MultiCD supports four algorithms for sound compression that are not always supported by the Mac OS. If you want to use these compressors you may need to install or update some system extensions to decompress the sounds. If you don't use the compressors during recording from CD you don't need a decompressor and can ignore this document and associated files.
There are four decompressors you may need to play sounds recorded by MultiCD:
- diff: implements a very simple differential decoder, converting 4 bit difference values back
into 8 bit sound samples. This encoding/decoding scheme is unlikely to ever be available
from Apple (or anyone else) and is available through the system extension called diff.
- IMA4: implements the Interactive Multimedia Association (IMA) adaptive differential decoder.
It works on blocks of data but is equivalent to converting 4 bit code words into 16 bit sound
samples. IMA4 is supported by MultiCD only if Sound manager version 3.2 or later is available.
- ALaw : implements A law decoding, converting 8 bit codewords back into 16 bit sound
samples. After decompression only 13 bits are actually valid, the low 3 bits are always 0.
This format is supported by Sound Manager version 3.3 as part of QuickTime version 3.0
so an separate decompressor is no longer really needed. I've included the decompressor system
extension (ALaw) for those who can't or don't want to use Quicktime 3.0.
- mu Law: implements mu (µ) law decoding, converting 8 bit codewords back into 16 bit sound
samples. After decompression only 14 bits are actually valid, the low 2 bits are always 0.
This format is supported by Sound Manager version 3.1 and later. It is recommended that
you use the Apple version of the decompressor. I wrote a mu law decompressor once upon a
time but it's easier to just install a newer version of the Sound Manager if you want to play
mu law sounds.
The rules for encoding/decoding both A and mu (µ) law are defined in ITU standard G.711. Both compress the sound on input so that small signals are encoded with finer granularity than large signals. This allows the use of fewer bits while preserving a low perceived noise level for quiet signals. A and µ law were originally designed for use in digital telephone systems but more recently have also been used for saving sound files on computers. Therefore these decoders may also be useful for encoded sounds from sources other than MultiCD. Until the release of QuickTime 3.0 the Mac OS supported mu law but not A law so I decided to write one to see what it sounds like (not much different from mu law to tell the truth).
Note that unless your Macintosh supports 16 bit sound output the A and µ law decoders will only result in at best 8 bit sound.
The differential encoding scheme replaces each 8 bit input sample with a 4 bit difference value between the current and previous sample giving a two to one compression factor. The decoder adds the difference to the previous recovered value to restore an approximation of the input signal. It relies on the fact that adjacent sound samples are typically quite correlated and don't differ that much in value. As a result the difference between samples can be encoded using fewer bits than the sample values themselves. In this implementation the difference value is encoded using a fixed coding scheme. Considerably more sophisticated schemes exist using A or µ law samples instead of linear 8 bit and adapting the coding of the difference values dynamically (IMA4 is one example of this). However, the simple scheme I use actually works quite well except for sounds with a lot of sudden changes (loud bangs, cuckoo clocks ...).
INSTALLATION
To install the diff or ALaw decompressors drag the one(s) you want to use to the extensions folder inside your system folder or simply drag them to the closed system folder and let the Mac figure out where to put them. The next time you restart your Mac the decompressors will be installed as sound components in the operating system, ready to handle compressed sounds.
NOTES:
- the diff and ALaw decompressors require Sound Manager 3.0 or later to operate. If Sound Manager 3.0 is
not present nothing catastophic happens but the decompressors won't install themselves and you will get
an error message when you try to play a sound that needs that decompressor.
- MultiCD will encode sounds to use the Apple A law encoder if it is installed on your computer. Sounds will
be coded by MultiCD to use my A law decompressor only if it is installed and the Apple decompressor is
not installed. If neither decompressor is installed then sounds are encoded to use the Apple decompressor.
- snd and AIFF files encoded for my A law decompressor can not be played by the Apple A law decompressor.
It is possible to convert the files from one format to the other, send me an email if you want directions on
how to do so.
COPYRIGHT AND LEGAL STUFF
Most of the code in the decompressors comes from MewLaw written by Kip Olson and copyright 1994 by Apple Computer. It is used in accordance with the terms of the Apple Software License Agreement on the Develop Bookmark CD. The actual code to do the decompression is copyright 1995 and 1997 by Alex Newcombe. The author is not aware of any other copyrights other than those listed above that are applicable to this software but makes no warranty of non-infringement of third parties' rights.
The decompressors are distributed as is with no warranties expressed or implied about what they may or may not do on your computer. In no event will Alex Newcombe accept liability for any damages or expenses you may incur from using or inability to use this software or associated compressed sound files, including loss of data, loss of profits, interruption of business or similar. If you are not willing to accept any risks associated with the use of the decompressors your only remedy is to not use the software.
Any distribution of this software must include unmodified versions of this document and warnings plus the program itself. This software is free in the sense that no payment is expected for it but Alex Newcombe reserves all rights to the software written by him.
