Electronic Arts AS4 / ASF Music
Contents |
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.
