This section is intended as a resource for folks who are trying to decide if they should use their OEM head unit, and in determining what's really involved in doing so. I will be drawing from information I've posted in the past on other forums, specifically, a site that I initiallyused as a forum - probably too early - and am in the process of converting to a wiki.

I will make a few statements with backup data, and a few assertions without (which I will try to remember to cover with the great term, in my experience").

Can it sound good?

Yes, using an OEM head unit can sound great. There are instances where you might have some limitations. In order to predict your results, you will need to understand the output types and the ways to work with them (more on this later).

OE audio systems aren't designed to easily allow the addition of aftermarket amplifiers. There are no RCA output connectors, and the signals are usually different electrically.

Aftermarket source units usually supply a "flat" signal across the audio range - each note is reproduced without emphasis or attenuation. OE source units may supply a flat signal - but they often may equalize the signal (making some notes louder than others), filter it into separate frequency bands with crossover high-pass or low-pass filters, and change the processing dynamically as the volume changes (usually by taking out the bass as the volume climbs).

We may need to de-process the signal, and we sometimes may need to re-combine these crossover bands into full-range bands (but not always!).

However, the amount and the nature of the OE processing being performed should dictate what interface approach you take - and what gear you need.

Why do I need to de-process the signal?

The salesman told me this Bose/JBL/Boston Acoustics/Mark Levinson/Logic 7 system is "tuned" to my car's acoustics. Isn't that processing needed to make any system sound better in my car's interior?

Well, first off, you may not need to - because a lot of that processing happens in thee OEM amplifier, and the signal from the HU to the amplifier is often just fine.

Secondly, not really. What that processing is usually doing is allowing your car's crappy OE speakers and crappy OE amplifier to sound not quite as crappy - at least for a little while.

It turns out that rudimentary processing can be lower-cost to manufacture than high-fidelity speakers and amplifiers. For that reason, most OE audio systems use very-low-quality speakers and amplifiers, and process the signal to the speakers in an attempt to force them to neutralize their worst electromechanical tendencies. It makes the speakers sound less bad, but it doesn't really sound good, either - and the speakers sound worse with age.

Ever see a Bose OE car speaker removed from the car? It's apparent to anyone schooled in speaker design that the per-unit cost of that speaker has been kept as low as possible - untreated paper cones of amazing flimsiness, stamped frames, cloth surrounds, etc. After a very initial point, processing is almost free. So Bose (and others) sell low-quality speakers, which change their mechnical tendencies very early in their lives (i.e., they sound even crappier very quickly).

Very rarely is any of that processing cabin-acoustic dependent, in my experience. Even when it is, it is based on the assumption that your speaker tendencies, locations, crossover points, and number of speaker drivers are all the same as the OE system. That's never the case. If the speakers are upgraded, and the OE processed signal is left alone, the new speakers often sound far worse than they should - so you are better off eliminating as much OE processing as you can.

I heard that if I'm doing an OE interface installation, I have to use a < Product X >, no matter which car I have - it's the best, and all cars need one.

Well, that's not really correct. I've heard that too - I've been told by certain manufacturers' employees that their OE interface device is needed by all cars.

This usually means that the person speaking has a hole - either in their OE interface knowledge or their ethical boundaries.

There is no OE interface on the market (as of this writing) which should be in every OE interface installation. (I include the JBL MS-8, even though it has value other than as an OE interface). Many OE systems can be interfaced with, with great results, with little or no special gear or widgets.

I've been told that Product X "restores" and "cleans up" the signal coming out of an OE source unit.

Yeah, not so much. None of these processors take anything out as far as background noise or distortion goes. If the OE source unit sends out a distorted, noisy signal, the OE interface will pass that noise and distortion through. If the input of the interface device is easier to drive than the OE amplifier or speaker, then the distortion may decrease to some degree.

But a lot of system noise - especially RPM-related whine and constant background hiss - are not coming out of the OE source unit. They are artifacts of the system's signal path (for instance, a ground loop is created between two components in the signal path - a single component doesn't generate a ground loop all on its own). There may be a input sensitivity ("gain") mismatch, there may be a ground loop, and the OE interface device may play a role in solving that particular problem. (Sometimes they make it worse, I'm afraid).