Loom CDDA Converter
This is a single-purpose command-line tool: To convert the CDDA.SOU file of the FM Towns-based VGA version of Loom (as sold on e.g. Steam or GOG) into a WAV file for further audio processing.
The Format
LOOM's VGA version uses a single CD audio track that contains all dialog as well as musical cues much like a radio play which the game seeks to upon corresponding events in-game.
The version distributed on Steam and GOG contains a single file called CDDA.SOU that contains this single track, but although its name suggests this to be a PCM-based CD audio (e.g. a simple AIFF or WAV file) it actually uses a bit-shifted 8-bit format:
| Byte(s) | Description |
|---|---|
| 1-800 | Unknown data, considered "garbage" |
| 801 | Bit shift value |
| 802-1978 | 1176 bytes of unshifted audio |
| 1979 | Bit shift value |
| 1980-3156 | 1176 bytes of unshifted audio |
The Conversion
To retrieve the 16-bit audio encoded in the file, each block of 1177 bytes needs to be processed in the same way:
- Read the first byte - it contains the shift values
- Shift the upper 4 bits down to yield the shift value for the left channel (
byte_value >> 4) - Mask the lower 4 bits to yield the shift value for the right channel (
byte_value & 0x0F) - Read the second byte and treat it as a signed 8-bit value
- Convert this value into a signed 16-bit value
- Left-shift this value by the amount of bits from step #2
- Read the third byte and repeat step #4
- Repeat step #5
- Repeat step #6, but this time use the amount of bits from step #3
- Continue until reaching a multiple of 1177 bytes, at which point repeat from step #1
Explanation
The CDDA.SOU file contains 16-bit stereo audio samples, but encoded to 8-bit values with their lower 8-bits removed. As such this compression is not lossless, as amplitude detail of the first 8 bits is lost - with 15 bits of amplitude in either direction, this comes down to a maximum accuracy loss of 0.7% (and that's at peak amplitude - the lower the volume, the more detail is retained).
Given two bytes (10011100 for the left channel, and 11100001 for the right channel) and the bit shift value 01000100, this would evaluate as follows:
uint8 bit_shift = 68; // 0100 0100
uint8 left_channel_shift = bit_shift >> 4; // 0100 => 4
uint8 right_channel_shift = bit_shift & 0x0F; // 0100 => 4
int8 left_channel_enc = -28 // 1001 1100 => -1 * 001 1100 => -28
int8 right_channel_enc = -113 // 1110 0001 => -1 * 111 0001 => -113
int16 left_channel_dec = -448 // 1000 0001 1100 0000 => -1 * 000 0001 1100 0000 => -448
int16 right_channel_dec = -1808 // 1000 0111 0001 0000 => -1 * 000 0111 0001 0000 => -1808
These decoded 16-bit values can now be written to a file with a proper header for 16-bit little-endian PCM audio and be played as-is.
A Different Kind Of Hello World
This is the first bit of Rust I've ever written, so code style inadequacies are to be expected š„“..
Acknowledgements
- Format discovered by user "apprentice_fu" on the official ScummVM forums
- Python 2-based extractor by Github user "symm" named LAAExtract