Let’s figure out where the noise is coming from. On the C16 and any other 264 series machine the TED chip does audio, video and I/O. Pin 33 on the chip is the sound output it seems. During a directory listing this stays flat on the scope. When I run a game we can see the waveform. So this rules out the TED chip I would say.
#commdore #c16 #plus4
The IEC serial bus on the other hand is handled by the CPU it seems. There is an inverter and some ferrite beads in there, and that’s it. A few pullup resistors as well, but nowhere does the bus cross paths with the audio signal. At least in the schematics.
@root42 The schematic is just a writeup of connections, it‘s important to know that it’s very well possible that the physical traces still cross.
@elosha obviously. There has to be some connection on the actual board. But if there was e.g. a shared IC pr something like that could have explained this.
@root42 It could even be AC coupling to a common plane/trace via bad cap …