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Thread: OEM signal analysis - "The 3 Questions"

  1. #1

    OEM signal analysis - "The 3 Questions"


    Aftermarket head units that would be used in a sound quality system all have RCA output connectors, carrying common-ground preamp signals, flat and full-range (barring internal crossovers). Some have other kinds besides, but practically all do have RCA outputs.

    RCA connectors are intended to carry a common-grounded signal. That is, the left outer shield of the connector is electrically common to the right outer shield of the connector. the left center pin and the right center pin are not common, they are separate conductors. So, a common-ground stereo signal has left (+), right (+), and signal ground.

    No OEM system in any mass-produced vehicle uses RCA connectors, either for the head unit output or the amplifier input. So you can't just plug in an aftermarket amplifier to the RCA outputs of the head unit.

    Some OEM systems use a common-grounded signal. Others use a balanced signal, with (+) and (-) signal legs on each channel. Some OEM amplifiers take speaker-level signals in, and amplify them again.

    As it turns out, there are three different aspects of the OE audio system that you need to understand in order to have predictable results with your installation:

    - Signal Type

    - Signal Range

    - Signal Response

    Signal Type can be balanced, or common-ground, or high-power speaker-level. Each requires a different means of interfacing, and can influence your amplifier choice.

    Signal Range is relative to the established 20 hertz to 20 khz audible range. A full-range signal contains every note between those two. A partial range system usually has the notes above a certain point on one set of outputs, and the notes below a certain point on another set of outputs.

    For instance, you could have a system where the high-pass channel has the notes above 80 cycles, and the low-pass channel has the notes below 80 cycles. For some systems, I would call that a partial-range signal with usable crossover points (many systems would work fine with a crossover point of 90 hertz, and if you were installing one of those systems, you could do so without necessarily having to sum those channels back together).

    However, in another example, there are partial-range systems with high-pass and low-pass crossovers at 300 hertz. Few aftermarket systems would use such a crossover point - so most systems would require some way to "knit" those signals back together into a full-range signal. I would call those systems "partial-range, unusable".

    So the possible Signal Types:

    -- Common-ground
    -- Balanced preamp
    -- High-power speaker level

    Signal Response refers to any emphasis or attenuation of certain notes over others. It's also called "equalization", but don't confuse it with an equalizer function that has user controls - it's not user-adjustable.

    In my experience, OEM equalization is usually intended to make the OEM speakers sound less heinous, and as soon as you upgrade the speakers (and sometimes even if you amplify the OEM speakers), that response curve is not appropriate, and you should defeat or "normalize" it somehow.

    I define the three kinds of Signal Response as:

    -- Flat (perfect)
    -- Fixed Equalization, constant with volume (allows OEM volume control retention)
    -- Volume-dependent Equalization (may require elimination of the stock volume control in order to keep the OEM equalization fixed)

    Note that OEM interface devices come with their own volume control, as a rule. That control only NEEDS to be used when the OEM processing changes when the value of the OEM volume setting is changed. In those systems, you need to set the OEM volume to one setting and leave it there, so your signal-reversal processor can hit a non-moving target. In the first two cases, take that volume knob and throw it away.

    Note: Auto-loudness can fool you here. What is "Loudness"? The Fletcher-Munson loudness curve is an equalization curve intended to make a speaker system sound "right" at lower volume levels, where our ears are more sensitive to midrange than to bass and treble. Whenever you see a Loudness button, it's intended to be used when the system is played quietly (one of the biggest counter-intuitive terms in electronics).

    Many OEM head units have preamp ICs that, in addition to handling volume and bass and treble and balance and fader duties, have an auto-loudness feature in them. When the volume is set low, bass and treble are boosted. When volume is high, bass and treble are flat.

    Auto-loudness is NOT harmful in my opinion. It more or less duplicates how we hear. However, determining its presence is a judgement call, and requires you to take your measurements at various volumes, and then to interpret the results. An "Auto-Loudness" EQ curve, for example, usually doesn't follow the Fletcher-Munson measurements too closely - they are usually more or less a happy-face curve.

    Fletcher-Munson curve

    Sony preamp IC auto-loudness EQ curve

    For example, it's my determination that the 2004-2008 Acura TSX head unit has a flat signal output, with an auto-loudness feature. I'm aware that NPDang reported a slightly different result. I've measured a number of these, and the output above a certain level is certainly flat.

    Acura TSX Gen 1, at lower volumes:

    Acura TSX Gen 1, higher volumes:

    Note that the nonlinearities in the bass region are due to the random nature of pink noise and the lower number of samples available with low-frequency information. This picture is of a test of the electrical signal coming out of the head unit, using a 1/3 octave NT Acoustilyzer.

    Sometimes there are ways to change the head unit's output to a flat one without an external processor. I have experience at this in certain German cars, and there may be more tricks up other sleeves as well.


    There are three questions you need to answer. If you don't know the answers, your OEM integration is a crap shoot, and you can't predict your results. If you know the answers, you can plan your install properly, and you can predict your results.

    - Signal Type (common-ground, preamp balanced, high-power speaker level)

    - Signal Range (full-range, partial-range usable, partial-range unusable)

    - Signal Response (flat, fixed equaliation, dynamic equalization with volume)

    Last edited by VP Electricity; 09-24-2010 at 03:58 PM.

  2. #2
    Founding Member
    Join Date
    Sep 2010
    VP, I can't see three of the images because I'm not registered at (I guess). could you upload them to MSS?

  3. #3
    My apologies for goofing that. Let me know if the initial post is fixed, thanks.

  4. #4
    Founding Member
    Join Date
    Sep 2010
    now I can see them. thanks!

  5. #5
    Founding Member
    Join Date
    Sep 2010
    and fantastic post, BTW.


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