(c) Copyright 1990 Commodore-Amiga, Inc. All rights reserved. The information contained herein is subject to change without notice, and is provided "as is" without warranty of any kind, either express or implied. The entire risk as to the use of this information is assumed by the user. Reading Mac Sound Files by John Orr There is little difference between the raw data of Amiga and Apple Macintosh (and Apple IIgs) sound samples. Both machines record and play digitized sound in 8-bit samples. The only difference between their raw data formats lies in how the sound wave is plotted. Sound samples on an Amiga are represented as a positive or negative offset from zero, ranging from -128 to 127. A value of zero represents no sound (see fig. 1). Mac samples on the other hand, are represented as a positive offset from zero, but here zero represents the part of a sound wave that is of the greatest possible negative amplitude. Macintosh sample data ranges from 0 to 255 with silence represented by a value of 128 (see fig. 2). Naturally, the sound data must be converted to cross from one platform to the other. Without any conversion, a sine wave sampled on one platform would look like figure 3 on the other platform. One simple way to convert a sample from Amiga to Apple format would be to add 128 to each byte in the sample. Conversely, to convert from Apple to Amiga, you could subtract 128 from each byte. But both cases can be handled more simply by reversing the high bit of each byte in the sample. This one process will convert a sample of one type to the other. Hence, performing an exclusive OR with 0x80 on each byte in the sample will generate the desired result. Transferring the Data Before converting a Mac audio sample to Amiga format, another problem has to be considered: obtaining Mac sound data. There are several straight-forward methods of transferring data from one platform to the other. A direct serial line transfer can be performed between the Amiga's RS-232 serial port and the Mac's RS-422 serial port. Another way the two machines can share data is through the use of a product such as Central Coast Software's Mac-2-DOS which comes with a disk drive that can read and write both Mac and Amiga formats. The A-Max Macintosh emulator from ReadySoft offers yet another convenient way of moving data across from one platform to the other. Macintosh File Conventions The Mac filing system has an unusual method of storing files. Files are broken up into two parts, a data fork and a resource fork. In the case of audio samples, the data fork typically contains the actual sound sample, while the the resource fork contains various parameters of the sound such as recording and playback rate. In general, when transferring Mac files to the Amiga, only the data fork is needed by the Amiga. Mac-2-DOS and most Mac terminal packages are capable of transferring just the data fork part of the file by itself. Another thing to note about Mac files is that they have a file type and creator associated with them. In the case of sampled sound files, the file type is FSSD. When transferring Amiga files over to the Mac, the file must be given the FSSD file type, otherwise Mac sound applications generally will not read the file. Mac-2-DOS is capable of specifying the file type field as it copies Amiga files to a Mac floppy. If you are using some other transfer method, then the file type can be changed to FSSD on the Mac end by using a utility such as ResEdit. The creator field depends on what application created the file. A popular sound sampler/editor for the Mac is SoundEdit, which is akin to the Amiga's AudioMaster II. SoundEdit's creator field is SFX!. It should be mentioned that in certain cases, sound data on a Mac is stored in the resource fork in an entity known as a sound resource. The Mac sys-beep is an example of this. It is possible to convert a Mac sound resource to the FSSD file type -- this is left as an exercise for the reader. Audio Sample File Formats: Raw vs. IFF When transferring Amiga sounds to the Mac, it's important to remember that Mac sound samples are usually saved as raw data. On the Amiga however, sampled sounds are usually stored in the IFF 8SVX format. The 8SVX format contains a header chunk called the VHDR which has global information about the sample such as the sampling rate and compression used. This is followed by a BODY chunk which contains the raw sound data. For the Mac to interpret an Amiga sample correctly, only the raw data should be present. Likewise, a Mac sample should be converted to 8SVX for use on the Amiga. SunRize Industries' Perfect Sound is capable of reading and writing both raw sound data and IFF 8SVX data which makes it a handy conversion tool. Another option is Aegis' AudioMaster II software which includes a utility that can strip an 8SVX file down to a raw data file. Note that for stereo samples, both platforms store the raw data in the same order -- the left channel first followed by the right channel. So, stereo and mono samples are converted using the same methods. Recently, Apple adopted another standard for storing sound samples. It is based on the IFF model of data storage and is called the Audio Interchange File Format (AIFF). It is similar in many ways to 8SVX, but is intended to be more versatile. The AIFF standard has not yet been widely accepted, and consequently, commonly found samples are not stored in this format. More information on this format can be found in Apple documentation. Sampling Speeds After a sound sample has been copied from one medium to the other, a playback rate has to be determined since this information is not preserved in transferring raw data. Mac sounds digitized with SoundEdit and MacRecorder -- a popular Mac sound digitizer -- are sampled at four different speeds: 22kHz, 11kHz, 7kHz, and 5kHz. Amiga audio samples normally vary in recording speed and can be as fast as 28,867 bytes/second. The table below shows the four MacRecorder sampling rates and their Amiga equivalents. The example code below will convert raw sound samples to and from the Amiga and Mac formats. It is important to remember that this program works only with raw data. 8SVX samples must be stripped of the added IFF information, before they can be converted. /* * convertsound.c - Converts raw sound data back and * forth from a Macintosh/Apple IIgs format to the Amiga * format. */ #include #include #define BUFSIZE 32767 struct FileHandle *Open(char *, LONG); UBYTE buffer[BUFSIZE]; void main(int argc, char **argv) { struct FileHandle *fh_in, *fh_out; WORD count_in, count_out, x = 0; if (argc != 3) { printf("usage:\n\t%s ",argv[0]); exit(30); } if (fh_in = Open(argv[1], MODE_OLDFILE)) { if (fh_out = Open(argv[2], MODE_NEWFILE)) { do { count_in = Read(fh_in, buffer, BUFSIZE); for (x=0; x < count_in; x++) buffer[x] ^= 0x80; count_out = Write(fh_out, buffer, count_in); } while (count_in > 0); Close(fh_out); } Close(fh_in); } }