Electronic Arts AS4 / ASF Music

Description
AS4 / ASF files are an audio file format used in many video games created by Electronic Arts. Note that extensions other than those listed may be AS4 / ASF files as Electronic Arts often change file extensions for different games.

Information
The basic structure of AS4 / ASF files is similar to that of RIFF, with each file divided into blocks (although, unlike RIFF, the format does not have a global file header).

Each block starts with a standard header:

struct ASFBlockHeader { char szBlockID[4]; DWORD dwSize; };

Where:


 * szBlockID gives the string ID for the block
 * dwSize gives the size of the block (including the header) in bytes.

Each file then consists of a number of blocks, each of which are described below.

Header Block
Block ID:1SNh

This is the first block in an ASF / AS4 file, describing the audio stream.

struct EACSHeader { char szID[4]; DWORD dwSampleRate; BYTE bBits; BYTE bChannels; BYTE bCompression; BYTE bType; DWORD dwNumSamples; DWORD dwLoopStart; DWORD dwLoopLength; DWORD dwDataStart; DWORD dwUnknown; };


 * szID - the ID string, should always be "EACS"
 * dwSampleRate - the sample rate for the file
 * bBits - the resolution of the decompressed sound data, divided by 8 (i.e. 1 = 8-bit, 2 = 16-bit)
 * bChannels - the number of channels. There are only two options: 1 for mono, 2 for stereo
 * bCompression - if this is equal to 0x00, the audio data consists of signed 8 or 16-bit PCM. If it is equal to 0x02, the audio data consists of compressed IMA ADPCM
 * bType - the type of file (this should always be 0x00 for AS4 / ASF files)
 * dwNumSamples - the number of samples in the file
 * dwLoopStart - where the beginning of the repeat loop should start (in terms of samples). If this is 0xFFFFFFFF, there is no loop
 * dwLoopLength - the length of the repeat loop (again in samples). If this is 0, there is no loop
 * dwDataStart - this is not used for AS4 / ASF files
 * dwUnknown - as the name suggests, the function of this would appear to be unknown

Sound Data
Block ID: 1SNd

The next block following the header is the sound data block. Confusingly, the first chunk of sound data will be contained in the 1SNh block.

struct ASFChunkHeader { DWORD dwOutSize; LONG lIndexLeft; LONG lIndexRight; LONG lCurSampleLeft; LONG lCurSampleRight; };


 * dwOutSize - the size of the uncompressed (8 or 16-bit PCM) data in the chunk (in terms of samples).
 * lIndexLeft, lIndexRight, lCurSampleLeft, lCurSampleRight - these are the initial values for the IMA ADPCM decompression routine.

As mentioned above, if no compression is used these blocks consist simply of signed 8 or 16-bit PCM audio. Also note that for mono files, there is no lIndexRight or lCurSampleRight value.

Loop Block
Block ID: 1SNl

This consists of a single value, DWORD value, which defines the looping jump position relative to the start of the song. This block may be followed by a number of further 1SNd blocks.

End Block
Block ID: 1SNe

An 8-byte block containing no data that defines the end of the audio stream. If there is a loop, the jump should be made from here. Note that some files contain further audio data beyond this.

Software

 * http://bim.km.ru/gap/

Links

 * http://www.extractor.ru/articles/electronic_arts_audio_file_formats_description