簡介

MP4只是一種container，可以想像成容器，裡面可以放mp3或aac等音樂。
關於MP4格式的spec可以在這裡下載。

Library

關於MP4檔案的操作，已經有現成的library可以使用囉～
https://code.google.com/p/mp4v2/

觀察MP4格式的工具

可以藉由atomicparsley這個工具來印出MP4檔案的內部結構

brew install atomicparsley
atomicparsley /path/to/mp4 -T 1

印出來的結果像這樣

Atom ftyp @ 0 of size: 24, ends @ 24
Atom moov @ 24 of size: 569, ends @ 593
     Atom mvhd @ 32 of size: 108, ends @ 140
     Atom iods @ 140 of size: 24, ends @ 164
     Atom trak @ 164 of size: 429, ends @ 593
         Atom tkhd @ 172 of size: 92, ends @ 264
         Atom mdia @ 264 of size: 329, ends @ 593
             Atom mdhd @ 272 of size: 32, ends @ 304
             Atom hdlr @ 304 of size: 33, ends @ 337
             Atom minf @ 337 of size: 256, ends @ 593
                 Atom smhd @ 345 of size: 16, ends @ 361
                 Atom dinf @ 361 of size: 36, ends @ 397
                     Atom dref @ 369 of size: 28, ends @ 397
                         Atom url  @ 385 of size: 12, ends @ 397
                 Atom stbl @ 397 of size: 196, ends @ 593
                     Atom stsd @ 405 of size: 96, ends @ 501
                         Atom mp4a @ 421 of size: 80, ends @ 501
                             Atom esds @ 457 of size: 44, ends @ 501
                     Atom stts @ 501 of size: 24, ends @ 525
                     Atom stsz @ 525 of size: 20, ends @ 545
                     Atom stsc @ 545 of size: 28, ends @ 573
                     Atom stco @ 573 of size: 20, ends @ 593
Atom mdat @ 593 of size: 9, ends @ 602
------------------------------------------------------
Total size: 602 bytes; 22 atoms total.
Media data: 0 bytes; 602 bytes all other atoms (100.000% atom overhead).
Total free atom space: 0 bytes; 0.000% waste.
------------------------------------------------------
AtomicParsley version: 0.9.6 (utf8)
------------------------------------------------------
Movie duration: 7504.597 seconds (02:05:04.60) - 0.00* kbp/sec bitrate (*=approximate)
Low-level details. Total tracks: 1
Trk  Type  Handler                    Kind  Lang  Bytes
1    soun  [none listed]              mp4a  und   1
     0.00* kbp/s  7504.597 sec  MPEG-4 Unknown profile: 0x0    channels: (2)