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Old 10th November 2012, 07:38 PM   #1
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Default Hacking auto-radio spdif input (need help)

Hi!

I'm trying to hack my car stereo adding an analog aux input to it. The way I'm doing it is by replacing the cd player's controller spdif output to the car stereo DSP's DAC by an ADC module (*). However, I am just getting white noise when connecting the ADC.

My car stereo has a SAA7326 "Digital servo processor and Compact Disc decoder" (https://dl.dropbox.com/u/112571/SAA7326.pdf) connected to a proprietary DSP (IC 2000 in https://dl.dropbox.com/u/112571/BE6021-schematic.pdf). The SAA7326's DOBM output (pin 51) connects to pin 24 of the DSP. What I'm doing is to intercept this link replacing the SAA7326's output by the spdif output of a Cirrus CS8427 (https://dl.dropbox.com/u/112571/CS8427_F5.pdf). However, by doing so I just get white noise from the car stereo.

When plugging the CS8427 spdif output into my home stereo DAC it works flawlessly, though.

I'm a noob here so I guess I'm probably missing something basic. You help would be much appreciated.

Thanks,
Rui

(*) The ADC module I bought and uses a ADC CS5340 plus the Digital Audio Interface Transceiver CS8427.
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Old 10th November 2012, 07:48 PM   #2
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Quote:
Originally Posted by roliveira View Post
Hi!

I'm trying to hack my car stereo adding an analog aux input to it..
The rest, I just don't understand !!
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Old 10th November 2012, 08:00 PM   #3
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Quote:
Originally Posted by picowallspeaker View Post
The rest, I just don't understand !!
You don't understand what I'm trying to do, or why I'm taking this approach?

My car stereo has a central DSP which gets the digital audio from the cd player (or external cd box ) and outputs the pre analgo signal to the output amp. There's no place I can add an analog source except with an awful hack straight to the power amp (no volume or equalizer control). Therefore, what I'm trying to do is to remove the cd transport (which connects to the DSP through a spdif signal) and replace it with an ADC component.

Did I manage to explain?...

Thanks!
Rui
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Old 10th November 2012, 08:54 PM   #4
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Yes, eventually I understand ...that the awful hack would still
be less ugly that what you intended to do and failed .
But I know that nowadays the circuits are much complicated and
it's not possible to just cut the wire from the potentiometer and route it
to some external jack . Simply because there's no pot and no wire !
But still there's the input of the amp, at least .
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Old 10th November 2012, 09:30 PM   #5
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I suspect the outboard CD changer has to talk to the head unit to enable playback on the spdif input. The head unit might even be sending from its master clock to the CD synchronise the datastream.
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Old 11th November 2012, 07:19 AM   #6
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For now I'm keeping the cd player as if it was the input source, I'm just intercepting the data signal.
As for the synchronizing clock, you may indeed be right. There's a clock being generated at the head unit that goes into the cd player outboard. Just don't know what to do with it. Need to investigate.

Cheers,
Rui
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Old 11th November 2012, 08:08 PM   #7
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What do you see on SR0 under normal and modded operation when looking at it with an oscilloscope? While SPDIF uses bi-phase encoding Mercedes' schematic does not show a 75 ohm termination to the line. So, even though the SAA7326 datasheet doesn't explicitly state the DOMB output uses the SPDIF protocol, I would tend to guess level problems due to impedance mismatch over a protocol mismatch.
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Old 12th November 2012, 10:18 AM   #8
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Yes, I'll borrow a digital analyzer to check both signals asap. I tried driving the ADC chip with the head unit's clock but no luck.

Thanks for the suggestion,
Rui
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