=============================================================================
		   FunCom ISS Audio File Format Description	    5-01-2002
=============================================================================

By Valery V. Anisimovsky (samael@avn.mccme.ru)

In this document I'll try to describe ISS audio file format used in the
FunCom game The Longest Journey for music, sound effects and speech. Other
FunCom games may also prove to be using that format.

The files this document deals with have extensions: .ISS, .XARC.

Throughout this document I use C-like notation.

All numbers in all structures described in this document are stored in files
using little-endian (Intel) byte order.

===================
1. ISS Audio Files
===================

The music, sfx and speech in The Longest Journey are .ISS files, either
stand-alone or stored in XARC archives.
The ISS file has some kind of a textual header -- the header fields are just
decimal string representations of correspondent fields values, separated
by spaces. So, to get the parameters of an ISS file, you should parse its
textual header, using the function like this one:

// read the file (f) into string (szValue) until the space character is
// encountered (taking into account the string size (dwSize) and EOF)
void ReadUntilSpace(FILE* f, char *szValue, DWORD dwSize)
{
  char	ch;
  DWORD i=0;

  while ((!feof(f)) && (i<dwSize-1))
  {
    fread(&ch,1,1,f);
    if (ch==' ') // space character (0x20)
       break;
    szValue[i++]=ch;
  }

  szValue[i]=0;
}

Of course, it's not the most optimal means to parse ISS header. Anyway,
once you've got the way to obtain the textual fields of ISS header, you'll
see the following structure of that header (all numbers are decimal!):
<szID> <dwBlockSize> <szFileID> <dwOutSize> <dwStereo> <dwUnknown1>
<dwRateDevider> <dwUnknown2> <szVersion> <dwSize>

szID -- ID string, which is "IMA_ADPCM_Sound".

dwBlockSize -- the block size for the audio stream in the file (see below).
The music (stereo) files has this value set to 2048 and the speech/sfx (mono)
files has this set to 512. Though, those values may vary.

szFileID -- the string ID of the given file. It seems to be the internal
name for the file (which coincides with the file title for stand-alone files).

dwOutSize -- number of samples in the file. May be used for song length
(in seconds) calculation.

dwStereo -- this seems to be boolean stereo flag: if this is not zero, the
audio stream in the file is stereo (music), otherwise it's mono (sfx, speech).

dwUnknown1 -- the unknown field. All ISS files in The Longest Journey have
this field set to 1.

dwRateDevider -- the value which determines sample rate for the file: the
sample rate is equal to (44100/dwRateDevider).

dwUnknown2 -- the unknown field. All ISS files in The Longest Journey have
this value set to 0.

szVersion -- version ID string, all files in The Longest Journey have this
set to "1.000". Note that this field may, in principle, vary.

dwSize -- the size of the audio stream in the file. The total filesize is
the sum of the header size and this value.

The resolution is NOT specified in the header, so the default value (16-bit)
should be used.

After the ISSHeader IMA ADPCM compressed sound data comes. The stream is
devided into the blocks each being (dwBlockSize) long. Each block begins
with the small header, containing initial values for IMA ADPCM decompression:

struct ISSBlockHeader
{
  SHORT iSample;
  SHORT iIndex;
};

For mono audio stream there's only one such header, while for stereo stream
there're two consecutive headers -- for left and right channels. Right after
the block header (or two of them for stereo stream) comes IMA ADPCM compressed
audio data. You may find IMA ADPCM decompression scheme description further
in this document.

=====================================
2. IMA ADPCM Decompression Algorithm
=====================================

During the decompression four LONG variables must be maintained for stereo
stream: lIndexLeft, lIndexRight, lCurSampleLeft, lCurSampleRight and two --
for mono stream: lIndex, lCurSample. At the beginning of each block you must
initialize lCurSampleLeft/Right and lIndexLeft/Right variables to the values
from ISSBlockHeader.
Note that both LONG and SHORT here are signed (so initialize LONG variables
by the SHORT initial values with sign extention).

Here's the code which decompresses one byte of IMA ADPCM compressed
stereo stream. Other bytes are processed in the same way.

BYTE Input; // current byte of compressed data
BYTE Code;
LONG Delta;

Code=HINIBBLE(Input); // get HIGHER 4-bit nibble

Delta=StepTable[lIndexLeft]>>3;
if (Code & 4)
   Delta+=StepTable[lIndexLeft];
if (Code & 2)
   Delta+=StepTable[lIndexLeft]>>1;
if (Code & 1)
   Delta+=StepTable[lIndexLeft]>>2;
if (Code & 8) // sign bit
   lCurSampleLeft-=Delta;
else
   lCurSampleLeft+=Delta;

// clip sample
if (lCurSampleLeft>32767)
   lCurSampleLeft=32767;
else if (lCurSampleLeft<-32768)
   lCurSampleLeft=-32768;

lIndexLeft+=IndexAdjust[Code]; // adjust index

// clip index
if (lIndexLeft<0)
   lIndexLeft=0;
else if (lIndexLeft>88)
   lIndexLeft=88;

Code=LONIBBLE(Input); // get LOWER 4-bit nibble

Delta=StepTable[lIndexRight]>>3;
if (Code & 4)
   Delta+=StepTable[lIndexRight];
if (Code & 2)
   Delta+=StepTable[lIndexRight]>>1;
if (Code & 1)
   Delta+=StepTable[lIndexRight]>>2;
if (Code & 8) // sign bit
   lCurSampleRight-=Delta;
else
   lCurSampleRight+=Delta;

// clip sample
if (lCurSampleRight>32767)
   lCurSampleRight=32767;
else if (lCurSampleRight<-32768)
   lCurSampleRight=-32768;

lIndexRight+=IndexAdjust[Code]; // adjust index

// clip index
if (lIndexRight<0)
   lIndexRight=0;
else if (lIndexRight>88)
   lIndexRight=88;

// Now we've got lCurSampleLeft and lCurSampleRight which form one stereo
// sample and all is set for the next input byte...
Output((SHORT)lCurSampleLeft,(SHORT)lCurSampleRight); // send the sample to output

HINIBBLE and LONIBBLE are higher and lower 4-bit nibbles:
#define HINIBBLE(byte) ((byte) >> 4)
#define LONIBBLE(byte) ((byte) & 0x0F)
Note that depending on your compiler you may need to use additional nibble
separation in these defines, e.g. (((byte) >> 4) & 0x0F).

StepTable and IndexAdjust are the tables given in the next section of
this document.

Output() is just a placeholder for any action you would like to perform for
decompressed sample value.

Of course, this decompression routine may be greatly optimized.

As to mono sound, it's just analoguous:

Code=LONIBBLE(Input); // get LOWER 4-bit nibble

Delta=StepTable[lIndex]>>3;
if (Code & 4)
   Delta+=StepTable[lIndex];
if (Code & 2)
   Delta+=StepTable[lIndex]>>1;
if (Code & 1)
   Delta+=StepTable[lIndex]>>2;
if (Code & 8) // sign bit
   lCurSample-=Delta;
else
   lCurSample+=Delta;

// clip sample
if (lCurSample>32767)
   lCurSample=32767;
else if (lCurSample<-32768)
   lCurSample=-32768;

lIndex+=IndexAdjust[Code]; // adjust index

// clip index
if (lIndex<0)
   lIndex=0;
else if (lIndex>88)
   lIndex=88;

Output((SHORT)lCurSample); // send the sample to output

Code=HINIBBLE(Input); // get HIGHER 4-bit nibble
// ...just the same as above for higher nibble

Note that LOWER nibble is processed first for mono sound while for stereo
sound HIGHER nibble corresponds to LEFT channel.

====================
3. IMA ADPCM Tables
====================

LONG IndexAdjust[]=
{
    -1,
    -1,
    -1,
    -1,
     2,
     4,
     6,
     8,
    -1,
    -1,
    -1,
    -1,
     2,
     4,
     6,
     8
};

LONG StepTable[]=
{
    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
};

====================================
4. ISS Audio Files in XARC Archives
====================================

When stored in .XARC resources, ISS audio files are stored "as is", without
compression or encryption. That means if you want to play/extract ISS
file from the XARC resource you just need to search for (szID) id-string
("IMA_ADPCM_Sound") and read ISS header starting at the beginning position of
found id-string. This will give you starting point of the file and the size
of the file will be the sum of header size (its size may vary!) and (dwSize).

===========
5. Credits
===========

Peter Pawlowski (peterpw666@hotmail.com, piotrpw@polbox.com)
http://members.fortunecity.com/pp666/
http://pp666.cjb.net/
http://www.geocities.com/pp_666/
Pointed out corrections in IMA ADPCM decoder.

Valentin Efimov (garden@postman.ru)
http://qmusic.wciom.ru
Provided me with The Longest Journey decoding stuff thereby inspired
me to decode the format and write the plug-in for GAP.

-------------------------------------------

Valery V. Anisimovsky (samael@avn.mccme.ru)
http://bim.km.ru/gap/
http://www.anxsoft.newmail.ru
http://anx.da.ru
On these sites you can find my GAP program which can search for ISS audio
files in game resources, extract them, convert them to WAV and play them back.
There's also complete source code of GAP and all its plug-ins there,
including ISS plug-in, which could be used for further details on how you
can deal with this format.