350z Radio Hack
Adding an aux input to the stock radio I have a 2003 350z with a stock non-Bose radio. This particular model does not have an aux input, which hampers my plan to get a computer sound output hooked up to the stereo. So I took apart the radio, looked up all the chips and was able to find the data sheets for the main audio chip. Yay!
The chip actually has multiple inputs available and mux that can switch between them. This is how the radio switches from radio to CD. It turns out the chip takes a serial command stream in on a couple pins. This stream is a fixed length (though I have forgotten the specifics) and includes things like volume, input selection, bass, treble and loudness controls.
My first idea was to hook the aux input to an unused input on the chip and then put a PIC between the on board micro and the serial bus input on the audio chip. This PIC would intercept the radio control bitstream, look at an input on a gpio and, if the gpio was high, modify the bitstream to switch the input to the aux in. I started coding the PIC and got something I thought would work, but when I hooked it up I got nothing. I played around for a while and tried running PIC emulators and such but I just couldn't get it to do anything. It turns out PICs are a pain to write for and it was taking too long and I was getting impatient. Plus it was going to be hard to lift the pins on the audio chip and solder to them in a reliable way. So I gave up on this approach.
The audio signal goes into the chip through one of the inputs and is switched internally by a mux controlled by the serial bit stream. After this the signal takes a detour outside the chip and then comes back in after passing through some sort of filter. This is where we decided to hook in.
We looked through the parts that my dad had lying around and found a couple old analog switches from some ancient project. We pulled out one side of the filter caps which gave us direct access to the post-mux audio path. These switches can choose 1 of 8 inputs. That is total overkill, but it is all he had and I didn't feel like waiting any longer. I also had to use 2 of them since they were mono. But it didn't matter too much. I stole power off the board and soldered everything up and it worked the first time.
After taking way too long to decide where to drill the holes in the back I finally got it done and installed the RCA jacks, soldered everything up and put it all back together.
It works like a charm.
The chip actually has multiple inputs available and mux that can switch between them. This is how the radio switches from radio to CD. It turns out the chip takes a serial command stream in on a couple pins. This stream is a fixed length (though I have forgotten the specifics) and includes things like volume, input selection, bass, treble and loudness controls.
My first idea was to hook the aux input to an unused input on the chip and then put a PIC between the on board micro and the serial bus input on the audio chip. This PIC would intercept the radio control bitstream, look at an input on a gpio and, if the gpio was high, modify the bitstream to switch the input to the aux in. I started coding the PIC and got something I thought would work, but when I hooked it up I got nothing. I played around for a while and tried running PIC emulators and such but I just couldn't get it to do anything. It turns out PICs are a pain to write for and it was taking too long and I was getting impatient. Plus it was going to be hard to lift the pins on the audio chip and solder to them in a reliable way. So I gave up on this approach.
The audio signal goes into the chip through one of the inputs and is switched internally by a mux controlled by the serial bit stream. After this the signal takes a detour outside the chip and then comes back in after passing through some sort of filter. This is where we decided to hook in.
We looked through the parts that my dad had lying around and found a couple old analog switches from some ancient project. We pulled out one side of the filter caps which gave us direct access to the post-mux audio path. These switches can choose 1 of 8 inputs. That is total overkill, but it is all he had and I didn't feel like waiting any longer. I also had to use 2 of them since they were mono. But it didn't matter too much. I stole power off the board and soldered everything up and it worked the first time.
After taking way too long to decide where to drill the holes in the back I finally got it done and installed the RCA jacks, soldered everything up and put it all back together.
It works like a charm.
