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It's so weird that this problem still didn't solved. Every time people ask, they got different answers and I don't remember anybody who reported about success.
I noticed that with Base Stereo system (Professional CD + 6 speakers, no tweeters, no amp) PDC sound DOESN'T changes it's volume when you turn volume up\down. So, I guess:
1. There are two output signals - audio + PDC. i.e. constant 2v for PDC and variable 0.1 - 9V for audio.
2. Two signals summing before going out from headunit.
Then, we can have E90 with Professional Audio, Front and rear PDC and three situations.
1. We add LOC, external amp with line level input and doesn't recode HU to Hi-Fi. We try to make output more loud and i.e. adding 2x gain level.
Then we got:
* LOC convert audio signal from 0.1 - 9V to i.e. 0.1 - 2V and PDC\gong from 2V to 0.6V.
* Amp makes makes output level >0.1 - 9V and more than 2V on PDC.
* Here we notice our gong level grew louder.
* But we got better audio quality, higher undistorted level.
2. We add external amp with high level input and doesn't recode HU to Hi-Fi. We try to make output more loud and i.e. adding 2x gain level.
Then we got:
* Amp makes makes output level >0.1 - 9V and more than 2V on PDC (remember we wanted more output, 2x gain).
* Here we notice our gong level grew louder.
* But we got better audio quality, higher undistorted level.
3. We add external amp with line level input and recode HU to Hi-Fi. We try to make output more loud and i.e. adding 2x gain level.
Then we got:
* 0.1 - ~4V line level audio output and ??? PDC output (2V or maybe line level, ~ 1V)
* Amp makes makes output level >0.1 - 9V and more than 2V on PDC. Same story.
Here i have only one solution:
Adjust level gain so amp outputs the save voltage as HU internal amp (9V). Then PDC\Gong volume will be ok.
So I have thee questions:
1. My guess about summing, is it true ?
2. PDC level. If it's level changes with recode to Hi-Fi or not?
3. Maybe someone have better solution?
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