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Introducing Project RAUX Box

My recently-acquired Volvo Wagon has one huge flaw: it has no AUX port. The primary reason I bought it is for road trips, where having all of my music at my fingertips is essential. I’m gonna put in an AUX one way or the other, and I’ll be telling y’all about it in excruciating detail. Welcome to Project RAUX Box.

In this introductory post, I’ll be surveying all the methods of putting in an AUX that the enthusiast community has figured out, outlining my plan for the method I’ve chosen, and showing you the major components involved.


Background: AUX Retrofit Options

The root cause of this problem is that the 2004-2007 P1 Volvos used an unusual MOST fiber optics interlink, and do not even have a factory AUX option available. The cheapest compatible kit I could find online is GROM Audio’s $300 adapter. Now, $80 for putting an AUX in my old Corolla was a little high, but worth it. But $300?! That’s good money I could put towards practical necessities, like a set of new tires, wheels that aren’t curb rashed, or gorgeous Nordic oak center stack and door pull trim. In my book, $300 is too much to pay for what should be more or less a minijack to RCA cable. At least, it’s too much without trying something else first. So I went digging.


I found a thread on SwedeSpeed where some folks have been figuring out different ways to retrofit an AUX input to the stereos in P1 Volvos for over 8 years. After reading through the thread’s 824 posts a few times, I’ve pieced together an understanding of all the different ways this could be achieved. If you don’t have a desire to understand the intricacies of signal paths in a car stereo’s head unit, you should skip ahead to the section called “The Plan”. Otherwise, read on as I survey all the methods the SwedeSpeeders came up with.

Method #1: AM Signal Patch

I could solder in to the AM analog signal path, between where it comes into the board and the Digital Signal Processor (DSP) that’s the nerve center of the head unit.



  • Easy to install (relative to all the other options, that is).


  • AM signals are pretty crappy, so the audio processing the DSP does on it means I’d lose the high frequencies of the AUX signal.
  • Mono only, no stereo.

Conclusion: Both of these drawbacks are unacceptable to me. Next!

Method #2: FM Signal Injection

This method involves getting a chip to encode the stereo AUX signal into the same frequency-modulated broadcast format that the DSP receives from the FM antenna. This is a little better than using an FM modulator hooked up to the antenna, but there is still some signal processing the DSP does on the FM signal to smooth over the limited bandwidth.



  • Better audio than AM.
  • It might be possible to encode track info into the FM signal and have it show up on the dash.


  • Still not that close to CD-quality.
  • Requires sourcing a not terribly common audio chip to the stereo analog -> FM broadcast signal encoding.
  • Harder to install than an FM modulator without offering that much of an advantage, or possibly any advantage.

Conclusion: Doesn’t seem worth the effort. Next!

Method #3: Bypass the Head Unit

Another option is to bypass the head unit entirely by cutting the signal between the DSP and the amp — which has expanded from a two-channel signal into a four-channel signal by the time it comes out of the DSP — and putting in relays to switch between the stock head unit feed and the AUX signal. This would involve bridging out the two stereo AUX channels to cover the four channels going into the amp.



  • Possibly better audio than using the radio signal paths, because there’s no AM or FM signal processing happening (but see below).
  • Simpler than any of the other options besides the AM patch.


  • Whatever processing the DSP does to shape each single stereo channel over the two different audio feeds it sends to the amp for that channel is lost, so just duplicating one stereo channel over both of those won’t sound as good, and maybe would sound worse than the FM method.
  • High potential for weird analog interference or ground loop problems.
  • Because the head unit gets entirely bypassed, all stock volume controls in the car will stop functioning (particularly the steering wheel controls), and you lose the ability to adjust bass & treble response and balance & fade.

Conclusion: Unacceptable! Next!

Method #4: Utilize the Hidden AUX Pins

The DSP chip is a fairly common one that’s used in many different models across a few manufacturers, so it does have pins on the chip for a dedicated AUX input. It’s possible to solder in the AUX signal to those pins, and then program a microcontroller to grab master on the communication bus and tell the head unit to switch over to them. This was initially very promising to me, but after more reading & thinking I’ve soured on it.


The primary problem is that, because the head unit was designed without an AUX input in mind — there wasn’t even a factory AUX option — the AUX signal goes through the same audio processing path as whatever input the head unit was using before you tell it to switch to AUX. For example, if it’s in FM, and then you make it switch to AUX, all the FM signal processing in the DSP is still active. Nobody on the forum seemed to try using CD as the prior mode, so I don’t know if that’s even an option. It’s possible that using the CD mode before switching doesn’t work at all, because the CD player has a digital output signal. This means that I’d still be subject to some of the same sound quality issues as in options #1 and #2.


  • Keeps all stock audio controls (including EQ and fade/balance) functional.


  • Long-term reports from people with this solut indicate that the strength of the FM/AM signal will affect the radio-compensation processing done by the DSP, so you get effectively get a random EQ on your AUX signal, and I’m not sure if it gets better or worse on an empty frequency.

Conclusion: If I was going to accept radio-ish quality and managing the radio frequency, I’d just get an FM modulator and save a lot of work. Butts!

None of These Options Satisfy Me

At this point I’m starting to get pessimistic. All of these solutions have major drawbacks that are making me lean towards just buying the dang GROM kit. And they were all figured out in the first two years of the thread (2008 to 2010), before there were even aftermarket solutions available. After a handful of people successfully implement option #4, there’s many years of talking about new aftermarket solutions and some futzing about with FM modulators. Then in 2016, a new guy called “wchpikus,” who clearly has embedded software experience, shows up. He resurrects the thread by going back to a technique that the people who developed the hidden AUX pin method dismissed six years prior as “ugly” and making it work.


Method #5: Pretend to Be the CD Player

The CD player sends a digital audio signal straight to the DSP, and the DSP doesn’t have to do any weird processing on it like with the radio signals. So this method encodes the AUX to the digital audio format the CD player uses (which is a very common one), and swaps the digitally-encoded AUX signal in to the head unit in place of the CD feed by way of some relays.


Now we’re talking! This is as close to CD-quality as you can get if you’re starting with an analog signal. It’s happening before the DSP, so all of the stock audio controls work (including EQ/fade/balance). An added bonus is that the digital encoding means I only have to worry about analog electrical interference in the cable itself and where it connects to the Audio to Digital Converter chip (ADC), which is great because I don’t actually understand how electricity behaves beyond I=RV.

There is a catch: the CD player mutes the DSP’s output by using a separate control channel. This happens whenever:

  • there’s no CD playing;
  • the CD is being inserted/removed;
  • between tracks with a gap;
  • and when the CD finishes and it loops back around to the start;

but I can burn a dummy CD with one long silent track, and only have the sound cut out for 5-10 seconds every hour. This is an acceptable downside given all the other upsides of this approach.


Even this catch is surmountable, because wchpikus and another guy managed to figure out the particular values in the I2C communication bus between the CD player and the DSP. This means I can program a microcontroller to play the part of the CD player that never sends a mute command when the AUX is being fed in.

In summary, this is the perfect solution: a wired AUX without any weird signal processing, that still preserves all the stock audio controls. In the final stage, it’s completely independent of the behavior of the stock audio inputs: the audio is unaffected by radio strength and not subject to random muting by the CD player. It also has the advantage that I can get 90% of the benefit by doing half (or less) of the work and burning a blank CD, at which point I can see whether I think doing the second round of work to prevent the few seconds of muting every hour or so is worth it.


The Plan

I haven’t done microcontroller programming and sniffing SPI & I2C communications in a few years now, but I’ve missed it and I’ve been looking for an excuse to get back in. This is it!


Because this sort of thing has been a hobby of mine for a while, I have most of the necessary pieces already.

The Parts

A tiny Arduino clone will serve as the brains. Much less powerful than your average coffee maker these days, but it’s amazing what you can do with a 16 MHz chip when your code controls literally everything it’s doing (no OS on it) and there’s no slow parts (even memory access only takes a handful of clock cycles!). It’ll be more than sufficient for this.

Illustration for article titled Introducing Project RAUX Box

The Analog to Digital Converter (ADC) is the only thing I didn’t have already. That will be handled by this pre-assembled board I bought from a guy in Portland who designs and sells all sorts of hobby electronics stuff:

Illustration for article titled Introducing Project RAUX Box

I even have a bunch of relays lying around, too. They’re not the best kind for this application, and they’re much beefier than they need to be, but for prototyping, they’ll do just fine while I wait for more suitable ones to be shipped from China.

Illustration for article titled Introducing Project RAUX Box

Then there’s, you know, a bunch of random things like proto boards, wires, resistors, capacitors, diodes, light-emitting diodes, etc. Boring stuff.

The Steps

  1. First I need to figure out how to get the toolchain set up to program my Arduino clone. It’s been a while since I did this on my Linux laptop, but I’d like to use that instead of my work Mac laptop. Hopefully this won’t take too long but, you know, Linux.
  2. Then I’ll program the Arduino to configure the audio encoder chip properly. It uses the common I2C/TWI protocol for control communications, but it’s capable of many different analog to digital or digital to analog conversions, as well as rudimentary signal processing, so I need to spend some quality time with the datasheet to figure out how to set it up to do nothing but convert the analog input to 16-bit right-justified I2S.
  3. Once the audio chip seems to be set up, I’ll assemble a big proto board with the Arduino, the audio chip, three relays, and something to provide power.
  4. The final step will be taking out the integrated CD player / head unit / amp, rerouting the three wires that carry the CD’s digital audio signal into the proto board, soldering the three wires of my board’s output into the head unit, and plugging the head unit back in. Then I turn it all on. If I’m lucky, I’ll be done heahahahaha, who am I kidding, even if I’m lucky there will be troubleshooting (hopefully not too much).

My deadline for this is the Spring Oppo Cruise in a bit over a month. It might be an optimistic schedule, especially considering that I’m trying to adopt a dog right now too, but I’ve done more complicated electronics projects than this before and I have plenty of free time at the moment. You’ll be sure to hear about it one way or the other. Wish me luck!

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