When I entered consumer electronics in 1986, I got a job in a “stereo store”.

The field we’re in is still often called “car stereo”. We install car stereos. The word “stereo” is so ingrained into our culture, we don’t talk about it much.

When I started doing OEM integration, and when I started explaining it to others, the assumption was always that we were starting with stereo – two channels, left and right, and two speaker systems left and right – which would give us a phantom stereo image in between the speakers if everything went right.

Then we came across some odd center-speaker systems.

Then we found out that Bose did some odd things.

Then we found out that Harman did some dumb things.

We have now identified six different stereo presentations used by automakers, and you should know what they are! One reason you should is that if you don’t know how they work, they can interfere with what you plan to do in your upgrade. Another reason is, if you know how they work, you can take them into account in your sale, and deliver a better-sounding system at a higher average price.

1. Stereo
2. Stereo with delay
3. Mono Center Speaker*
4. Mono Center Midrange*
5. Upmixed 2-Seat*
6. Phase Equalized 2-Seat

*Presentations using a center speaker

Let’s look at each of these:

Stereo has a left channel, and a right channel. It presumes that we have two identically-sounding speaker systems. It also presumes that we sit centered, as shown below – the speakers should be mirrored in orientation and equidistant from us.

OEMs don’t use this much. We in the aftermarket use it all the time – but that’s because we don’t mind great sound for one seat that sucks for the passenger.

The two vehicles I’ve seen test results for with OEM delay are the 2018 Subaru Outback with base audio, and the 2018 Ford F-150 with base audio. Those were both using “Driver” buttons in the menu which you could turn off, and defeat the delay.

Frankly, I hate mono centers – they don’t help – but they are usually easy to get around.

Sounds on the left channel come out the left speaker and the center speaker. Sounds on the right channel come out the right speaker and the center speaker. Sounds on both channels come out all three speakers. If that sounds like a confused presentation, I’ve explained it properly.

There are three things that are attractive about mono centers to an automaker:

1. They are cheap to do. No IP licensing, and you can run it off the front channels if you have to.
2. It sounds similar from either front seat. Automakers like that. They don’t demand excellence, but they do like symmetry.
3. It uses the speaker location in the top of the dash which is really there for the premium audio package.

Mono center speakers can simply be unplugged, as long as the center isn’t playing non-entertainment sounds (handsfree call audio, chimes, etc.) that don’t also come out of the sides. (And that almost never happens.)

I hate this one even more. All midrange – between 200 and 2000 Hz, in my testing – comes out the center speaker, and not out the sides – so the system is essentially mono in the midrange. It’s like fake upmixing. If you run into this one, you can’t get stereo out of it, so you’re either using an external preamp, or a parallel source that IS in stereo.

If your customer doesn’t need stereo, then you can use these signals. Load them properly, as that Harman GreenEdge amp has been finicky in the field.

Vehicles with this: Toyota Highlander with JBL (2G), 2020 Subaru Outback with HK.

Upmixing is when you take a two-channel stereo signal and convert it to a multichannel signal. This has most often been done for movies, and I used to think that’s all it was for – but there are now several upmixing technologies which are intended for music listening. The purpose of upmixing for music is to make seating position much less critical. Because of that, it’s very attractive for use in cars, and many premium automakers use it for this reason.

The center speaker plays only sounds which are contained in both the left and right channels. Sounds only on one channel are played by that channel, and not the center. There are differences in how this is accomplished and what happens to the sides when a sound is played by the center, but this is the essence of the upmixed result – and how it differs from mono centers.

Because of how upmixers work, the center speaker is really important – if you unplug it, you lose any information intended for the center of the stage. Many newer systems don’t allow the upmixing to be defeated, so you’re stuck with it.

Many upmixers also have two sets of rear speakers, and the second set is used to play ambiance information to help the cabin feel larger, without pulling the stereo image aft. That’s not strictly related to the center processing, but they do seem to go together in most implementations.

This one is the most complex, so buckle up!

We’re going to have to talk about phase.

Most of the time we talk about phase in car audio, we are really talking about polarity. If we swap the speaker (+) and (-) wires, we are inverting the polarity.

Any of us who’ve connected very many subwoofer enclosures with two woofers will remember what happens when we get one backwards – the two speakers cancel each other out, and we don’t hear any bass. Same thing if we accidentally wire one voice coil backwards on a dual-voice-coil woofer – the speaker doesn’t play at all.

When we invert polarity, we invert phase. Since phase has 360 degrees to it, inverting phase means we go from 0 degrees to 180 degrees.

When we sit off center, sounds intended for the center of the stage reach us at two different times. This is because sound travels relatively slowly, and we are sitting closer to one side than the other. This difference is in the neighborhood of 2 milliseconds for door speakers.

We don’t hear the different arrival times as two separate events. Research has shown that the difference between the two arrival times has to be a lot more than 2 milliseconds for humans to discern the two sounds as different sounds.

But, when two signals arrive at slightly different times, they interfere with each other in predictable ways. There are periodic phase cancellations, similar to when we wire one speaker backwards – and here is a chart showing those phase cancellations for a set of imaginary door speakers in an average imaginary car (reflections have been removed for clarity):

This is what we fix with delay in many of our audio systems – the fact that sound intended for the center of the stage sounds horrible and is often out of phase.

But using delay for this is a one-seat solution, and OEMs don’t like those.

Phase equalization does what it sounds like it does – it manipulates the phase of the signals, in this case to correct some of the cancellations caused by the off-center seating positions. (Since cars are roughly symmetrical on the long axis, the cancellations are roughly the same for both the left and right seats, and so phase equalization is comparably effective for both seats).

So, here’s the graph of phase cancellations again. This measurement is taken with both channels playing the same pink noise test tone.

Now, let’s apply an all-pass filter to one of these two channels. We will invert the phase 180 degrees at the point of the largest and lowest cancellation, in this case at 250 Hz.

Now, that first, and largest, cancellation has disappeared – and it has in both front  seats.

This use of phase equalization is the most basic we’ve found, and so I call it Simple Phase EQ. It improves the mid bass and the male vocals in both seats. It doesn’t do anything for the cancellations at 750 and above. We often don’t worry much about the cancellations above 1500 or so, because our brains don’t notice them much – we cause similar cancellations in the higher frequencies with normal small movements of our heads.

So, if the OEM uses Simple Phase EQ and we don’t notice it, and then we try to apply delay to the signal (as we often do), we end up correcting every cancellation left, but then we accidentally bring the big cancellation back!

There are a few ways to address this, and that’s going to have to be the subject of another article – but first we have to know it’s there.

I recommend you play a mono mid bass pink noise test tone (there’s one on the Educar smartphone app). If it sounds roughly centered, two-seat phase EQ is in use. If it sounds split apart, then it isn’t.

As a rule, if a system uses Phase EQ and it also uses active crossovers, don’t sum. Summing channels together which are not in phase with each other at all frequencies causes more cancellations. This chart shows a lowpass channel (red line) and highpass channel (green line) summed back together, and the resulting blue line shows the cancellations that result.

Design a system which doesn’t sum channels back together (that often forces us to use OEM-sized speakers).

So that’s the Six Stereo Presentations. If you know which one you’re starting with, you can design a system which fits. If you can’t get delay to work for you, or if your system is missing all the vocals, perhaps this explains why.

How to learn more:

Educar Integration group on Facebook  –  https://www.facebook.com/groups/EducarGroup/

Educar online courses (link)  –  https://educarlabs.com/educar-online-courses/

Audiofrog U  –  https://www.audiofrog.com/u/