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Cryo Interactive APC Audio File Format Description

Автор материала: Valery V. Anisimovsky ([email protected])


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 ([email protected], [email protected])
http://members.fortunecity.com/pp666/
http://pp666.cjb.net/
http://www.geocities.com/pp_666/
Pointed out corrections in IMA ADPCM decoder.

Valentin Efimov ([email protected])
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 ([email protected])
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.


 
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