Sierra Audio

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This page describes audio format and codecs used in different games by Sierra. Multi-byte numbers are stored in little-endian format.

Note

Basic information about the format is below; a more detailed description is further down the page.

File Format

 Byte Value
 ----------------
 0    Version
 1    Header size
 2-5  String "SOL"
 6-7  Sample rate
 8    Flags
 9-10 Size

Flags are used to determine audio format and compression:

 0x01 DPCM
 0x04 Stereo
 0x10 16-bit

Working scheme to determine what decompressor to use:

 if(Version == 0x8D) {
   if(Flags & USE_DPCM)
     use old DPCM variant;
   else
     use 8-bit unsigned mono PCM;
 }
 if(Flags == USE_DPCM | USE_16BIT)
   use 8->16 bit DPCM;
 else if(Flags == USE_DPCM)
   use new DPCM variant;
 else if(Flags & USE_16BIT)
   use 16-bit mono or stereo PCM;
 else
   use 8-bit mono PCM;


Credit

The information below was originally based on one of the many format documents written up by Valery V. Anisimovsky, available on http://wotsit.org/ and many other sites across the internet.

AUD File Header

The AUD file has the following header:

struct AUDHeader
{
 BYTE	bID;
 BYTE	bShift;
 char	szID[4];
 WORD	wSampleRate;
 BYTE	bFlags;
 DWORD dwDataSize;
};

bID -- is equal to 0x8D in all Sierra On-Line games I've seen, except for King's Quest 8, where it equals to 0x0D.

bShift -- defines where audio data starts: (bShift+2) is the starting position of the audio data relative to the file start (NOT to the start of RESOURCE.SFX/RESOURCE.AUD containing this file).

szID -- always "SOL\0". Note that there're four bytes including terminating zero!

wSampleRate -- sample rate for the file.

bFlags -- bit-mapped flags: bit 0 -- if set, audio data is compressed (otherwise it's PCM), bit 1 -- ??? (I've never seen it set), bit 2 -- if set, audio data is 16-bit (8-bit otherwise), bit 3 -- if set, audio data is in signed format (unsigned otherwise): 16-bit sound is signed and 8-bit is unsigned, bit 4 -- if set, sound is stereo (mono otherwise).

dwDataSize -- size of the audio data (in bytes).

AUD File Data

Starting at (bShift+2) from the file start, comes AUD audio data. If bit 0 of bFlags is not set, it's just PCM: 8-bit or 16-bit, signed or unsigned. Otherwise it's compressed with the algorithm, which I refer to as SOL ADPCM. SOL ADPCM has two types: 8-bit (for 8-bit sound) and 16-bit (for 16-bit sound).

8-bit SOL ADPCM Decompression Algorithm

Let's (CurSample) be current sample value and (InputBuffer) contain SOL ADPCM compressed data:

SHORT CurSample;
BYTE  InputBuffer[InputBufferSize];
BYTE  code;
DWORD i; // index into InputBuffer

CurSample=0x80; // unsigned 8-bit

for (i=0;i<InputBufferSize;i++)
{
 code=HINIBBLE(InputBuffer[i]); // get HIGHER 4-bit nibble

 if (code & 8) // sign bit
    CurSample-=SOLTable3bit[INDEX4(code)];
 else
    CurSample+=SOLTable3bit[code];

 CurSample=Clip8BitSample(CurSample); // clip to 8-bit unsigned value range
 Output((BYTE)CurSample); // send to the output stream

 code=LONIBBLE(InputBuffer[i]); // get LOWER 4-bit nibble

 ...the same for lower nibble
}

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).

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

SOLTable3bit is the delta table given near the end of this document.

INDEX4(code) is really a tricky thing. In some games (mostly older ones) it should be the following:

#define INDEX4(code) (0xF-(code))

While in some other games it's the following:

#define INDEX4(code) ((code) & 7)

"Old" INDEX4 is used, for example, in King's Quest 6, Quest For Glory 3, Gabriel Knight. "New" INDEX4 is used in Torin's Passage, maybe in other games. I do not know the reliable way to figure out which of those you should use for particular file, but currently I use the simplest technique: I just decode first, say, 1Kb of data using both approaches and look if one of them results in the output stream which is far from reasonable 8-bit unsigned sound (that is, its mean sample value is far from 0x80).

Clip8BitSample is quite evident:

SHORT Clip8BitSample(SHORT sample)
{
 if (sample>255)
    return 255;
 else if (sample<0)
    return 0;
 else
    return sample;
}

Note that the HIGHER nibble is processed first.

16-bit SOL ADPCM Decompression Algorithm

It's just analoguous to the 8-bit decompression scheme:

LONG  CurSample;
BYTE  InputBuffer[InputBufferSize];
BYTE  code;
DWORD i;

CurSample=0x0000; // signed 16-bit

for (i=0;i<InputBufferSize;i++)
{
 code=InputBuffer[i];

 if (code & 0x80) // sign bit
    CurSample-=SOLTable7bit[INDEX8(code)];
 else
    CurSample+=SOLTable7bit[code];

 CurSample=Clip16BitSample(CurSample); // clip to 16-bit signed value range
 Output((SHORT)CurSample); // send to the output stream
}

SOLTable7bit is the delta table given near the end of this document.

INDEX8(code) might be as tricky as for 8-bit sound. But in all games I've seen where compressed 16-bit sound is used it's just the following:

#define INDEX8(code) ((code) & 0x7F)

At least it's true for Torin's Passage, King's Quest 8, Gabriel Knight, etc.

Clip16BitSample is quite evident, too:

LONG Clip16BitSample(LONG sample)
{
 if (sample>32767)
    return 32767;
 else if (sample<-32768)
    return (-32768);
 else
    return sample;
}

Note that the decompression schemes are given ONLY for unsigned 8-bit sound and signed 16-bit sound. I've never seen signed 8-bit or unsigned 16-bit sound in AUD format, but to support these you should only support the correspondent clipping (-128..127 for signed 8-bit and 0..65535 for unsigned 16-bit) and make additional conversion before outputting the sample value: signed->unsigned for 8-bit sound or unsigned->signed for 16-bit sound, provided that you've initialized (CurSample) to the correspondent value: 0x00 for signed 8-bit and 0x8000 for unsigned 16-bit.

Also, those algorithms are ONLY for mono sound, but their improvement for stereo is simple: for 8-bit sound left channel is in HIGHER nibble and right is in LOWER one, while for 16-bit sound left channel is first byte and right channel is second one. Note that you should maintain two different (CurSample) variables for left and right channels: (CurSampleLeft) and (CurSampleRight).

Of course, both decompression routines described above may be greatly optimized.

SOL ADPCM Tables

BYTE SOLTable3bit[]=
{
   0,
   1,
   2,
   3,
   6,
   0xA,
   0xF,
   0x15
};

WORD SOLTable7bit[]=
{
   0x0,   0x8,    0x10,   0x20,   0x30,   0x40,   0x50,   0x60,
   0x70,  0x80,   0x90,   0xA0,   0xB0,   0xC0,   0xD0,   0xE0,
   0xF0,  0x100,  0x110,  0x120,  0x130,  0x140,  0x150,  0x160,
   0x170, 0x180,  0x190,  0x1A0,  0x1B0,  0x1C0,  0x1D0,  0x1E0,
   0x1F0, 0x200,  0x208,  0x210,  0x218,  0x220,  0x228,  0x230,
   0x238, 0x240,  0x248,  0x250,  0x258,  0x260,  0x268,  0x270,
   0x278, 0x280,  0x288,  0x290,  0x298,  0x2A0,  0x2A8,  0x2B0,
   0x2B8, 0x2C0,  0x2C8,  0x2D0,  0x2D8,  0x2E0,  0x2E8,  0x2F0,
   0x2F8, 0x300,  0x308,  0x310,  0x318,  0x320,  0x328,  0x330,
   0x338, 0x340,  0x348,  0x350,  0x358,  0x360,  0x368,  0x370,
   0x378, 0x380,  0x388,  0x390,  0x398,  0x3A0,  0x3A8,  0x3B0,
   0x3B8, 0x3C0,  0x3C8,  0x3D0,  0x3D8,  0x3E0,  0x3E8,  0x3F0,
   0x3F8, 0x400,  0x440,  0x480,  0x4C0,  0x500,  0x540,  0x580,
   0x5C0, 0x600,  0x640,  0x680,  0x6C0,  0x700,  0x740,  0x780,
   0x7C0, 0x800,  0x900,  0xA00,  0xB00,  0xC00,  0xD00,  0xE00,
   0xF00, 0x1000, 0x1400, 0x1800, 0x1C00, 0x2000, 0x3000, 0x4000
};

SOL Low-Pass Filter

If decompressed as described above, SOL ADPCM compressed 8-bit files from Torin's Passage seem to have some minor noise. It appears that some Sierra On-Line games use a kind of strange low-pass filter after the decompression of SOL APDCM compressed 8-bit files. SOL ADPCM compressed 16-bit files does not seem to need such enhancement (as well as SOL ADPCM compressed 8-bit files in some other Sierra games). The SOL LPF scheme looks like to be the following. For unsigned 8-bit sound:

BYTE Sound[BufferSize]; 	// decompressed sound buffer
BYTE FilteredSound[BufferSize]; // filtered sound buffer

DWORD i; // index

for (i=0;i<(BufferSize-2);i++)
   FilteredSound[i]=(BYTE)((WORD)Sound[i]+(WORD)Sound[i+2])/2;

For 16-bit sound the implementation is just analoguous.

AUD Resources: RESOURCE.AUD and RESOURCE.SFX

When stored in .SFX/.AUD resources, the audio files are stored "as is", without compression (unlike other Sierra On-Line resource files) or encryption. That means if you want to play/extract AUD file from the RESOURCE.SFX/.AUD resource you just need to search for szID id-string ("SOL\0") and read AUDHeader starting at the position two bytes before found id-string. This will give you starting point of the file and the size of the file will be (dwDataSize+bShift+2).


Games Using This Format