This applies to the original iMac with model identifier iMac,1. It requires the "iMac Firmware 3.0" file from the iMac Firmware Update 1.2.
Type make to build this program.
Set up the sound in Audacity. It will need to be a 44.1 KHz mono sound with a maximum length of just under 2.5 seconds. Export it in Audacity by going to File -> Export -> Export Audio and choosing the following settings:
- Format: Other uncompressed files
- Header: RAW (header-less)
- Encoding: Signed 16-bit PCM
For the rest of the example commands, I will assume you named the file sound.raw.
This will likely export the sound as little-endian, but we need big-endian. You can make a big-endian copy using the following command:
dd conv=swab < sound.raw > sound_be.raw
This final sound_be.raw file is the file you will pass to the command in the next step.
Starting from the original iMac Firmware 3.0 file and your custom sound_be.raw file, run the following command:
mkdir patched
./inject_chime iMac\ Firmware\ 3.0 sound_be.raw patched/iMac\ Firmware\ 3.0
The new version of iMac Firmware 3.0 in the patched directory is the newly patched output firmware file.
Make sure to preserve the resource fork of the file when you copy it back to the Mac. I handle this by storing the firmware file on a netatalk server and modifying the file through a Linux or Windows computer.
The firmware updater program won't allow you to install the patched firmware because it's already up to date.
The updater program has an allow list of firmware versions that can be patched. If you open it in a hex editor, you should see the list with entries such as:
Apple iMac Open Firmware 3.0.f8 built on 08/06/98 at 12:04:16
Simply modify one of these entries (I picked the last one) to be the following:
Apple iMac Open Firmware 3.0.f2 built on 04/23/99 at 14:31:03
Just like with the update file, make sure to preserve the resource fork when you make this change.
Note that after installing the update, it will warn you that the update was unsuccessful. This is a false alarm. If it persists at every boot, make sure your System Folder:Startup Items folder doesn't contain a copy of the firmware updater.