Background

Disclaimer

This specification was created by analyzing data contained in freely-available media files. No reverse-engineering or other illegal activity took place during collection of this information. Neither author nor any contributors guarantee that any bit of this information is correct.

Data types

UINT8, UINT16, UINT32, UINT64 - unsigned integer values, 8, 16, 32 or 64-bit long. In GNU C compiler they are represented by types 'unsigned char', 'unsigned short', 'unsigned long' and 'unsigned long long'.
FILETIME - unsigned 64-bit integer. Number of 100-nanosecond intervals since midnignt, January 1, 1601, GMT.
GUID - 128-bit value, that can be generated on any system using special algorithm. The algorithm guarantees uniqueness of any such value ( it means that two different computers or even the same computer in different moments of time cannot generate the same GUIDs ).
BITMAPINFOHEADER - universal structure that describes format of a ( compressed ) image.

typedef struct
{
    long 	biSize; // sizeof(BITMAPINFOHEADER)
    long  	biWidth;
    long  	biHeight;
    short 	biPlanes; // unused
    short 	biBitCount;
    long 	biCompression; // fourcc of image
    long 	biSizeImage;   // size of image. For uncompressed images
			       // ( biCompression 0 or 3 ) can be zero.
			       
			      
    long  	biXPelsPerMeter; // unused
    long  	biYPelsPerMeter; // unused
    long 	biClrUsed;     // valid only for palettized images.
			       // Number of colors in palette.
    long 	biClrImportant;
} BITMAPINFOHEADER;

typedef struct
{
  short   wFormatTag; // value that identifies compression format
  short   nChannels;
  long  nSamplesPerSec;
  long  nAvgBytesPerSec;
  short   nBlockAlign; // size of a data sample
  short   wBitsPerSample;
  short   cbSize;    // size of format-specific data
} WAVEFORMATEX;
This structure is immediately followed with an array of bytes of size cbSize.

Basic information

   ASF 1.0 file consists of 'chunks'. They are similar to chunks from AVI format, but size of their fields was increased.
Chunk:

Field	Type	Size (bytes)
Chunk type	GUID	16
Chunk length	UINT64	8
Data	-	Variable

Chunk type describes type of content in the chunk. See below for list of known chunk type GUIDs.
Chunk length corresponds to the entire chunk ( i.e. length of data only is chunk length minus 24 ).
The other important concept is 'packet'. Since the format is supposed to be streamable, all actual data, such as compressed audio or video, is stored in 'packets'. Unlike in ASF 2.0, all packets have fixed size.
Each valid file should contain at least two chunks. They are File Header Chunk and Data Chunk. File Header Chunk contains all the information required to start processing actual data, while Data Chunk contains data packets.

Headers

Data chunk

Packets

Audio error concealment

Data sent to decoder: [0] [4] [8] [1] [5] [9] [2] [6] [10] [3] [7] [11]
width=4
height=3

[0] [1] [2] [3]
[4] [5] [6] [7]
[8] [9] [10][11]

Data stored in the stream: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Streaming over the Internet

Streaming using HTTP protocol

Known GUIDs

struct GUID 
{
    long v1;
    short v2;
    short v3;
    unsigned char v4[8];
    int operator==(const GUID& guid) const{return !memcmp(this, &guid, sizeof(GUID));}
};

/* GUID indicating audio stream header */
const GUID guid_audio_stream=
	{ 0xF8699E40, 0x5B4D, 0x11CF, 0xA8, 0xFD, 0x00, 0x80, 0x5F, 0x5C, 0x44, 0x2B };

/* GUID indicating video stream header */
const GUID guid_video_stream=
	{ 0xBC19EFC0, 0x5B4D, 0x11CF, 0xA8, 0xFD, 0x00, 0x80, 0x5F, 0x5C, 0x44, 0x2B };

/* GUID indicating that audio error concealment is absent */
const GUID guid_audio_conceal_none=
	{ 0x49f1a440, 0x4ece, 0x11d0, 0xa3, 0xac, 0x00, 0xa0, 0xc9, 0x03, 0x48, 0xf6 };

/* GUID indicating that interleaved audio error concealment is present */
const GUID guid_audio_conceal_interleave=
	{ 0xbfc3cd50, 0x618f, 0x11cf, 0x8b, 0xb2, 0x00, 0xaa, 0x00, 0xb4, 0xe2, 0x20 };

/* GUID for header chunk */
const GUID guid_header=
	{0x75B22630, 0x668E, 0x11CF, 0xA6, 0xD9, 0x00, 0xAA, 0x00, 0x62, 0xCE, 0x6C};

/* GUID for data chunk */
const GUID guid_data_chunk=
	{0x75b22636, 0x668e, 0x11cf, 0xa6, 0xd9, 0x00, 0xaa, 0x00, 0x62, 0xce, 0x6c};

/* GUID for index chunk */
const GUID guid_index_chunk=
	{0x33000890, 0xe5b1, 0x11cf, 0x89, 0xf4, 0x00, 0xa0, 0xc9, 0x03, 0x49, 0xcb};

/* GUID for stream header chunk */
const GUID guid_stream_header=
	{0xB7DC0791, 0xA9B7, 0x11CF, 0x8E, 0xE6, 0x00, 0xC0, 0x0C, 0x20, 0x53, 0x65};

/* ASF 2.0 header */
const GUID guid_header_2_0=
	{0xD6E229D1, 0x35da, 0x11d1, 0x90, 0x34, 0x00, 0xa0, 0xc9, 0x03, 0x49, 0xbe};

/* File header object */
const GUID guid_file_header=
	{0x8CABDCA1, 0xA947, 0x11CF, 0x8E, 0xE4, 0x00, 0xC0, 0x0C, 0x20, 0x53, 0x65};

Credits

Most of the information contained in this document was collected by Avery Lee <uleea05 at umail.ucsb.edu> and by unknown author of ASFRecorder program. Translated from C/C++ into readable English by yours, truly <divx at euro.ru>. Comments and improvements are welcome.

Last modified on April 5, 2001