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THE AUD FILE FORMAT

Revision 3

by Vladan Bato ([email protected])


In this document I'll try to describe the AUD file format used in
Command & Conquer and Redalert.

Command & Conquer is a trademark of Westwood Studios, Inc.
Command & Conquer is Copyright (C)1995 Westwood Studios, Inc.
Command & Conquer: Red Alert is a trademark of Westwood Studios, Inc.
Command & Conquer: Red Alert is Copyright (C)1995,1996 Westwood Studios, Inc.

The information provided here is for anyone who would like to make an
AUD player program or AUD to WAV or WAV to AUD converters.

Most information about AUD files and IMA-ADPCM compression has been
provided by Douglas McFadyen.

I won't explain here the format of the WAV files. You'll have to find this
info yourself. I'll just tell you how to obtain 16-bit PCM data and how
to encode it.

I will use Pascal-like notation throughout this document.

===============================
0. IMPRTANT NOTE - WHAT'S NEW
===============================

This revision contains an important difference in the IMA-ADPCM compression
routine. Instead of computing the diffrence between the current and
previous sample, it computes the difference between the current sample
and the value that the decoding routine will predict for the previous
sample.
This is the way the algorithm is implemented in C&C.
If you implement it the way it was in previous revisions of this document,
the sound will be the same but there will be a "pop" sound at the end.


==============
1. AUD FILES
==============

The AUD files have the following header :

Header : record
SamplesPerSec : word; {Frequency}
Size : longint; {Size of file (without header)}
OutSize : longint; {Size of ouput data}
Flags : byte; {bit 0=stereo, bit 1=16bit}
Typ : byte; {1=WW compressed, 99=IMA ADPCM}
end;

There are two types of compression. The first is the IMA-ADPCM compression
used for 16-bit sound. It's used in most AUD files.

The other one is a Westwood's proprietary compression for 8-bit sound and
is used only for death screams. I won't describe it in this document
because I don't know how it works.

The rest of the AUD files is divided in chunks. These are usually 512
bytes long, except for the last one.

Each chunk has the following header :

ChunkHd : record
Size : word; {Size of compressed data}
OutSize : word; {Size of ouput data}
ID : longint; {Always $0000DEAF}
end;

The IMA-ADPCM compression compresses 16-bit samples to 4 bits. This means
that OutSize will be apporximately 4*Size.

The IMA-ADPCM compression and decompression are described in the following
sections.

Note that the current sample value and index into the Step Table should
be initialized to 0 at the start and are mantained across the chunks
(see below).

==========================
2. IMA-ADPCM COMPRESSION
==========================

I won't describe the theory behind the IMA-ADPCM compression. I will just
give some pseudo code to compress and decompress data.

The compression algorithm takes a stream of signed 16-bit samples in input
and produces a stream of 4-bit codes in output.
The 4-bit codes are stored in pairs (two codes in one byte). The first one
is stored in the lower four bits.

Two varaibles must be mantained while compressing : the previous sample
value and the current index into the step table.

You can find the Step Table in Appendix B.
The Index adjustment table is in Appendix A.

Here's the pseudo-code that will do the compression :

Index:=0;
Prev_Sample:=0;

while there_is_more_data do
begin
Cur_Sample:=Get_Next_Sample;
Delta:=Cur_Sample-Prev_Sample;
if Delta<0 then
begin
Delta:=-Delta;
Sb:=1;
end else Sb:=0;
{Sb is bit 4 of the output Code (sign bit)}

Code := 4*Delta div Step_Table[Index];
if Code>7 then Code:=7;
{These are the 3 low-order bits of output code}

Index:=Index+Index_Adjust[Code];
if Index<0 then Index:=0;
if Index>88 the Index:=88;

Predicted_Delta:=(Step_Table[Index]*Code) div 4 +
Step_Table[Index] div 8;

{This is the Delta that decoding routine will compute}

Prev_Sample:=Prev_Sample+Predicted_Delta;
if Prev_Sample>32767 then Prev_Sample:=32767
else if Prev_Sample<-32768 then Prev_Sample:=-32768;

{Prev_Sample is the sample value that the decoding routine
will compute}


Output_Code(Code+Sb*8);
end;

Note that this code is usually implemented in more efficient manner
(No need to divide).

The Get_Next_Sample function should return the next sample from the input
buffer.
The Output_Code function should store the 4-bit code to the output buffer.
One byte contains two 4-bit codes, and this function should take care of
this.

============================
3. IMA-ADPCM DECOMPRESSION
============================

It is the exact opposite of the above. It receives 4-bit codes in input
and produce 16-bit samples in output.

Again you have to mantain an Index into the Step Table an the current
sample value.

The tables used are the same as for compression.

Here's the code :

Index:=0;
Cur_Sample:=0;

while there_is_more_data do
begin
Code:=Get_Next_Code;

if (Code and $8) <> 0 then Sb:=1 else Sb:=0;
Code:=Code and $7;
{Separate the sign bit from the rest}

Delta:=(Step_Table[Index]*Code) div 4 + Step_Table[Index] div 8;
{The last one is to minimize errors}

if Sb=1 then Delta:=-Delta;

Cur_Sample:=Cur_Sample+Delta;
if Cur_Sample>32767 then Cur_Sample:=32767
else if Cur_Sample<-32768 then Cur_Sample:=-32768;

Output_Sample(Cur_Sample);

Index:=Index+Index_Adjust[Code];
if Index<0 then Index:=0;
if Index>88 the Index:=88;
end;

Again, this can be done more efficiently (no need for multiplication).

The Get_Next_Code function should return the next 4-bit code. It must
extract it from the input buffer (note that two 4-bit codes are stored
in the same byte, the first one in the lower bits).

The Output_Sample function should write the signed 16-bit sample to the
output buffer.

=========================================
Appendix A : THE INDEX ADJUSTMENT TABLE
=========================================

Index_Adjust : array [0..7] of integer = (-1,-1,-1,-1,2,4,6,8);

=============================
Appendix B : THE STEP TABLE
=============================

Steps_Table : array [0..88] of integer =(
7, 8, 9, 10, 11, 12, 13, 14, 16,
17, 19, 21, 23, 25, 28, 31, 34, 37,
41, 45, 50, 55, 60, 66, 73, 80, 88,
97, 107, 118, 130, 143, 157, 173, 190, 209,
230, 253, 279, 307, 337, 371, 408, 449, 494,
544, 598, 658, 724, 796, 876, 963, 1060, 1166,
1282, 1411, 1552, 1707, 1878, 2066, 2272, 2499, 2749,
3024, 3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484,
7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899, 15289,
16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767 );


---

Vladan Bato ([email protected])
http://www.geocities.com/SiliconValley/8682