Introduction

go-mpg123 is a library that provides bindings to libmpg123.

Not all library functions are present, but there are enough bindings to decode an MP3 file using mpg123_open and mpg123_read. However, decoding from a file reader and feeding data directly to the decoder are not yet supported. Meta-data reading are also not yet supported.

This library is still very much a work in progress. Currently this library is a test bed for all the forks merged together

Usage

Decoding a file

The mpg123 library is accessed via a Decoder struct containing a C pointer to an instance of the library. To decode a file, first create an instance of the decoder, then tell it to open the file.

decoder, err := mpg123.NewDecoder("")
err = decoder.Open("test.mp3")

At this point you should have the decoder peek into the file and find the format it is encoded in. You may also want to lock this format in as it may change later if you do not do so.

rate, channels, encoding := decoder.GetFormat()
// clear list of formats and only allow the current settings
decoder.FormatNone()
decoder.Format(rate, channels, encoding)

Now you are ready to start decoding the file. Simply create a buffer and read data into it. Note that there may still be data in the buffer when EOF is returned, so check for errors after processing the buffer.

buf := make([]byte, 1024*16)
for {
	len, err := decoder.Read(buf)
	// do something with the PCM data
	if err != nil {
		break
	}
}

Decoding a Reader

Useful when working with custom audio-protocols!

decoder.OpenFeed() // You must call this manually first
// Get a DecoderReader for an output format
outputReader := decoder.DecoderReader(inputReader, 44100, 1, mpg123.ENC_SIGNED_16)

buf := make([]byte, 16*1024)
for {
	if n, err := outputReader.Read(buf); err != io.EOF {
		// Do your thing with buf[0:n]
	} else {
		break
	}
}
// outputReader will Close and Delete itself automatically when data is over 😇

Seek stream to sample from current position

// move forward for 11 sec
// multiply seconds to sample rate (44100 or 48000)
off := int64(11 * 44100)
whence := os.SEEK_CUR
pos, err := decoder.Seek(off, whence)

Examples

An example program is included in examples/mp3dump. This program decodes an MP3 file and writes the raw PCM data to a file.

go get github.com/SiloCityLabs/go-mpg123/examples/mp3dump
mp3dump <file.mp3> <outfile.raw>

This raw audio file may be played using mplayer:

mplayer -demuxer rawaudio -rawaudio rate=<samplerate>:channels=<channels> out.raw

or 

ffplay -ar 44100 -ac 2 -f s16le test_1.raw