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Discussion Starter #1 (Edited)
Hello everyone. I have a question that is bugging me. I read @mthomas47's Guide to Subwoofer Calibration and Bass Preferences which is a good guide. I understood that a high trim on the AVR can cause clipping. But I want to learn when exactly clipping occurs. What determines WHEN the signal starts clipping? I'm not looking for an answer like "when the amp in the receiver is no longer able to send a clean signal". WHEN does an AVR no longer send a clean signal to the subwoofer? Is it when you're at reference volume and the subwoofer trim is above 0, and similarly, if you're at -5 MV and the subwoofer trim is above +5? If not, what exactly causes an AVR to send a clipped signal?

If nobody knows the answer to this, then it seems like the recommendation of keeping the trim at -5 or less is pulled out of thin air, because how can you pick a recommended number to stay under if you don't understand when exactly clipping occurs?

Surely there's a particular point when clipping occurs, such as "when the signal is x dB over x" or "when the signal is amplified by more than x".
 

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Hello everyone. I have a question that is bugging me. I read @mthomas47 's Guide to Subwoofer Calibration and Bass Preferences which is a good guide. I understood that a high trim on the AVR can cause clipping. But I want to learn when exactly clipping occurs. What determines WHEN the signal starts clipping? I'm not looking for an answer like "when the amp in the receiver is no longer able to send a clean signal". WHEN does an AVR no longer send a clean signal to the subwoofer? Is it when you're at reference volume and the subwoofer trim is above 0, and similarly, if you're at -5 MV and the subwoofer trim is above +5? If not, what exactly causes an AVR to send a clipped signal?

If nobody knows the answer to this, then it seems like the recommendation of keeping the trim at -5 or less is pulled out of thin air, because how can you pick a recommended number to stay under if you don't understand when exactly clipping occurs?

Surely there's a particular point when clipping occurs, such as "when the signal is x dB over x" or "when the signal is amplified by more than x".

Hi,

You have asked a great question, for which no one I know has a great answer. There are multiple variables in play which could all affect when clipping occurs: the type of AVR; the capability of the subwoofer(s), and the gain setting; the master volume; the total amount of subwoofer boost, including both things like DEQ, and independent sub boosts; the nature of the bass content, and how demanding it is at lower frequencies, etc.

The -5 to -6 AVR trim recommendation is just a general best practice guideline, which was arrived at through anecdotal experience. Several subwoofer makers suggested it several years ago, and subsequent experience seems to have demonstrated its utility. The reality is that if you are playing at very near Reference volumes, with a significant subwoofer boost, you may be better off with an AVR trim of -8 or -9. And, if you are playing at -20 MV or lower, without much subwoofer boost, you may never experience any clipping at all, even with a trim level in positive numbers.

But, we have to start somewhere, and we need to establish some kind of general standard, because too many people really have experienced clipping, when the gain/trim balances got too far out of alignment. So, the Guide offers this as a best practice principle to begin with, and everyone is certainly invited to experiment as carefully as he wishes, to determine whether the number recommended in the Guide pertains to his specific circumstances or not.

As I am sure you have already discovered in other aspects of audio/HT, we can start with some basic principles, regarding HT system calibration, the use of room correction, subwoofer placement, basic settings, etc. But, hard-and-fast rules, and extremely predictable outcomes, are much more difficult to establish. Good subwoofer gain/trim protocols are no different.

If we try to keep AVR sub trim levels well in the negative (somewhere around -5 or -6 seems to work well in most cases) especially as we get above about -10 or -15 MV, and especially if we are using significant subwoofer boosts, it will do no harm, and it may do some good. For instance, as the Guide also points out, some subwoofers will never be able to reach their max output capabilities without fairly high subwoofer gain levels. And, as consumers, we won't always know which specific subwoofer models that may apply to. So, keeping AVR trim levels well in the negative, and remembering to add most of our subwoofer boost with our sub gain, may make sense from that perspective as well.

But, all of these are just general best practice recommendations to offer us a good starting point for the operation of our HT systems. People are always free to disregard those recommendations, and/or to experiment to determine for themselves what works best for their specific systems. For those who want to understand more about the recommendations, which led to these questions, they may be found in Section II of the Guide linked below. The Cliff Notes at the very beginning of the Guide also briefly address the issue.

Regards,
Mike
 

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It's really just an easy to implement safety measure for people who push their systems hard
@desertdome (and maybe shreds) tested the SW preouts on several brands of AVRs using an oscilloscope, and found that an AVR SW trim above -7dB * with the MV at 0dB (calibrated to Ref Level) will send a clipped signal during some of the WCS (worst case scenario) scenes like HTTD finale and "Interstellar, Spectre, OZ The Great and Powerful, and Lone Survivor."

It's been discussed a few times over the years, both here and on dataBass forums

here's 2 I could find

https://www.avsforum.com/forum/113-subwoofers-bass-transducers/2673865-official-jtr-speakers-subwoofer-thread-174.html#post54500873

https://www.avsforum.com/forum/113-subwoofers-bass-transducers/2673865-official-jtr-speakers-subwoofer-thread-173.html#post54494049
 

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I don't rely on the AVR gain level indicator to keep me from getting into the clip zone.


You never know what the AVR is internally doing with the signal levels based on bass management, internal channel EQ, internal channel level trim, source signal (line-in, 7.1, 2.0, etc). Too many variables to keep track of.


I have all the preamp channels hooked to mixer/splitter units just for the level monitoring - each channel signal monitored independently.



That gives instant visual confirmation and great piece of mind.
 
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