_file_info_t contains decoded samples and info,
       use free(info.buffer) to deallocate samples */

Example of file decoding with seek capability:

    mp3dec_ex_t dec;
    if (mp3dec_ex_open(&dec, input_file_name, MP3D_SEEK_TO_SAMPLE))
    {
        /* error */
    }
    /* dec.samples, dec.info.hz, dec.info.layer, dec.info.channels should be filled */
    if (mp3dec_ex_seek(&dec, position))
    {
        /* error */
    }
    mp3d_sample_t *buffer = malloc(dec.samples*sizeof(mp3d_sample_t));
    size_t readed = mp3dec_ex_read(&dec, buffer, dec.samples);
    if (readed != dec.samples) /* normal eof or error condition */
    {
        if (dec.last_error)
        {
            /* error */
        }
    }

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