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.


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;
     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;
   use 8-bit mono PCM;


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

AUD File Header

The AUD file has the following header:

struct AUDHeader
 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=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;
    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;

CurSample=0x0000; // signed 16-bit

for (i=0;i<InputBufferSize;i++)

 if (code & 0x80) // sign bit

 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);
    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.


BYTE SOLTable3bit[]=

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++)

For 16-bit sound the implementation is just analoguous.


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