need software help with digital aux-in

I'm working on a digital aux-in project. I've been using the Eutronix digital out board for a while now and wanted to add digital in from my XM unit.

I lifted the CD2 input pins of the 7705 DSP and routed an I2S signal in from the XM unit. I then modified the hijack kernel DSP programming in empeg_mixer.c to select CD2 when aux-in is selected.

However, there seems to be some sort of issue with a mute function. When I turn on the XM unit I get an audio click and then silence. The display will increment "XM 1", "XM 2", etc... so I know it's seeing something.

I know the CS4231A can be programmed to sample the analog aux in and display bar graphs, etc. If no analog signal is seen, will it automatically mute? If so, where is this code?

Thanks for the help...
-Jon

You can see what the player s/w is doing much of the time by running it under "strace" (need to compile it and upload to the player).

This works best on an upgraded-memory unit, but may still do the job on a 16MB player if ReserveCache=96 is used. Oh, a copy of the strace.binary for the Empeg is here.

Well, after further investigation I believe the software hack is fine but I have signal integrity issues. I was able to get the digital aux to work for brief moments on the bench. By "work" I mean the empeg bar graph lit up and I witnessed bits going from the DSP to my digital out board. In the car it was hopeless.

To be fair, my method of communication is a hack. The real solution would be an SPDIF board in the XM unit, and a corresponding SPDIF receiver (reverse of the Eutronix board) in the empeg. Right now I'm running a simple single ended driver/receiver pair for the I2S signals (WS,CLK,DATA).

I can now confirm the software hack works great! I've been listening to digital aux-in reliably on the bench now for a while. The level bar graphs even work well.

Good time to take a break and watch the super bowl. Hopefully this week I can clean up the in-car signal quality and install this permanently.

Very cool! Please keep us updated. I just got my tuner module and assorted cables for the testing on my desktop. Let me know if I can help.

ISTR the digital in pins are on the docking connector (well, CLK, DATA & WS), two of them multiplexed onto the analogue signal/signal strength pins and one separately.

This might be a bit easier than lifting pins? ISTR that they might be inverted though (input buffers).

Hugo