Hi everyone,

I've used the wonderful Time Alignment feature for years. I just got my empeg installed into the new car (a retired cop car), and am using the rear output to drive the sub. Time alignment will thus send out-of-phase signals to the sub, according to the guy who installed my sub and sub amp.

Is there any hijack option to have time alignment adjust front outputs only, and leave the rear in phase? I tried search, but couldn't find anything.

Thanks in advance,

Jim

Your installer guy is right about the time alignment sending an out-of-phase signal to the sub. That's all that the time alignment feature does, it delays one channel so that it's slightly out of phase.

I'm reasonably sure that what you're asking for can't be done with the current empeg hardware. Here's the reason:

1. The MP3 gets decoded into two channel (left and right) wave audio.

2. That wave audio is then sent to the DSP, which is an ordinary off-the-shelf car stereo DSP, so it has only two inputs (left and right).

3. The DSP is what controls the front/rear balance and the equalization, and then sends the audio to the outputs.

I'm pretty sure that the time alignment trick is done by the kernel between steps 1 and 2. It simply delays one of the two audio channels slightly.

Since there are only two audio channels that can be worked with by the kernel or the player software, then it can't separate it to have the fronts do something different from the rears.

The only things that you can do differently between the fronts and the rears are the things already built-in to the DSP, i.e., the fader and EQ features.

All is not lost, though.

Despite what your installer guy said, it might not be that big of a deal. The *only* thing that having the left and rights being out of phase does is reduce the volume of the bass. The worst thing you'll need to do is turn up the subwoofer a bit louder, and maybe readjust the subwoofer volume each time you readjust the phase. And you might not even need to do any of the above. I say, just try it: Listen to some music and alter the time alignment. If you notice the subwoofer volume seeming to decrease, then you're hearing the thing your installer warned you about. But you might not hear any change at all, the phase difference might be quite small.

The truth is that the thing I just said happens anyway, with any audio signal, with or without electronic phase modification. As you change your position relative to a pair of stereo speakers, you're getting the same thing.

There will be "nodes" in any stereo pairing where you'll be at the opposite phase for any given frequency. It's most noticeable in the bass frequencies, of course... Once I had accidentally mis-wired one of my two rear speakers in the car and wondered why the bass response sucked back there. As I moved left and right in the back seat, I could hear the bass getting louder as I moved to each side. After I fixed the wiring, then the bass was louder in the middle and softer on either side. So it was six of one half dozen of the other.

For that matter, the subwoofer gets out-of-phase signals all the time anyway. All music recorded in stereo is a constant stream of out-of-phase signals. It's (partially) those differences in phase that make us perceive a 3D image at all.

So just try it, it might not be as bad as you're worrying about.

If all the feature does is delay the audio then it would be logical to assume that only one channel ever has delay applied to it. Depending on what delay setting you've configured, connect your sub to the channel that is not delayed.

This makes the incorrect assumption that all low frequencies on an album are panned center. That's just not the case.

Example: Sometimes the toms on a drum kit are panned in the stereo field, so that when you get to the low, boomy-sounding floor toms, they're off to one side at least somewhat. And the choice as to whether to pan them as drummer's POV or audience's POV is a subjective one, resulting in different results in different recordnigs.

...to a different extent at different frequencies, though, right?


But, especially inside a complex shape such as a car, reflected sounds and so on mean that nodes aren't very sharply defined: the bass got softer in the middle of your back seat, it didn't disappear altogether. With misaligned phase going to the subwoofer, certain frequencies (the ones whose wavelengths are half-integer multiples of the path difference) will be at zero volume throughout the car. For instance, if your channel delay is 4ms, corresponding to about 5ft of sound path difference, then your subwoofer's frequency response acquires utter dropouts at 125Hz, 375Hz, 625Hz, and so on.

Or is the answer that in practice these dropout frequencies are so high that they aren't being emitted by a subwoofer to start with?

Peter

You have a point... The signal to the subwoofer would be precisely out of phase in a very pure fashion, so the nodes would be very sharply defined. But only on purely mono material and only in those sharply defined frequency bands. I suppose most bass guitars and kick drums are going to fall into that "pure mono" category, so perhaps it will be an actual issue, then.

If that turns out to be the case, I say just set the time alignment to zero and be a cowboy about it. Most folks' car stereos don't even have the time alignment feature; they just live with slightly-out-of-phase front speakers "AND THEY LIKED IT!"

I would've said the only way to do it properly would be to do time alignment externally and tap the sub off before the players output reaches the processor.

Of course 125Hz is pretty high, you'll probably find that it's near the upper limit of low pass filters on most sub amps.

I wrote the time alignment stuff initially and Tony's right. Mark did re-write some of it for speed, but I'm sure he didn't change the concept - there really is no way with the current hardware.

One solution is to go mono - that is pick one or the other channel and use it for the sub. There is not much direction in sub level bass, so either side is OK. You could y-split the input if you needed a stereo feed in to your amp.

The side-advantage of this is that you get to choose which phase (==channel) you want. You know that one is x msec off the other, and the sub tuning may benefit from the delay or advance.

Just finished a eq tune in my Mini using white band noise, ear-omerty and fine tune with my favourite music. The cabin is so small and my head is right in a trough between my front 6.5" and my rear 6x9s :| Was hoping I could get front-rear adjustment.