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LFE vs Diverted Bass - Is your processor WRONG? (Updated with custom test tones)

Discussion in 'Speakers' started by Arnel, Apr 15, 2002.

  1. Arnel

    Arnel Active Member

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    We all know that the 0.1 channel of a soundtrack is a separate/discrete channel for LFE and being reproduced by our sub. The sub would reproduce only LFE if the rest of the speakers were set to Large. But if any speaker were set to small (say at the 90hz crossover), the sub will reproduce the LFE (0.1) plus the diverted bass (90hz and below) from the speakers set to small.

    Now AVIA DVD has a test tone (under evaluation menu), where you can check the level of the diverted bass and the LFE. On my first test, my RS meter drop to about 10db when the tone reaches LFE. The note says that I have to set my LFE to normal or 0db on my system. So my former setting of -10db (this is the mid setting of my Yamaha RX-V800, where I can easily adjust sub level), was now change to 0db (max setting on Dolby, while DTS was up to +10db). The AVIA test again indicates a drop of about 4db on the LFE. I know that the result indicates that I have a poor bass management, since diverted basses were overpowering my LFE. I don’t know were else can I modify to make my bass properly leveled. Please help.
     
  2. Harold_C

    Harold_C Well-Known Member

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    You should have two different adjustments on the Yamaha, one for the subwoofer level and one for the LFE levels (separate settings for Dolby Digital and DTS).

    The Yamaha manual is not terribly clear, so I can't give you specific instructions. However, you want to use the procedure described on page 22 of your manual for setting the level of the subwoofer (and perhaps the Lo Freq test procedure described on page 36). Use these adjustments to set your subwoofer to the same 85 dB output as any other channel on the Avia test disc.

    The LFE level procedure described on page 39 is different. This is an LFE adjustment. It must be set to 0 db when you are calibrating your subwoofer levels. It should be left at 0dB for both Dolby Digital and DTS, unless you find that the loudest bass peaks on soundtracks are causing audible distress to your subwoofer or to your ears! Other than using it as a sort of "limiter" to protect your subwoofer, there would be no reason that you should ever set this to less than 0dB for Dolby or DTS -- with the exception of certain DTS music recordings that were mixed incorrectly and require a -10db LFE setting to compensate for DTS's screw up in mixing these recordings. (It's possible that Yamaha is mislabeling the DTS 0 db setting as +10 db -- a quick listen to a DTS soundtrack would confirm.)

    As far as the 4 dB discrepancy -- I will have to play with that portion of the AVIA disc and get back to you. The wife is asleep, so I don't think I can do it now! In any case, if you have any of your speakers set to LARGE, this could be the problem as redirected bass would to to the LARGE speakers AND to the subwoofer. In 99 out of 100 systems, you shouldn't have any of your speakers set to LARGE. You really don't want the surrounds and center set to SMALL and the fronts set to LARGE because this will produce better bass from the surrounds and center than the fronts. Not good. Try setting every thing to SMALL and try the test again. Also, make sure that the LFE/BASS output is set to SUBW -- not the MAIN or the default BOTH settings.
     
  3. Arnel

    Arnel Active Member

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    Thank you Harold for your quick response. Below are my current settings:

    1. All speakers were set to small and my LFE/Bass Out was set only to SWFR – this means that my LFE plus the 90 Hz and below frequencies from L, C, R, LS & RS will be reproduced only by my Subwoofer.

    2. LFE Level for DD and DTS is set to 0 db – this is the default. DD range is from -20 to 0 db, while DTS is from -10 to +10 db.

    3. SWFR Level is set to 0 db at the Speaker Level Settings.

    Under Audio Test Patterns of AVIA DVD, you will find the “Low Frequency Pink Noise, 6 Channel Pan” at Verification/Evaluation Chapter. This is a circulating low frequency test pattern from R, C, L, RS, LS to SWFR/LFE. Since all my speakers were set to small, naturally all sounds were emanating from my SWFR. My RS SPL meter reading is the same all through out the 5 speakers but dropped by about 4db when it reaches the LFE. I don’t know how to compensate for it.
     
  4. greg_t

    greg_t Well-Known Member

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    I think you should have your lfe level set at it's highest level for both DD and DTS. So for DD, set at 0, and for dts, +10. The LFE channel is meant to be 10db higher than the other channels. A setting of 0 on DD adds the proper 10db boost, and a setting of +10db will do it for DTS. You then need to recalibrate your overall subwoofer level. The subwoofer level can be the same as the main channels, but many here prefer to run 2-5db hot. For a much better explanation on all this, do a search by name for "Vince Maskeeper". He has several excellent posts on how this all works.
     
  5. Harold_C

    Harold_C Well-Known Member

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    Arnel:

    Thanks. I just wanted to clarify your settings -- Dolby Digital does some various combinations of attenuation on various channels with bass redirection. I wanted to look these up and see if that could be it and all SMALL is simplest configuration. Everything looks exactly right in your settings. I'm puzzled by the 4 dB shift and agree with you that it should be the same levels.

    I looked at the documentation for bass management and specifically the bass mixer. I see nothing that would account for the discrepancy.

    The only other setting that I can think of that might affect the results might be an LFE peak limiter (which I don't think the Yamaha has) or a dynamic range setting or, conceivably, the dialog normalization. However, I can't really think of why these wouldn't change all channels simulataneously.

    The other thing to confirm is to make sure that you don't have a bass control or "loudness" curve activated by accident. I've often run through all the settings on my system and found inadvertant settings memorized -- the result of mashing buttons on the remote in a dark room in the wrong remote mode. However, these shouldn't affect Dolby Digital.

    I have found the tones on my Avia disk and want to see if I'm experiencing the same thing you are -- we have the same settings, but different brands of processors. I'll get back with the results.

    BTW, there are register settings in the bass management software that would cause the results you've seen. At the input to the bass mixer (with all SMALL speakers), the five main channels are attenuated 10 dB and the LFE is attenuated 5 dB -- this is actually the mechanism the decoder uses to get the extra 10 dB of LFE level during playback. It is possible that Yamaha has used different settings that the Dolby recommendation for these individual channel levels at the bass mixer input. I don't think this is likely, since "ALL SMALL" is one of Dolby's mandatory published configurations, required for certification and detailed in the Dolby chipset documentation. But, anything is possible.

    It is also possible that you are seeing some kind of crossover "artifact". For example, if the pink noise test tones are bandwidth limited to 20 - 200 Hz, you would be getting some output from the main speakers as well as the sub. Given that the Radio Shack meter is not "flat" this could account for some higher readings. This would not impact the LFE channel because it goes through a brick wall digital filter at 120 Hz during encoding and is almost always passed through an 80 Hz lo-pass filter as well. If I get the same imbalance as you, I want to try the test with a couple of main channel speakers actually unplugged to eliminate that possibility.

    Anyway...all I have for now is speculation and a curiousity about the mystery.
     
  6. Harold_C

    Harold_C Well-Known Member

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    Arnel:

    I'm getting the same sort of thing on my system with those test tones. I dont' know that I could peg it as a 4 dB drop -- I can't get sufficient precision from the Radio Shack to call things that closely, too much needle movement.

    I'm goint to defer to the Avia folk on this one.

    I do know that, even witha 100 Hz crossover, there is still output from the main speakers on this test. So the test signal extends at least up to 120 Hz or so, perhaps higher. This energy won't be there on the LFE track.

    I don't have any real confidence in the Radio Shack SPL meter below 100 Hz. In addition to the needle moving too much to get an accurate reading, there are serious issues decay time issue trying to measure wavelengths that long with simple detector circuits and the weighting built into the mic and the meter itself confuses things.

    For exmaple, I noticed that just before the LFE track ended, the needle was rising rapidly. It could very well be that if the LFE test tone is filtered at 80 Hz, the length of time the test tone plays may not be long enough for the SPL meter to get an accurate read. I notice this phenomenom with guitar tuners. The tuner has a delayed response to low E bass string because it's got to process "x" number of cycles before it can lock on.

    If this were the case, the SPL meter could be basing it's reading of the main channels based mostly on higher frequency tones (say 100 to 200 Hz). Then, when faced with the LFE tones (sharply filtered in the encoding process above 80 Hz), the meter itself is simply not able to provide a similarly accurate reading.

    I think the note on the Avia instructions that "some variations are acceptable" indicates to me that they know this particular test is fraught with measurement error.

    I go back to Dolby own tech papers. While they highly recommend the Radio Shack SPL meter for setting levels and channel balancing of the main 5 channels, they flat out state that this type of meter is essentially useless for pink noise measurements below 100 Hz. I think that's right and that's why I would very wary of trying to EQ a subwoofer based on measurements from the Radio Shack SPL.

    You can tell that something is funky from the "correction factor" charts published here and there for this meter. In the bass, the correction factors are all over the board, with wildly different "correction" factors for 30 Hz, 40 Hz, 50 Hz, etc. If the measurements are valid, the correction factors should be a fairly smooth curve -- not jumping all over the board.

    You can also see the problem inherent with this test by moving the meter 2 feet and repeating it. I bet that you get significant different results.
     
  7. Arnel

    Arnel Active Member

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    Harold, Thank you very much for the time you've spent trying to understand my system's problem.

     
  8. Harold_C

    Harold_C Well-Known Member

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  9. Arnel

    Arnel Active Member

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  10. Harold_C

    Harold_C Well-Known Member

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    Having seen that the individual channel bass sweeps perform as they are supposed to (same levels for LFE and individual channels, assumng Avia recorded the LFE channel 10 dB down), my assumption is that the pink noise test tones are just plain wrong on the disc.

    There's no reason that one set of tones would work and the other wouldn't -- allowing for some discrepancy inherent in the meter.
     
  11. Harold_C

    Harold_C Well-Known Member

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    Hey Arnel:

    I figured out, in part, why that Avia test was behaving so strangely.

    That is a PAN test. It's hard to tell from that one, but if you play with some of the broadband "pan" test tones, you'll see that these don't just jump from one speaker to the next as the speaker calibration tests do. Instead the "panning" tests smoothly and slowly pan the signal from one channel to the next.

    The end result of this is that you get just an instant of single channel sound at the start and finish of the pan with all the time in between taken up with various levels of mixing between the two channels. Obviously, this introduces a whole 'nother dimension of room effects and speaker interaction into the equation plus the fact that the absolute levels vary during the pan (depending on the taper of the panning controls).

    I believe the fact that you are never looking at a pure "single channel" signal during that test could easily account for the 4 dB discrepancy you were seeing.

    The Avia records the LFE test tones 10 dB lower than the individual channel test tones. For the sake of discussion, lets say that the center channel test tone is recorded at -20dB and the same test tone recorded in the LFE channel is recorded at -30dB.

    Now, those signals are attenuated at the input of the bass mixer bass management module. The center channel signal is attenuated by 15 dB, so our orignal -20 dB signal is now attenuated to -35 dB. The LFE signal is attenuated by 5 dB, so our original -30 dB signal also ends up at -35dB. All fine and dandy if we are comparing two separate channels back and forth like a light switch. But, who the heck can predict what is happening when you PAN between those two signals (each with different levels of attenuation and each affected by the taper of the panning volume control). On top of that, I'm pretty sure that it is panning from a 20-200Hz signal to a 20-80Hz signal.

    Just to see how the panning changes things, watch your SPL meter on the regular 5 channel pink noise panning tests. It will be moving all over the place as the signal is shared by two adjacent speakers.
     
  12. Bill Kane

    Bill Kane Well-Known Member

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    Great dialog you guys.
    Arnel, I run RXV-800 and take an interest here. One thing bothered me (and I dont know if it's still in the equation).
    It's the balance between AVR sub level and sub amp "volume" setting.
     
  13. Harold_C

    Harold_C Well-Known Member

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  14. Arnel

    Arnel Active Member

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  15. Bill Kane

    Bill Kane Well-Known Member

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    I had never used the AVIA evaluation "LF Pink Noise, 6 CHL Pan" and just tested.
    First, I recalibrated all speakers again at 75dB incl sub. DD LFE at 0(default). DTS LFE is irrelevant in these settings since AVIA is DD only. (I left it at +10 and will come back later with my Sound&Vision test disc that has a DTS track.)
    In the pan test, I too saw a ~5dB meter drop at SWFR. Absent any equalizer in my system, I don't see there's a huge "problem" to concern me. Arnel may feel differently.
    Harold wrote:
     
  16. Harold_C

    Harold_C Well-Known Member

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  17. Vince Maskeeper

    Vince Maskeeper Well-Known Member

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    I would certainly like to comment on the meat of this discussion- however while I do own Avia and find it useful for some things- I usually use VE for audio calibrations. That said, you each keep referring to comparing a BAss Mgmnt tone to a LFE tone- and as of yet, I had been unaware of a dedicated LFE tone on the Avia disc that was designed with calibration in mind (accurate).
    If someone could point out to me exactly what tone you're using, exactly where you found it on the disc- I'd be happy to check into the reason for the problem.
     
  18. Harold_C

    Harold_C Well-Known Member

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  19. Vince Maskeeper

    Vince Maskeeper Well-Known Member

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  20. Richard Greene

    Richard Greene Well-Known Member

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    Hey, stop beating up that Radio Shack sound meter!

    Quote from my Radio Shack 33-2050 meter owners manual

    (copyright 1989):

    "With C-weighting, the Sound Level Meter

    frequency response is flat from 32 to 10,000Hz. (+/-3dB). Above 10kHz., the frequency response of the Sound Level Meter drops off rather rapidly ..."

    ------

    The meters do read too low below 32Hz. and especially above 10kHz., so ignore those frequencies ... but I believe they meet their specifications from 32Hz. to 10kHz. (and are probably within +/-2db at bass frequencies from 32Hz. to 100Hz. -- the treble has larger deviations from the official C-Weighting curve).

    That's a good deal for $40.
     

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