How does this Pioneer car head unit know the RPMS?

[Rattlehead02]

My friend just got a new kick ass head unit for his 2003 Hyundai Tiburon. It has a volt meter for the car’s battery AND an RPM gauge.

http://www.pioneerelectronics.com/pna/press/release/detail/0,,2076_4313_291832622,00.html

Vehicle Dynamics Gauges – Further utilizing the OEL technology, the players feature basic vehicle dynamics gauges for cool visual effects. The three modes are:

* Tachometer meter – RPM graph showing peaks and valleys
* Triple gauge display – RPM, voltage and sound meter simulates analog gauges
* Running tachometer – Digital representation RPM, voltage and wattage

How does it know the RPMs? It was installed at Best Buy. Does it have a connection to the car’s computer? Does it judge based on engine noise? I’m pretty knowledgeable about electronics and car stereos, but this has me puzzled.

Thanks,

Jeff

[Baron]

I’d think that it would be able to detect the feedback from the ignition system, but this is just a WAG.

[Rattlehead02]

Would that require tapping directly into the ignition system? Or can it detect it from the power off the battery since it is all part of a big web of connections?

[Absolute]

You got the ‘noise’ part right, just not the type of noise.

It uses electrical noise in the car’s power supply, caused by the alternator. The alternator produces AC current, which is converted to DC. The conversion is imperfect, and produces identifiable patterns in the output waveform that change depending on how fast the alternator (and therefore the engine) is turning.

You have to calibrate it, and it doesn’t work with all cars.

EDIT: I have also heard of the ignition system being used, but that does not work for diesels (which do not use electrical ignition), while this unit claims to support those as well.