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Thread: Two seat tuning guide / tips

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    Two seat tuning guide / tips

    Hey all,

    Do we have a list of resources, videos, guides, tips for doing a two seat tune with a physical center channel and processor with an up mixer?

    If not, can we put something together with this thread?

    I’d love to start learning as I start acquiring the gear I need to do this. Information seems pretty sparse but things I have questions about are:

    1) do you do any time alignment on the front stage?

    2) how are all pass filters used? Is this done to address the phase issues that come up because the speakers aren’t time aligned? If so, what are people’s strategies for identifying these issues and the correct filters to address them?

    3) the microphone is put in the driver seat and then in the passenger to confirm?

    4) how are virtual channels best used (for the helix processors)

    It seems like we have a ton of great resources for one seat tunes and it would be great to start collecting tips / creating a guide for two seat tunes.

    Thanks!


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    Re: Two seat tuning guide / tips

    For example, here is one tip from I think Doug from Audiotec distributor in North America on Facebook with regard to the helix up mixer :

    “It is an upmixer, it just doesn’t affect the left and right channel natively Its better when you subtract 20% of center from left and right. Virtual Channels are pretty killer. “

    I am interested in if anyone has tried this and how exactly this was done. So the center was created with the up mixer or Real Center feature and then routed into a virtual channel for center, and then 20% of this center channel information is subtracted from the L and R channels and routed to L and R virtual channels?


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    Re: Two seat tuning guide / tips

    Another example from Skiz on Facebook:

    “done it a few times. Using the helix processors and a bit of math and measurements, its not that bad. But having 2 people makes it quicker and a bit easier “

    What are the sort of measurements and math that help you do this?

    Is the allpass usually 1st order? 2nd order?

    Would love to hear your process / thoughts on how to approach it.


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    Noob Ge0's Avatar
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    Re: Two seat tuning guide / tips

    Good thread. Exactly the questions I was about to start asking. Subscribed.

    Ge0
    Scanspeak - Dynaudio - Helix - Bose

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    Re: Two seat tuning guide / tips

    Here is another tidbit I found on Facebook about someone explaining a center channel:

    “in a car with an actual centre channel, you use no time alignment and level matching and the centre channel plays only parts of the music which is common to both left and right speakers, this then doesn’t create sound spread it creates a focused centre using processing and gives a centre image from both front seats dues to sound being physically created in the centre of the car”

    So, it sounds like you don’t level match the front stage?

    This kind of makes sense to me because if you level match to your position in the drivers seat, the speaker near the passenger seat is going to sound much louder to the person sitting in that passenger seat.

    With that said, how are the levels managed?

    I could see maybe measuring the L side of the car from the L side and level matching the left side as you would with a one seat tune.

    Then you could do the same on the R side when sitting in the R side. You could even match up those L and R curves to each other. In a way this would be level matching the L and R side but the key being that you measured each side from the same “relative position.” - NOT the same absolute position (the driver seat for example.

    I imagine you would want to make sure that your install has the speakers aimed such that when you are sitting on the L side or the R side, the angles that each sides speakers are playing (their axis) is the same.

    I think this might be some of what Nick is saying with a bit of math and measurement, but this is speculation and me trying to figure this out.

    Now the big question, among many, is where does the center channel fall into this? I imagine you could boost the level of the center a bit and at the higher frequencies where IID dominates, this would pull the image to the center.

    For the lower frequencies where ITD dominates, it will be critical for phase to align as that will localize the image to the speaker that is out of phase (or both); like when our sub is out of phase with our mid bass - we hear the bass localize to both the door and trunk when we really want it on the dash.

    So, I’d imagine you could then add the center channel to each sides tune and measure and fix the phase issues (that will show up with constructive/deconstructive peaks and valleys) and focus on addressing the phase issues with all pass filters at ~2kHz and below (or at least 500 and below) and then focus more with levels at frequencies above 2 kHz to get the center image where you want it.

    The above is my guess on how it might work.

    Anyone, please chime in


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    Re: Two seat tuning guide / tips

    When the real center functionality on the helix processors came out Doug described his process for time alignment as such:

    “Two way active plus a center and a sub is exactly how I have my Kia set up.

    As far as setting up for signal delay, I prefer to align everything to a spot directly between the headrests.”

    Is this how those with experience do it?

    Anyone doing it differently?


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    Re: Two seat tuning guide / tips

    Quote Originally Posted by mauian View Post
    I am interested in if anyone has tried this and how exactly this was done. So the center was created with the up mixer or Real Center feature and then routed into a virtual channel for center, and then 20% of this center channel information is subtracted from the L and R channels and routed to L and R virtual channels?
    Yep. You can play around with the % that is subtracted.

    As far as time alignment for 2 seat tune, I abide by the school of thought for time aligning all drivers in the system including the center channel itself to a center point between both headrests.

    As far as levels I tend to personally like the center channel to be 3 to 6 dB louder than the R and L channels. But if you go too high it distinctly/artificially localizes (similar to turning the sub level too high localizes to the rear).

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    Re: Two seat tuning guide / tips

    Quote Originally Posted by Bnlcmbcar View Post
    Yep. You can play around with the % that is subtracted.

    As far as time alignment for 2 seat tune, I abide by the school of thought for time aligning all drivers in the system including the center channel itself to a center point between both headrests.

    As far as levels I tend to personally like the center channel to be 3 to 6 dB louder than the R and L channels. But if you go too high it distinctly/artificially localizes (similar to turning the sub level too high localizes to the rear).
    Awesome. Very helpful.

    So the process is something like:

    1) set TA using point between head rests
    2) tune individual drivers to match their target curves from drivers seat

    Then this is where I am (more) unsure:

    Do you
    3)keep the R and L channel levels (as set in the dsp) the same but try to match the shape of the frequency response curves for driver pairs (L vs R); same for the center channel but it’s level is going to be a bit higher

    Or,

    Do you try tuning the whole L side from L side of car, then whole R side from R side of car, then center channel alone (from the drivers seat), then (while in the driver seat) play L and R together and address phase issues (and ignore level matching issues), then add the center and address phase issues, then use levels to have the center be a bit (3-6 dB) higher than L or R?

    Sorry, just trying to understand the process as people say it’s very similar to a one seat tune but there has to be a few key differences as you can’t level match the R, L, and C channels from one measurement position as you would in a one seat tune (can you?)

    I’m wondering about the order in which people tackle each driver, then L, R, and C channels together and where the measurements are taken when doing each of these steps.

    Thanks again!


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  9. Back To Top    #9

    Re: Two seat tuning guide / tips

    Well this where threads like this could/should be awesome. Somehow though it seems like the cat bites everyone’s tongue when it comes to sharing. I’m glad you started this here and I’m hoping many of the people who will view and read this thread, actually contribute.

    That said, there doesn’t seem to be 1 way to skin the cat as they say.

    In my observation, a properly up-mixed center channel is most useful for doing 2 seat tunes, but they can also be applied to one seat tunes as well with great results. No stereo recordings were not mastered with center channel in mind, and yes it will not be so successful for some songs. Algorithms are not perfect but they can be pretty damn close.

    When I do a 1 seat tune with center channel:

    Before any upmixed signals and matrixed signals are applied, I use plain R signal, plain L signal, and R+L mono sum for center to perform measurements. I level everything to the listening position so that L, R, and C measure at the same level. I T/A each driver to the listening position. Then perform the EQ method you subscribe to. Some do each driver individually, then link the entire L side to EQ, then link entire R side to EQ, then link all the drivers that make up center to EQ. Some may also then link everything together L, R, & C to perform house curve EQ for the entire front stage. Probably where I assume most are implementing all pass filters. But let’s see what people contribute

    Once I have my drivers tuned like instruments to the listening position, I give them the notes to play for the refinement adjustments. So I remove mono sum R+L for center and enable the upmixed center channel. Play around with center channel levels in relation to the sides and pretty much go by ear as to what I prefer. Then the same with subtracting center info from R and L. Play around with % until you are pleased. I wish I had a more scientific methodology but it’s going to be very subjective at that point. Keep in mind what Doug mentioned in regards to the Helix Ultra with Real Center only affecting the Center channel. Any matrixing and subtracting percentages of center from the R or L will be static adjustments on the Helix. I believe (correct me if I’m wrong) true multi channel upmixers (PL2, L7, and Penteo) are even more sophisticated than what RealCenter does in the sense that they will attenuate the L,R, and C levels actively on the fly based the signals fed into the algorithm.

    For a 2 seat tune with center channel, I pretty much do the same as above but all the measurements are taken at that central point between the seats (theoretical listening position).
    Last edited by Bnlcmbcar; 06-23-2020 at 09:39 AM.

  10. Back To Top    #10

    Re: Two seat tuning guide / tips

    I'll contribute, for the people that will actually benefit from and attempt to apply this information. Two seat tuning is a balance of compromise.

    1) Delay should be used to align groups of speakers that are on the same side. Example: Left midbass 50in, Left midrange 40in, Left Tweeter 30in. You still need to align them with each other to arrive at the same time. This is most easily done by entering the distances to a point in between the headrests. However, you do not use delays to align the left and right side. Doing so will create a biased listening experience in only one seat and ruin the opposite seat. Aligning the sub to midbass using delay is still necessary as well.

    2) All pass filters are used to compensate for the destructive interference caused by the path length differences of speakers between the left and right. Example: Path length difference of left to right side is 20 inches. There will be a frequency where half the wavelength will be 20 inches. Half a wavelength = 180 degrees out of phase = Complete cancellation. That is the frequency where you apply a 2nd order all pass filter to just one side. What you've effective done is correct the phase cancellation in both seats at the same time, since the path length difference is 20 inches from both seats. This has a similar effect as time alignment (not quite as exact) Which side you apply it to, and what the Q should be, I can't answer accurately at the moment.

    3) The mic should be placed at the drivers seat, then moved to the passenger seat. The two measurements should be then averaged. Huge difference compared to placing the mic in the point between the headrests. Matching the left and right side's frequency response from both seats is impossible, as one change will skew the opposite listening position. The main objective of this step is overall level matching, problem area cutting, and global tonality adjustment. You and your passenger aren't going to listen with your heads smashed together in the middle of the car while you're driving. You want to mic to pick up the unique acoustical behavior at each headrest (reflections off the side window, etc). Luckily, the center channel will have almost identical response from either seat because the reflections are identical.

    4) You can use the Helix Virtual Channels to remove Mono (Center) information from the left and right virtual outputs. If you don't attenuate it, you will likely hear the mono information coming from the speaker closest to you because it's too loud (improper imaging). You want to center channel to dominate the center info and pull it to the center of the car. All you have to do is sum the Left channel with Center flipped in polarity (Maybe 80% left and -20% center), with real center FX turned on). This will remove some center information from the side channels.

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