Tones to test the position on your LFE pad

Discussion in 'AV Receivers' started by Vince Maskeeper, Jul 9, 2002.

  1. Vince Maskeeper

    Vince Maskeeper Producer

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    In reaction to the fact that the 6 channel pan on Avia appears to be flawed, I have authored 2 DD test tones which will allow people to test the level of LFE vs the level of rerouted bass from other channels.
    The idea is that the 40hz sine wav in any channel, with the speakers set to small, will be rerouted to the sub. Since it is the same freq, coming from the same sub, with the meter in the same position- the only variable in the level of output would be the amount of signal being sent by the preamp. If the decoding is done correctly, the main channel tones and the sub tone should be within 1 db of one another.
    1) dd140hz1.wav: A 40hz tone at -20 which repeats in each of the 6 channels, 5 seconds at a time (Left, Center, Right, Left Surround, Right Surround and LFE).
    The exact same wav file was imported for each of the channels- so it is absolutely identical. Because the LFE track in a dolby playback system (if configred correctly) boosts the LFE by 10db- a correct playback should keep the tone even until it reaches the LFE, where it will jump by several DB (approx 10). If your final tone does not jump obviously- there might be an incorrect setting in your processor.
    2) dd140hz2.wav: A 40hz tone at -20 in the mains/ -30 in the LFE which repeats in each of the 6 channels, 5 seconds a time (Left, Center, Right, Left Surround, Right Surround and LFE).
    The exact same wav file was imported for each of the main channels- so it is absolutely identical (aside from the LFE tone which is 10db down- but all tones were generated using professional tone generation software). Because the LFE track in a dolby playback system (if configred correctly) boosts the LFE by 10db- a correct playback should keep the tone even through all 6 speakers. If the final tone drops off significantly in your system, there is a LFE error.
    -The tone is 40hz sine wave- so for this experiment you will have to have your speakers set to small (the whole point is testing rerouted bass vs lfe!) and you might want to start out with your x-over point around 80, and if you usually run lower- work your way down and see what effect it has.
    - Tones are 5 megs a piece, so broadband is probably a must.
    - These two files are DD encoded wav files. It is possible to burn them to a CD, and play them in a cd player or a dvd player with a digital output- and get DD. This does not work with every player. However- if you have a PC with Spidf output, you should be able to use windows media player to play the files and get DD on your processor (it worked for me with the Maudio Audiophile card). Note, if you try to play these back in the analog domain or if your player can't deal with the format you will get very displeasing noise! Be careful, turn the volume all the way down until the processor confirms a DD lock!
    If anyone has any questions- feel free to post.
     
  2. Vince Maskeeper

    Vince Maskeeper Producer

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    PS: I used the tones in my system, using the Outlaw 950 preamp and found that the L/R speaker rerouted bass was slightly louder than the surround speakers:

    L: +1db
    C: +.5db
    R: +1db
    Rs: -1db
    Ls: -1db
    LFE: 0

    I have always wondered if the Speaker volume setting has any effect on the rerouted bass info sent to the sub- on my 950 is does not. I tried adjusting the rear output volume up and down, and it did not affect the level of tone sent to the sub.
     
  3. Vince Maskeeper

    Vince Maskeeper Producer

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    Links are fixed.

    -V
     
  4. Doug_B

    Doug_B Screenwriter

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    Good stuff, Vince. I'll try one of your 44.1 KHz files later this week once you post it. Maybe I'll vary the xover as well.

    Doug
     
  5. Vince Maskeeper

    Vince Maskeeper Producer

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    Some interesting findings:

    If I set the center to "none", the center tone rerouted to the sub comes up 3db hotter. Seems like it is summing the center to both L/R, and then rerouting from BOTH to the sub- giving a 3db boost!

    A similar thing is true using the Avia main setup tones- with center set to NONE the downmix of the center to the l/r speakers actually ends up boosting it by 2-3 db (because it coming out of both the left and the right speaker).

    Though, dolby flags a center downmix level (how much it should reduce the center when downmixing) which is supposed to "fix" this- but I'm wondering if decoders ignore it.

    I will have to tinker with some settings and try some different center downmix levels and see if the processor actually does it or ignores it...
     
  6. Dan Hine

    Dan Hine Screenwriter

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    Thanks Vince for both the test tones and helping me sort through exactly what it all meant.

    For those interested I have a Pioneer Elite VSX 35tx. My values for the first tone (LFE at same level on recording) came out as follows (using the Mains as reference point) also note that I run my sub about 1-2db hot:

    L: 0

    C: +3db

    R: 0

    LS: -3db

    LS: -3db

    LFE: + 10db

    On the second recording (with the LFE -10db) everything was the same except for the LFE which was only 1-2db hot...which coincides with the way my system is set up with the sub calibrated about 2db hot. These show that the decoder is processing the LFE correctly since I do not have an LFE pad, only the general subwoofer level setting.

    I find it interesting though that on the first recording it only measured as +10db even though I do run my sub hot as was reflected in the second recording. All in all a fun test and very informative about the receiver. In fact, the next time I audition a receiver or processor this will be an asset. I will be able to test and see if I'll be able to still get accurate calibration from the system in case it has no LFE pad. And even if it does you can test its accuracy.

    Thanks again Vince!


    Dan Hine


    P.S.

    Does anyone have an idea as to why it reroutes the bass from the center a little high and the rears a little low? You can see this reflected in Vince's numbers as well though not it was not as drastic as mine. Thanks.
     
  7. Doug_NHT

    Doug_NHT Stunt Coordinator

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    What a pleasant surprise. My experiences and results are surprisingly similar to the rest of you in this thread. I went thru this so many times that I just finally decided to let it go. The setup I have done sounds killer so I'm happy even though this was so frustrating.
     
  8. Warren_Sc

    Warren_Sc Second Unit

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    I am afraid to use this...
    It opens the door to too many tweaks!
    (Still, I downloaded it.)
     
  9. Doug_B

    Doug_B Screenwriter

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    OK, I fooled around with this somewhat. Used different xover freqs on my pre/pro, all with 12db slope. Didn't record my findings, but have some interesting results.

    At 80 Hz xover, the main and center signals were within 1 db of each other. The rears were 2-3db hotter, though. At 60 Hz xover, the rears pulled out even more ahead of the other channels (> 5db for sure, but forgot the exact number). At 40 Hz xover, I only did quick measurements, as they started getting erratic. I went the other way as well. At 100 Hz xover, a small rear perturbation remained. At 160 Hz xover, all was within 1/2 db.

    I'm pretty sure that my room acoustics had a primary role in the readings. Here are all the contributing factors that resulted in the rear (really side) surround "anomolies".

    1) Via Avia's calibration tests (white noise, pink noise, or whatever it is), I have my side surrounds hot by 5 - 5.5db relative to my mains.

    2) Really the number 1 issue: my sub in its current position in front corner results in a 40Hz response at the sweet spot that is 8db below the 40Hz response when the sub is in the rear corner (which is near one of the side surrounds). I have played around with both positions in the past and have graphed the response for many low freq sine waves. Sweet spot response of 40Hz signals from the position of my mains is also about the same as front corner; in fact the entire front section of my room has a very similar low freq response at the sweet spot, based on a number of measurements I performed in the past while moving my sub around.

    3) My side surrounds are no slouches at low freqs, with each having two 6.5" woofers. I believe the -3db manufacturer rating is in the low to mid 40's.

    Bottom line IMO: better 40 Hz response from rear room sources (although I haven't tested the response exactly where my surrounds reside), coupled with an added calibration boost for my side surrounds for white noise test signal results in the side surrounds having a non-negigible influence on the results, even when the xover is an octave above the sine wave used.

    Doug
     
  10. Vince Maskeeper

    Vince Maskeeper Producer

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    Sure- and this is the prime reason I suggested using an 80hz xover. If you can insure that the majority of the 40hz signal is coming from the sub position- then you overcome the acoustics issue (because all the sound is coming from one position).

    The key element of the test is just to make sure the LFE tone does not jump way out in comparison to the main tones. Certainly you can use the tone to play around in any way you wish- I just wanted to make it clear for anyone reading that the simply use (80 xover, small, measure) will give them info on how their processor handles LFE material.

    -V
     
  11. Vince Maskeeper

    Vince Maskeeper Producer

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    Figured I'd bump this post since it had been a year- thought someone might find this helpful.

    -Vince
     
  12. JimmyK

    JimmyK Second Unit

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    Thanks Vince.
    When I get a chance, I'll download the files and see what results I get.
    Or then again, maybe I don't want to know?[​IMG]
    BTW, what is meant by LFE "pad"?
    JimmyK
     
  13. Vince Maskeeper

    Vince Maskeeper Producer

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    A control in bass management- sometimes called LFE trim, that specifically controls level of Bass on the dedicated LFE channel (not the overall sub volume whioch includes rerouted bass).

    The problem was, and often is, that Dolby and DTS used two different ways of describing this control- resulting in a confusing situation. On the majority of processors that offer the LFE pad, the proper position for both is in the HIGHEST available position- but often this means 0 for DD and +10 for DTS...

    If you do a search on LFE Pad and my name (make sure to set it for old searches)-- you'll find a boatload of info.

    -Vince
     

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