=============================================================================
	    Cryo Interactive APC Audio File Format Description	    5-01-2002
=============================================================================

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

In this document I'll try to describe APC audio file format used in many
Cryo Interactive games for music, sound effects, speech and movie soundtracks.
Also described is the format of ZIK audio files used in Cryo game Chine.

The games using these formats include: Atlantis: The Lost Tale, Atlantis 2,
Aztec: The Curse in the Heart of the "City of Gold", Egypte 2, Odyssee, Chine.
Maybe, some more.

The files this document deals with have extensions: .APC, .HNM, .BF, .ZIK.

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. APC Audio Files
===================

The music, sfx, speech and video (.HNM) soundtracks in some Cryo Interactive
games are .APC stand-alone files. APC file has the following header:

struct APCHeader
{
  char	szID[8];
  char	szVersion[4];
  DWORD dwOutSize;
  DWORD dwSampleRate;
  LONG	lSampleLeft;
  LONG	lSampleRight;
  DWORD dwStereo;
};

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

szVersion -- version ID string, all files in the mentioned games have this
set to "1.20". Note that this field may, in principle, vary.

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

dwSampleRate -- sample rate for the file.

lSampleLeft -- the initial value for the left sample (see below).

lSampleRight -- the initial value for the right sample (see below).

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

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

After the APCHeader IMA ADPCM compressed sound data comes. 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 the file you must
initialize lCurSampleLeft/Right variables to the values from APCHeader while
lIndexLeft/Right are initialized to zeroes.
Note that LONG here is signed.

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

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

====================
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. HNM Movie Soundtracks
=========================

.HNM movie hasn't got soundtrack inside the HNM movie file itself.
However the soundtrack for HNM movie is a stand-alone APC file, as a rule,
with the same file title. The format of such APC soundtracks is just the
same as described above.

==================================
5. APC Audio Files in BF Archives
==================================

When stored in .BF resources, APC audio files are stored "as is", without
compression or encryption. That means if you want to play/extract APC
file from the BF resource you just need to search for (szID) id-string
("CRYO_APC") and read APC 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 APCHeader size and the compressed audio stream
size correspondent to (dwOutSize) header field (that is, (dwOutSize) for
stereo stream and (dwOutSize)/2 for mono stream).

===================
6. ZIK Audio Files
===================

In the game Chine music is in ZIK files. Most of them are simple RAW files:
16-bit signed stereo 22050 Hz (no header), except for only one which is a
regular WAV file. So, you can just load and play those ZIKs as RAWs (except
for one which is WAV).

===========
7. 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 Atlantis 2 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 APC 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 APC plug-in, which could be used for further details on how you
can deal with this format.