Speaker distance question when calibrating

Discussion in 'Archived Threads 2001-2004' started by Rhett_Y, Mar 25, 2002.

  1. Rhett_Y

    Rhett_Y Screenwriter

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    Hi all..

    I have a couple of questions on calibrating my system.

    1st-Do you actually tape measure your speaker distance from the tv and then enter that into your setup?

    2nd-referance level, does it have to be set to "0" on the receiver, or can it be say "-10" etc. My receiver goes from -50 to +10

    3rd-on the sub, what volume do you set it to, right now I have mine at 50%?

    Thanks

    R~
     
  2. John Doran

    John Doran Screenwriter

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    rhett,

    1. measuring the distance from the speakers to your listening position is NOT necessary for proper calibration - all that matters is that each channel measures the same SPL fromthat position.

    2. you can measure reference level at whatever point on the volume control you want - just remember it so you'll know when you're actually at reference, or when you're above it or below. people often calibrate at "00" just for convenience sake.

    3. as long as the sub-level is calibrated to be equal to the SPL of your other channels, you can run the sub's volume at whatever level you like.

    some around here recommend actually reducing the sub-level on your receiver quite substantially (more than halfway) and compensating by turning the volume on the sub UP when calibrating. doing so will allegedly give you better bass response.

    but try both ways and make up your own mind which sounds better to you.

    hope this helps.

    - jd
     
  3. Jeremy Anderson

    Jeremy Anderson Screenwriter

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    I have to absolutely DISagree with John on point 1. Proper speaker distance settings are crucial to calibration, since they determine the delay times applied by your receiver. The point of this is to ensure that sound from each speaker arrives simultaneously at your listening position. If all of your speakers are equidistant from the listening position (which is the ideal situation, but next to impossible in most rooms), then you don't need to bother with speaker distance/delay... however, since most rooms do not allow for equidistant speaker placement, the delay settings on your receiver allow your receiver to effectively make it sound like all your speakers are at the same distance from you.

    The rough formula for this is that 1 foot difference in distance = 1ms of delay time. For instance, in my setup my mains are at 12', center at 11' and surrounds at 9'. Therefore in my case, I am adding 1ms of delay to the center channel and 3ms to the surrounds so that the sound from those channels arrives at the same instant as the mains. To simplify things, most receivers now let you put in the distance of each speaker in feet or meters as opposed to the # of ms delay, but both ways will achieve the same effect.

    If speaker delays are not properly set, you will likely hear many problems with your system. Dialogue can sound muddy when shared between the front channels, center channel data can sound overly distant and unintelligible (which is one of the primary reasons some people overcrank their centers), and sound from the surrounds can seem to have a reverb effect. These settings are especially important to Dolby Pro-Logic II, since the simultaneous arrival of sounds at the listening position is so crucial to the proper effect.

    In short, YES... sit at your listening position and have someone drag the other end of a measuring tape across your room to each speaker, then set your receiver accordingly.
     
  4. John Kotches

    John Kotches Cinematographer

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    John D,
    As Jeremy pointed out your statement 1 is grossly incorrect. Then again your statment two indicates you aren't particularly familiar with how a THX-certified receiver (or preamp/processor
    The reason for time alignment (aka speaker distance) being important is twofold:
    1) Maintain timing integrity for events in surround sound modes (although this is equally valid for stereo as well).
    Verbose mode on
    Let's assume you have an event that's supposed to happen in all 5 speakers simultaneously, but your speakers are:
    7.5' front L+R
    7.0' CC
    5.5' Surrounds
    6.5' Subwoofer
    Without correcting for time differentials this coincident sonic event will occur across the span of 2 milliseconds (give or take) -- not a good thing. Psychoacoustic research has shown that we have sensitivity to < 1 millisecond for timing differences.
    verbose mode off
    2) Correcting phase on "small speakers" so that the low pass accounts for phase discrepancies due to distance differences of the small speaker and the subwoofer.
    Your 2nd statement with respect to reference level is at best misleading, and possibly just plain wrong depending on the product in use.
    THX mandates the following for speaker calibration:
    1) Master volume control is disabled.
    2) Main speaker levels are at -30dB FS (30 deciBels below the loudest possible signal). Subwoofer level is -40dB FS.
    So, the behaviour of a receiver/processor is dependent on a variety of things, including whether it is THX certified or not.
    Further, with respect to volume controls, THX allows for relative values to "reference level", which you calibrated to earlier. When using a receiver/processor with such a volume control, values are listed from -x.xdB to +x.xdB. I have seen non-THX products adhere to these standards as well.
    It is possible to use relative values on the volume control in dB and not be relative to THX/Dolby reference levels. The numbers can be relative to unity gain (aka 1:1). In this case, the volume control for - indicates input attenuation, 0.0dB would indicate unity gain, and + would indicate gain is in use.
    Regards,
     
  5. Rhett_Y

    Rhett_Y Screenwriter

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    So when measuring I measure distance from my speakers to my sitting spot correct?

    Thanks

    R~
     
  6. John Kotches

    John Kotches Cinematographer

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    Rhett,
    Yes, you measure to the "best seat in the house". The tradeoff (and you probably knew this was coming) is that every other seat will be slightly misaligned [​IMG]
    Regards,
     
  7. Bob McElfresh

    Bob McElfresh Producer

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    Hey fellas. You guys are a bit harsh correcting John D's advice.

    We have had a bit of trouble lately with people who ENJOY finding fault with others and telling them why they are wrong. These people are now banned.

    I can think of many ways to say:

    "Time alignment is important because ..."

    rather than

    "..you aren't particularly familiar with.."

    The second example is a bit of a slap, and will now inhibit others from offering advice for fear of getting something wrong and getting jumped on.

    Help us keep HTF a flame-free zone where people are not afraid to post advice/opinions.
     
  8. LowellG

    LowellG Stunt Coordinator

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    I have a question to tag on to this if you don't mind. I can make my surround speakers the same physical distance from my listening position. However, the way I have to run my speaker wire around the room makes my right surround speaker wire about 15' longer than my left speaker wire. Do I need to compensate for that.

    Thanks, Lowell
     
  9. ColinM

    ColinM Cinematographer

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    No need to compensate for long wire.

    Wire = nearly the speed of light.
     
  10. Roger Kint

    Roger Kint Stunt Coordinator

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    The speed of sound is 1125ft/sec which means it takes 1.125 microseconds for the sound to travel 1 foot. Setting the delay is important for ITD (Interaural Time Difference) localization. ITD is how our ears figure out where the low frequency sound is coming from due to the difference in the time that the sounds reach us.
     
  11. John Doran

    John Doran Screenwriter

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    john and jeremy,

    you are right - i had (perhaps mistakenly) assumed that each of the speakers was equidistant from the listening position, as it is in my setup. if there ARE inequalities in the various distances, then correcting for them is certainly critical.

    i thought the question was whether there was just some independent value in knowing how far you were sitting from the speakers (given that you were sitting equally distant from each).

    john,

     
  12. John Kotches

    John Kotches Cinematographer

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    John,
    THX mandates that the master volume control be disabled during speaker calibration.
    You have the ability to trim an individual channels output level to match the others, with some allowing +/- 10dB others allowing +/- 15dB. Are you confusing channel trim with volume?
    While I'm not the biggest fan of some aspects of THX' Certification program, the standardization of speaker level calibrations are a good thing.
    With respect to time alignment, there are a very small number of rooms that can accomodate equidistant speakers from the listening position. I wish mine could, as it is I have the Front L/C/R equidistant, Rear R/L and subwoofer equidistant.
    Regards,
     
  13. John Doran

    John Doran Screenwriter

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    john,
    i have no idea how to disable the master volume of my receiver.
    does having it disabled somehow affect calibration? i mean, since i didn't have it disabled when i calibrated my HT, is my system now miscalibrated? i'm not sure how it could be (it's difficult to understand how getting 105db at -10 on my receiver could be "wrong"). but then there's certainly all kinds of stuff i don't know, and i'm always looking to learn.
    at any rate, all the stuff i've ever read about calibration has never mentioned it before - it has always stipulated that any point on the volume control can be used, with the result that, on your receiver, reference level will be at THAT volume level. it's at -10 on mine.
    be that as it may, i was simply trying to provide the same basic answer to the same basic question asked all the time on this and other forums - even if there ARE differences in calibration with regard to THX certified receivers, no one (or almost no one) else ever mentions them in "how do i calibrate?" threads, and i've never seen them take any heat for the ommission....
    as far as time-alignment is concerned, i know it's a significant issue, and my failing to mention it was just a (gross) oversight on my part. :b
    i'll try harder next time.
    - jd
     
  14. John Kotches

    John Kotches Cinematographer

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    John,

    You don't have to disable the master volume control during calibration, the system does it automatically.

    All that should be accessible during calibration is channel level trims, as the system outputs a -30dB FS (covered earlier) signal.

    Channel trim gets an individual speaker levels calibration to correct.

    This assumes you are using something that's THX badged, sorry don't know what you're using.

    As far as reading 105dB at -10, LFE output should be 105dB at the listening position at -10dB. This will far outweigh the other channels.

    Regards,
     
  15. Phil Iturralde

    Phil Iturralde Screenwriter

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  16. John Doran

    John Doran Screenwriter

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    john,

     
  17. Rhett_Y

    Rhett_Y Screenwriter

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    One more question, just want to make sure I am doing this right.

    When I look at my spl meter, it only has 60.70.80 etc. No 75, so this is what I am assuming one does in order to reach the 75 level. On the meter it has a + and - on either side of the "0". Do I take the sound up to +5 on SPL meter? This is how I calibrated my system, if not what Do I need to do to get the 75 mark?

    Also after the calibration, I did a test on a movie and it seemed really low compared to what I had it at, although I did notice a lot more action in my rears then before, that part I like, but the overall sound seems to be a bit low.

    Now once I turned up the volume say 5 points form the reference level that I calibrated at the sound was incredable!

    Did I do something wrong, am I missing something?

    Thanks

    R~
     
  18. John Kotches

    John Kotches Cinematographer

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    Rhett,

    Nope -- you've got the process right.

    Regards,
     
  19. John Kotches

    John Kotches Cinematographer

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    John,

    We still haven't covered what piece of equipment you're using (receiver or processor). This question isn't a case of "mine's bigger than yours" so to speak, rather I'm just curious if your equipment is THX badged or not.

    Yes, if you calibrate AVIA with the volume set at -10, and the requested levels of 85dB calibrated are utilized then this will be reference level. One of the central issues though is what is -10 referenced against? Full available output? Dolby reference -10dB? Is it just -10 on a unitless volume control?

    In addition to master volume you have channel level trims. This makes it possible to calibrate a THX badged piece of equipment "hot", provided you have sufficient channel trim available.

    Regards,
     
  20. John Doran

    John Doran Screenwriter

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    john,

    i'm using a denon 5800.

    and i've left the channel levels (trim settings) flat (0db) on everything except the sub, which i have at -12db and for which i have compensated by turning up the sub volume. i heard that setting the sub this way had some advantages and i'm giving it a shot to see for myself.

    - jd
     

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