That was me.<p>The iPod 4G was very similar to its predecessors. The software was stored on the internal drive, kind of like the MBR of a PC. In addition, the iPod had a mode where it would act as a regular USB storage device (or firewire). That firmware was stored in a flash. Even with a blank drive the USB storage mode was always accessible. This made this whole hack somewhat safe.<p>With custom code put at the correct offset with some magic numbers in the right places the iPod would start executing code from the first sectors of its drive. This was just a dd if=mycode.bin of=/dev/sdb, so no exploit or anything special was needed. A few people on the #ipodlinux channel helped me with C and creating position-independent binaries without any external libraries. I had some experience writing a toy OS in assembler on x86 which came in handy.<p>The piezo was controlled by writing to some memory addresses. Someone else already figured that out. I toyed around with various values until I had two distinct noises, one for 0 and the other for 1. Then I write code for loop over a memory region one bit at a time while the piezo played either of those noises. I had audacity (an audio recorder) open on my computer and just started recording using a cheap dynamic microphone. The decoding software was embarrassingly stupid but I had no clue about signal analysis in 2005. It would have been possible to use a modulation method to speed up the process quite a lot. I opted for a compression algorithm instead.<p>I think I tried different memory regions until I found one that started with data that looked like ARM opcodes. When I found that region the final dump took a few hours over night.<p>After sending the extracted binary over to the other devs we had a kernel running not much later.<p>This project taught me a lot: ARM assembler, C, SDL (for visualization), sox (audio processing tools) and patching/porting the linux kernel for new hardware. It also got me job offers from a few big companies but I did not take any of them as I had different plans already. I kept on hacking and contributing to various open-source projects over the years, spent way too much time on hyper-optimizing crypto mining algorithms, and eventually got back to hacking more meaningful code again. Basically, my career after this hack has been almost exclusively open-source projects.