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What does THX BGC really do?

Discussion in 'Integrated Amplifiers & Receivers' started by PEPPY52, May 18, 2013.

  1. PEPPY52

    PEPPY52 Member

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    I have been going crazy over this and there is no accurate explanation on the thx website, and no one seems to have posted results of BGC on vs off to make a conclusion myself. I do not have an SPL and i really would like to know if my system could benefit from this setting. It helps get rid of most of my subs passive radiator noise (passive radiator vibrates at high volumes mkaing noise), but the rattling also goes away from deactivating audyssey eq (it is not bottoming out, it is what could be called the port distortion of a PR sub, I have an earthquake supernova MKV 10" btw.
    I believe BGC rolls off some frequencies at the lower end, wich audyssey boosts after room correction. Does anyone know exactly what BGC (boundary gain compensation) actually does? If someone could post some results (BGC on with audyssey eq on and viceversa) it would be a lot of help. Thanks guys!
  2. Dr Manhattan

    Dr Manhattan Active Member

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    THX Ultra2 BGC applies a high pass filter at around 50-55hz and rolls off the sub bass response. You shouldn't need to use it if you've run Audyssey and room EQ is switched on.

    You'll pretty much kill your subs low end output if you use Audyssey in conjunction with BGC. No harm done in trying it out for yourself to see what you think.

    It was quite useful back in the day when there was no room EQ or subwoofer EQ available. It could take the edge off a boomy subwoofer or if the low end output overpowered the rest of the frequency response.

    Make sure BGC is OFF when you run the Audyssey Auto Setup or you'll just end up with Audyssey trying to boost what BGC has cut/filtered.
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    Last edited: May 19, 2013
  3. PEPPY52

    PEPPY52 Member

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    Thanks for the advice! Doesnt Audyssey actually boost the low frequencies to get a flatter respone? With audysset on, I can sometimes hear my passive radiator vibrating. Without Audyssey I hardly get any passive radiator distortion and with audyssey on and bgc it is very similar to audyssey off. Is it possible that the BGC on top of the audyssey balances out audysseys LF boosting?
  4. Dr Manhattan

    Dr Manhattan Active Member

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    What Audyssey does will be totally dependent on the actual frequency response of the sub etc in your room. Problem is, unless you can measure what's going on you won't know what Audyssey is doing.

    Audyssey doesn't just boost the low end it will also cut any frequencies to reach it's target response. If I remember correctly Audyssey can cut by up to -20db but will only boost by +6db.

    If it sounds better to you with BGC on as well then by all means run it like that. Alternatively, trim the subwoofer level down a bit so the noise is reduced.
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  5. PEPPY52

    PEPPY52 Member

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    I added another 10" subwoofer and it helped by letting me trim the subwoofer level down. Got rid of most of the distortion and I'm getting better output than before! Thanks for the advice!
  6. Mark.Yudkin

    Mark.Yudkin Active Member

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    From Jamo:
    Most modern AV amps also incorporate more sophisticated room equalization technologies such as Audyssey. In addition, better subs have sophisticated equalization built in (e.g. Velodyne DD series) or you can add it as an option (e.g. Velodyne SMS-1, Anti-Mode). These more modern technologies have made the ca. 20 year old THX BGC essentially obsolete.

    In general though, provided the EQ setup is made with the BGC option already enabled or disabled, it should be of no consequence.

    When you add a second subwoofer, you need to drop the level of each subwoofer by 3dB to keep the sum correct. Amps with multiple subwoofer support automatically do this when you reconfigure the number of subwoofers, but if you're using a splitter you'll need to recalibrate the trims yourself. Enabling Boundary Gain Compensation is not the resolution, as it is non-linear.

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