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Old 27th September 2015, 09:09 PM   #1
Trileru is offline Trileru  Romania
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Default Hacking a factory head unit for AUX input

Hello,

I just got a Renault Laguna 2. It has the factory head unit, it says "Tuner List" on it's front. Nothing fancy, CD-Player and AM/FM radio, with an optional CD-Changer (that it also has in the car, but is missing the CD storage device, has only the player).

I really like the way it is integrated with the car, I have the remote on the steering column, also uses the HUD of the car for info.
I'm not a fan of the Christmas lights like aftermarket players but I do need an aux input into this unit.
The only option that this unit provides is the input from the CD changer, but that is spdif based, and also has some serial data RX/TX happening between the units. And that would seem like the long way to hack, also I need something that can take digital sound from my phone and convert it to spdif etc. Too much trouble.

The easy way for me looked like I could just hack another input into the dsp chip.
Looking at it's schematic, the chip has many inputs. The only problem is that the unit does not go through all of the inputs. If no CD is inside the unit, it defaults to radio, and I can only switch between AM/FM. The CD unit inside this player also uses a spdif input on the dsp chip. So no luck there. I would have happily removed the cd-unit but no luck.
So because the unit talks to the cd-player or cd changer via serial to determine what's happnening, I can only use the two active inputs that are available always. AM and FM radio. I need FM but I don't need AM. So looking at the schematic, I noticed that AM audio signal goes into one pin, the Right Channel/Mono pin. The left channel is floating and is conveniently broke out into a small test pad.
I decided to make a test, and inject another signal on that pin. That way I figured that the DSP detects signal on that pin and starts a stereo output. But sadly that isn't the case. I couldn't get my test signal on either channel.
What else should I do? Is there something that sets the DSP on mono/stereo for AM input?
If my test worked, I could have cut the trace leading to the AM input pin and then inject left/right signal with a plain cable that would go out of the unit to my phone.
Any ideas?
On the picture, I circled with red the left channel (not connected one) and with yellow the active right/mono channel.
Is there another radio signal that tells something to the DSP about mono/stereo? If so, is that signal on the same line with the FM data? I see a line named Level, what does that do?

edit: I had a look on the other side of the board, it's two layers and those are not vias, they are indeed pads, or test pads or whatever. They are not going on the other side, so the Left channel AM input is indeed floating. Along with the other inputs.
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File Type: jpg IMG_3864.JPG (478.2 KB, 104 views)
File Type: jpg IMG_3863.jpg (204.5 KB, 100 views)

Last edited by Trileru; 27th September 2015 at 09:20 PM.
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Old 28th September 2015, 05:30 AM   #2
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Bluetooth to FM adapter?

Use a relay to switch from FM to phone signals going into the DSP IC?
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Old 28th September 2015, 10:23 AM   #3
Trileru is offline Trileru  Romania
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I just saw in the chip datasheet that the mono/stereo is controlled by IC, so no way of activating that.
ImageUploadedByTapatalk1443435651.052916.jpg
I was also thinking of replacing the amplifying chip to something better (tda7850 from tda7385) but the heatsink is really small.
ImageUploadedByTapatalk1443435631.070302.jpg

I need to rethink this.
What bugs me is that a different head unit would kill my remote controls and dash display.
There are adaptors for the controls, and they are around 30$, there are also adapters for the display, but those are 130$.
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Old 28th September 2015, 10:38 AM   #4
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Increasing the amplifier might destroy the PSU.

You could consider adding an aftermarket amplifier and adding in the aux input between the two.
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Old 28th September 2015, 10:50 AM   #5
Trileru is offline Trileru  Romania
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Quote:
Originally Posted by KatieandDad View Post
Increasing the amplifier might destroy the PSU.

You could consider adding an aftermarket amplifier and adding in the aux input between the two.
There is nothing to be destroyed. Maybe the filtering inductor? But let's calculate:

TDA7385 is a 4x23W device at 14.4V (at 10% distortion, that's at max, and I wouldn't listen at that volume). So that translates into 4x1.6A = 6.4A.
TDA7850 is a 4x30W device at 14.4V (same 10% distortion) That is 4x2.1A=8.4A. At max volume with the more powerful chip I would have a total extra increase of 2A, that is continuous. I guess the inductor can handle it, especially as I'm not going to max it.

But yes, I have considered the other option, of adding another amplifier. Just tap the chip inputs and route that to another 4in/4out amplifier. I still need to figure how to inject stereo signal, as FM even if it's stereo, the dsp chip does the decoding (RDS/MPX lines input the chip).
The modulator is not a viable solution, I've used them before and they are total crap. On bass I'd get bells instead.

edit: Forgot to add the schematic
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File Type: png Screen Shot 2015-09-28 at 13.42.09.png (34.9 KB, 69 views)

Last edited by Trileru; 28th September 2015 at 11:15 AM.
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Old 28th September 2015, 12:00 PM   #6
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Most external amps will have high level inputs as well as low level. The L/S output of the head unit can be used.
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Old 28th September 2015, 12:17 PM   #7
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I would use an analogue signal to FM transmitter and select a spare RF channel on the radio to listen to your AUX signal.
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Old 28th September 2015, 12:28 PM   #8
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The problem with FM transmitters is interference. I've never found a good one.
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Old 28th September 2015, 05:48 PM   #9
Trileru is offline Trileru  Romania
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Me neither, and I've tried several.
Seems like there's this guy that hacked the FM part. And used a switch between the AUX input signal and FM so he can have both.
https://docs.google.com/document/d/1...hiE/edit?pli=1

Basically he's tapping on the FM MPX line and encodes his aux signal to be decoded back by the dsp. Lots of work.
Having a CDC option, and not using it, I figured I might go that route. There is some info on this on the Renault forums. I may update if I make some progress for the AUX input.
On the amplifying part, I will first try to replace that chip and see if it's any better and if I have any trouble with extra heat, or I may just use an external one.
First, I need to find a way to inject my signal into this unit, then I decide how I will amplify.
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Old 28th September 2015, 07:35 PM   #10
sreten is offline sreten  United Kingdom
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Hi,

There is no point upgrading the amplifier. The more powerful
amplifier achieves it by being lower than 4 ohm capable. It
will probably be no louder than the stock amplifier at clipping.
They are both BTL that can swing rail to rail.

rgds, sreten.
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