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Rich Lafferty's Journal

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aftermarket audio blues
disbelief
mendel
Out of boredom I just spent some time learning about the audio system on my car (a '99 Saab 9-3), with an eye towards seeing what I can do in terms of aftermarket kit. MP3s and satellite radio might be nice, or just a newer non-OEM head unit, etc. But Saab seems to have tried really, really hard to make sure I don't touch anything. Viz:
  • The amplifier uses balanced connections
  • The speakers have odd impedances: 1.8, 2.5, 5.5 ohms
  • There is no line-in
  • The CD-changer audio-in ports are not activated unless a CD changer is present
  • Communication between the CD player and the head unit is via a Saab-specific packet-based protocol, "I-BUS"
  • The display is a separate system (SID) from the head unit, and they communicate over I-BUS too
  • The steering-wheel controls send control voltages to the SID, which instructs the head unit via I-BUS
Volume controls which send control voltages through an ADC to talk to the head unit via packets might be a sign of overengineering. So it looks like the only realistic upgrade options I have are to add a separate mp3 player or satellite radio unit via an FM modulator. The stock stereo sounds ok, so that's probably fine anyhow -- I just couldn't believe the level of complexity in all of its interfaces.

In other news: This weekend I'm heading back to Belleville to order a suit for the wedding from my father's store, made-to-measure. Yay, new things! Yay, not paying retail!


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I-Bus is not that uncommon. Many car manufacturers use it. I have a friend with an electric car that has some sort of aftermarket I-Bus device that pulls diagnostic data from the car computer.

Also, using control voltages for remote buttons is not terribly uncommon, either. Running a data bus up through the steering wheel adds cost and complexity. As a firmware engineer, I cringe at the thought of putting integrated circuits into something like a steering wheel. Not only would the cabling be a nasty point of failure, but the "disposability" (sure, they're not disposable as such, but they're an easily removed/broken/cut part) of the wheel also raises some hardware engineering red flags. A single wire, some buttons, and some fraction-of-a-penny resistors, on the other hand, seem prefect for such a location.

As for the audio thing, I do seem to recall several aftermarket kits that get between the CD changer and the head unit which basically inject your own audio in place of the CD. The theory is that you put a CD you don't care about in the changer, plug in the iPod (or similar), tell the head unit to start playing the CD, then hit play on the iPod. Sure, it's not an ideal solution, but the sound quality is certainly better than those FM transmitter things.

If you really want to get into the electrical engineering of things, it's usually not too difficult [for someone who knows what they're doing] to open up a component (amplifier or head), tap a few traces, and basically hack a line-in into the system.

Ooooh new suit. I've been plotting and planning a suit acquisition for the past several months. Pictures will be necessary!

Dude, it's for my wedding. Trust me, there'll be pictures. :-)

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