Can anyone help with my DIY room correction process ?...

Discussion in 'Home Theater Projects' started by Jones_Rush, Jul 26, 2003.

  1. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Hi,
    I am running my audio system from a HTPC. I have an unlimited band Parametric Eq software (which uses fir filters and not IIR filters, so you can use it also with the main speakers and not only with the subwoofer). I would like to use this software in order to apply room correction for my listening spot, but only below 200hz (maybe even lower).

    The question is this: what is the best process to do it ?. Should I calibrate both speakers at the same time (using the same signal) ?, or, should I calibrate each speaker individually ?.

    I know that when it comes to professional RCS systems, they calibrate each speaker individually (since high frequencies are involved, meaning, both speakers won't play it at the same time). OTOH, I know that regarding dual subwoofers, the notion is that you need to calibrate them together at the same time (using the same signal) because in real life they will be playing the same signal at the same time (since low frequencies are non directional). So, what do you think I should do in my situation ?.
     
  2. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Anyone ?.
     
  3. Wayne A. Pflughaupt

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    I EQ’d mine separately first, then turned them both on for the final tweaking.

    Regards,
    Wayne A. Pflughaupt
     
  4. Jones_Rush

    Jones_Rush Stunt Coordinator

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  5. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Wayne, I've checked with Tact Audio, and their Room correction products only calibrate each speaker individually, and never together. Am I missing something ?.
     
  6. Todd Shore

    Todd Shore Stunt Coordinator

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    "...for tweaking."

    Small adjustments.

    Another reason to at least test them together is to see if cumlative output might excite any resonances.
     
  7. Todd Shore

    Todd Shore Stunt Coordinator

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    How flat is flat? With a single subwoofer, not very. Half decibel variation would be considered ideal. I wonder how many people are getting there.

    With dual subwoofers you can also have cancellation effects so testing a single channel at a time might not give you a good indication of what the summed response is.
     
  8. Jones_Rush

    Jones_Rush Stunt Coordinator

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  9. KevinJP

    KevinJP Auditioning

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  10. Pete Mazz

    Pete Mazz Supporting Actor

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  11. Stephen Dodds

    Stephen Dodds Second Unit

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    I do a similar thing to what you are talking about using the dbx Driverack 260. I also used to own a TACT.

    I measure and correct each main speaker separately, then do the sub with a BFD.

    The most important thing is knowing what you are correcting, which means an accurate measuring system, and getting the right correction values.

    There is a very good article on how to do this using ETF 5.0 on the Tag Mclaren website (www.tagmaclaren.com).

    You'll find it by going into the VIP Club and checking the TMREQ stuff.

    Steve
     
  12. Jones_Rush

    Jones_Rush Stunt Coordinator

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  13. Terry Montlick

    Terry Montlick Stunt Coordinator

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    This is a great question!

    If:

    1. You are EQing for a single listening position, and
    2. that listening position is equidistant from both speakers, and
    3. the speakers have identical characteristics, and
    4. you are hearing the near-field response of the speakers (i.e., not the room), then

    equalized from the same signal, they should have exactly the same filter settings. You can EQ them both together or separately, as long as you adjust the filters the same. It doesn't matter. The same sine wave played through either speaker will have the same phase at the listening position, so there will be no cancellation. And under these conditions, it doesn't matter whether you use IIR or FIR filters.

    Of course, if the above 4 conditions are not met, then all bets are off. [​IMG]

    I know, this seems like a cop-out, and it is. There's no particular reason for choosing your one channel over the two 80 Hz signal configuration. This situation seems to be a good argument for having a single LF channel (and a high sub crossover frequency). At least you can EQ it rationally!

    Regards,
    Terry
     
  14. Terry Montlick

    Terry Montlick Stunt Coordinator

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  15. KevinJP

    KevinJP Auditioning

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    hm supereq for winamp looks interesting. takes it a good second to take effect huh? will play with this...

    what about eq for DVD playback? I have been able to get Zoomplayer to use the directshow eq filters from SonicFoundry Soundforge. are these the good type of eq filters? paragraphic, parametric, plus 10 and 20 band graphic eqs can be used here. anyone know about this?
     
  16. Terry Montlick

    Terry Montlick Stunt Coordinator

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    OK, I'm a little slow tonight, but now I see what Jones_Rush is getting at. An FIR filter can be a linear phase device, so one COULD equalize for any listening position by adding the correct relative delay. You don't have to be equidistant from the speakers, because the right delay correction can be precisely added without messing up phase.

    But here's a wrinkle. Assume the signals applied to all the nasty mechanical stuff (speakers cones, speaker enclosures, room, etc.) start off as nice and flat in the purely electronic world. Then the effects of all this non-ideal stuff (room mode resonances, for example) are due to good old-fashioned, causal IIR (albeit electro-mechanical) analog filtering. An ideal equalizer would be an inverse filter that is similarly causal, set up to cancel the poles and zeros of the ugly, real-world processing. But this inverse filter, while appearing to introduce frequency-dependent phase shift, should actually correct it! A typical minimum-phase IIR equalizer is probably a good approximation to this.

    And a linear-phase FIR filter would NOT correct the phase problems, but simply preserve them.

    That's enough thinking for tonight. I'm going to sleep.

    Goodnight,
    Terry
     
  17. Dennis XYZ

    Dennis XYZ Stunt Coordinator

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  18. Stephen Dodds

    Stephen Dodds Second Unit

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    Jones,

    The Driverack is indeed a nice piece of gear, altho at $679 it isn't really high end in price.

    It gives 13 up to 21 bands of parametric EQ a side, although that depends how it is configured, and is also a three way digital crossover with phase and delay adjustments.

    For straight EQ the Behringer DEQ2496 might be better, and is much cheaper.

    I gave up the TACT because I built my own speakers which need to be actively triamped, and actively EQed before even starting on room EQ.

    The TACT doesn't do crossovers and I wanted everything in one box. The TACT is a better room correction system, but the Driverack can fix up the bass enough so that it is close.

    Steve
     
  19. Terry Montlick

    Terry Montlick Stunt Coordinator

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    Dennis and I are saying the same thing. IIR equalization filters correct real-world IIR effects, where time and phase necessarily change together.

    Regards,
    Terry
     
  20. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Terry Montlick and Dennis XYZ,
    I am not sure that you are right about IIR filters, maybe in theory you do (or just in basic bass eq duties), but in practice, for full range correction, Fir filters sound way more neutral than IIR filters.

    There is another thing: I've contacted more than a few software companies which create software parametric eq's using IIR filters (I had no choice, since I found none who use FIR filters), and asked them what are the advantages of IIR filters over FIR, which made them chose one over the other. What they all basically said is that FIR filters are more appropriate for eq tasks. They ALL agreed that IIR introduce audible phase artifacts, but they also never forgot to mention that the phase artifacs are no worse than the products of the competition. Most of them said that they ARE working on FIR filter usage, and are pretty enthusiastic about its capabilities, when implemented correctly. And just that you won't say that I'm talking in the air, here is one of the email correspondence, which I have currently in hand:
     

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