How are files (especially audio files) organized internally?

Question

I try to grok that: Apple is talking about "packets" in audio files, and there is a fancy function called AudioFileReadPackets which takes a lot of arguments. One of them specifies the "start packet", and another one the number of packets which you want to read.

So I imagine an audio file to look like this, internally: It's made up of a lot of packets. If it's an audio file which has an variable bit rate format, then every packet may have a different size. If the file has an constant bit rate format, then every packet is the same size. So an audio file is like a truck full of boxes, and every box contains some interesting stuff.

Is that correct? Does it apply to any kind of file? Is this how files actually look like?

Answer 1

That's pretty much what audio files look like: a series of chunks of data, or frames. AudioFileReadPacketData and AudioFileReadPackets shield you from the details of, for instance, how big a frame might be in bytes (because you might be reading from a WAV file, which has a different structure to an MP3 file, or your MP3 file uses a variable bit rate).

The concept of frames doesn't apply in general to any file, but then you wouldn't be using the Audio File Services API to access just any old file.

Answer 2

For MP3 (and MP1, MP2) the file consists of frames. And yes, your understanding is correct - in VBR files packets have different size. In WAV files packets have the same length if memory serves (I wrote a decoder / player 11 years ago,).

Answer 3

"Packet" looks to me like Apple-specific terminology. I just did a lot of reading and coding to process WAV and MP3 files and I don't believe I saw the term "packet" once.

Answer 4

The question (even with the "especially audio files" qualification) is far too broad; different file formats are, well, different!

So to answer the question you will first have to specify a particular file type; then the answer to the question will invariably to look at its specification. Proprietary formats may not have a publicly available specification.

Specifications for many files (official and reverse engineered) can be found at the brilliant Wotsit's Format site.

AAC used by Apple iTunes and others is defined by ISO/IEC 13818-7:2006. The document will cost you 252 Swiss Francs (about US$233)! You'd have to be really interested (commercially) to pay that rather than use an existing AAC Codec.

"Packet" is a term commonly used in data transmission, so may be more applicable to audio streaming than audio files, where a "frame" may be more appropriate, or for data files in general a "record", but the terminology is flexible because it means whatever the person that wrote it thought it meant! If enough people misuse a term, it essentially becomes redefined (or multiply defined) to mean that, so I would not get too hung up on that. The author was do doubt using it to define a unit that has a defined format within a file that has multiple such units repeated sequentially.

Answer 5

Files contain whatever the application that created them chose to place in them. Files are essentially a sequence of bytes. Any further organisation is a semantic distinction made by the program that created them. It is untrue to think of all files containing the same structure.

That said, certain data storage problems are similar enough to be solved in similar ways, and patterns start to emerge. Splitting data into records or packets is an example of that.