Missing polarity switch on 8331 / 7350 - Forum
Missing polarity switch on 8331 / 7350
I recently did some measurements to verify the automatic phase settings chosen by GLM between my 8331 and 7350. What I discovered was quite disappointing from my point of view.
The "phase shift" in the 7350 (and probably 7360 etc.) actually is a simple delay. It only adds about 0.5ms per 15 degree step. And since it's not possible to add negative delay, the steps -15 till -165 add even more delay. From 0 degree..90..180=-180..-90..-15 it's adding up to 12ms. Of course for the exact crossover frequency this will result in the desired phase shift and maximum addition. You can hear this during calibration. But that's about all it does. And two things keep me worrying here:
1. if your sub is slightly behind and all you would need is a minor delay of your main monitor, then the attempt to correct it with a phase shift of -15 degrees in the sub results in a delay of 12ms. This is in phase, but a complete cycle too late compared to your mains. I guess people who buy the 8331 and 8341 love their excellent timing and transparency. I cannot imagine anybody wants to ruin this on purpose.
2. even if the crossover frequency is in phase with the added delay, due to the added cycle we now have comb filtering below and above that crossover frequency. Due to the steep filters this is not very audible, but still, why ruining your frequency response around the crossover frequency? The 8331 is a master piece of engineering, and then the crossover is treated with such disrespect?!
Well, I went on even further and tried to fix the problem by adjusting the necessary delay inside the 8331. I added 3.1ms while both 8331 and 7350 were standing exactly next to each other. Then they were perfectly aligned to each other at 80Hz. But now there is an even bigger problem: while the Low Pass in the subs hasn't changed its polarity at the crossover frequency, the 8331 have a 180 degree phase shift when you add the High Pass Filter. This means even if they are both perfectly time aligned, there is also a perfect cancellation at 80Hz. Even worse than what we got before.
This leads to my question: why is there no polarity switch for the subs? Especially knowing that the built-in crossover design of the 8331 makes this absolutely necessary! The 180 degree setting in the phase settings only adds delay, but does not invert the audio signal.
A simple button would solve a lot of the before mentioned problems. And then users could at least setup their speakers manually, getting perfect results.
Anybody from Genelec who can advise here?
I have been very happy with the sound of the 8331 so far, but without a proper sub integration this makes me quite unhappy with my investment. I hope for a quick firmware update.
I had to consult our R&D engineers, and the response I received is below:
Autocal does not correct the subwoofer phase by adding a delay to the speaker, as additional delay is even more harmful to many users. If there are more subwoofers, then the second phase inversion will result in reversals between the subwoofers. Because of that, it has not been used either. Of course, the user interface could have an adjustment for the user, and we appreciate your feedback and we can consider this for the future. There is also group of people, for whom the absolute phase of the subwoofer and the speakers is important and the polarity can't be reversed.
Currently the subwoofer phase can be reversed from XLR cable if required. If digital signal is enabled, then the subwoofer must have its own channel where the phase is reversed from the sound card.
Customer Support Team
Thanks for your quick reply, Jani!
First of all, I appreciate how simple and user-friendly the whole automation in GLM works and how great the result gets for the average user! So my whole point is not to discuss the software nor what "most people" expect regarding phase. Totally understand that nobody wants to switch polarity on a sub without a reason.
Regarding your comment: well, if you suggest using a phase-reverse XLR cable then you obviously agree that a 180 degree phase shift in the software does NOT apply a 180 degree phase shift. Otherwise you would have suggested the software option, wouldn't you? :) And that is exactly what I could verify with the measurements. The phase does not change at all, only delay.
And yes, I'm using full digital. And of course my CD-player and laptop do both not have a second digital output that I could phase reverse.
The only option that I see to setup the 8331 with the 7350 properly is to use the 8331 full range. I already tried using a low shelf filter to get some lows cut, but the Q of the shelf filter is way too low. Lowering -6dB at 40Hz already starts way above 100Hz. So only a normal peak filter could help here.
Once again a phase reverse switch like all subwoofers on this planet have for decades, would easily solve this issue. Or you change the design of the high pass filter and don't reverse phase at the crossover frequency..
Thanks for your support anyway, Jani! I appreciate having this forum for direct communication!