Python library to split and join mp3 files

I wrote a library (pydub) for pretty much this exact use case:

from pydub import AudioSegmentsound = AudioSegment.from_mp3("/path/to/file.mp3")# len() and slicing are in millisecondshalfway_point = len(sound) / 2second_half = sound[halfway_point:]# Concatenation is just addingsecond_half_3_times = second_half + second_half + second_half# writing mp3 files is a one linersecond_half_3_times.export("/path/to/new/file.mp3", format="mp3")

Adding a silent gap

If you'd like to add silence between parts of a sound:

two_sec_silence = AudioSegment.silent(duration=2000)sound_with_gap = sound[:1000] + two_sec_silence + sound[1000:]

Have a look at the MP3 file structure on Wikipedia. Use binary read mode in python to edit the MP3 file. s = open(file_name, 'rb').read() will put the whole file into a string object representing the raw bytes in your file (e.g. \xeb\xfe\x80). You can then search and edit the string, addressing the byte offsets with indeces using brackets: s[n]. Finally, just do a binary write of the MP3 frames you want in your new file(s), appending the ID3 header to the set of frames that you want to make up each file.

Here is my attempt at splitting an MP3 using python without re-encoding. Not all varieties of MP3 files are supported and I would gladly welcome suggestions or improvements. The script is hard coded to split at 55 seconds but the code demonstrates the general principles.

from __future__ import print_functionimport structimport sys#MP3 frames are not independent because of the byte reservoir. This script does not account for#that in determining where to do the split.def SplitMp3(fi, splitSec, out):    #Constants for MP3    bitrates = {0x0: "free", 0x1: 32, 0x2: 40, 0x3: 48, 0x4: 56, 0x5: 64, 0x6: 80, 0x7: 96, 0x8: 112,        0x9: 128, 0xa: 160, 0xb: 192, 0xc: 224, 0xd: 256, 0xe: 320, 0xf: "bad"}    freqrates = {0x0: 44100, 0x1: 48000, 0x2: 32000, 0x3: "reserved"}    countMpegFrames = 0    frameDuration = 0.026    unrecognizedBytes = 0    splitFrame = int(round(splitSec / frameDuration))    while True:        startPos = fi.tell()        #Check for 3 byte headers        id3Start = fi.read(3)        if len(id3Start) == 3:            if id3Start == b'TAG':                print ("Found ID3 v1/1.1 header")                fi.seek(startPos + 256)                continue            if id3Start == b'ID3':                #Possibly a ID3v2 header                majorVer, minorVer, flags, encSize = struct.unpack(">BBBI", fi.read(7))                if majorVer != 0xFF and minorVer != 0xFF:                    encSize1 = (encSize & 0x7f000000) >> 24                    encSize2 = (encSize & 0x7f0000) >> 16                    encSize3 = (encSize & 0x7f00) >> 8                    encSize4 = (encSize & 0x7f)                    if encSize1 < 0x80 and encSize2 < 0x80 and encSize3 < 0x80 and encSize4 < 0x80:                        size = ((encSize & 0x7f000000) >> 3) + ((encSize & 0x7f0000) >> 2) + ((encSize & 0x7f00) >> 1) + (encSize & 0x7f)                        unsync = (flags >> 7) & 0x1                        extendedHeader = (flags >> 6) & 0x1                        experimental = (flags >> 5) & 0x1                        print ("Found ID3v2 header")                        print ("version", majorVer, minorVer, unsync, extendedHeader, experimental)                        print ("size", size)                        #TODO extendedHeader not supported yet                        fi.seek(startPos + 10 + size)                        continue        #Check for 4 byte headers        fi.seek(startPos)        headerRaw = fi.read(4)        if len(headerRaw) == 4:            headerWord = struct.unpack(">I", headerRaw)[0]            #Check for MPEG-1 audio frame            if headerWord & 0xfff00000 == 0xfff00000:                print ("Possible MPEG-1 audio header", hex(headerWord))                countMpegFrames += 1                ver = (headerWord & 0xf0000) >> 16                bitrateEnc = (headerWord & 0xf000) >> 12                freqEnc = (headerWord & 0xf00) >> 8                mode = (headerWord & 0xf0) >> 4                cpy = (headerWord & 0xf)                if ver & 0xe == 0xa and freqEnc != 0xf:                    print ("Probably an MP3 frame")                    bitrate = bitrates[bitrateEnc]                    freq = freqrates[freqEnc >> 2]                    padding = ((freqEnc >> 1) & 0x1) == 1                    print ("bitrate", bitrate, "kbps")                    print ("freq", freq, "Hz")                    print ("padding", padding)                    frameLen = int((144 * bitrate * 1000 / freq ) + padding)                    #Copy frame to output                    fi.seek(startPos)                    frameData = fi.read(frameLen)                    if countMpegFrames >= splitFrame:                        out.write(frameData)                    fi.seek(startPos + frameLen)                    continue                else:                    raise RuntimeError("Unsupported format:", hex(ver), "header:", hex(headerWord))        #If no header can be detected, move on to the next byte        fi.seek(startPos)        nextByteRaw = fi.read(1)        if len(nextByteRaw) == 0:            break #End of file        unrecognizedBytes += 1    print ("unrecognizedBytes", unrecognizedBytes)    print ("countMpegFrames", countMpegFrames)    print ("duration", countMpegFrames * frameDuration, "sec")if __name__=="__main__":    fi = open(sys.argv[1], "rb")    out = open("test.mp3", "wb")    SplitMp3(fi, 55.0, out)    out.close()

Merging would be a similar case of extracting and appending frames from two separate MP3 input files.