low x-over = missing LFE?

It's a good question, and may depend on your receiver. It is easy enough to test if you get a test disc with an LFE-only channel sweep, and measure your system and see what it does. That way you'll know for sure. Realistically though, i think it's fair to say that most content will have only the lowest and loudest bass on the LFE track, and significant other bass in the main channels, so even if your processor is rolling off the LFE at that xover frequency, it may not really do much if any damage and still may be a preferred xover if it works better in your system otherwise.

AVIA has a "Low Frequency Sweep, LFE" track.

Sorry I can't answer your question. It's a good question. It has certainly been hinted at in several discussions and articles that a receiver's crossover setting can chop off the LFE channel at that same setting. There has even been quite a bit of discussion as to how high the LFE channel actually goes. I think 120Hz is commonly sited as a frequency that engineers often use when the 6th channel is used as an LFE channel, but my understanding is that it can go higher than that when used as an LFE channel and I think it can actually be used full-range as a non-LFE channel.

Some receiver's have an "LFE cutoff" setting that can be set as high as 120Hz (maybe even higher). Whether this allows the LFE channel to squeeze through properly when the receiver's crossover is set lower, I don't know, but I think that is what it does. But what setting for this option is "correct" with different crossover settings is a mystery to me. I don't understand why it is even an option. I would think you would want the entire LFE channel no matter what, so why have an option of setting this in the first place? Beats me.

Would setting LFE+main help eliminate any x-over gaps that may exist? This would be provided that your mains can go down sufficiently low.

What brand and model of BD player do you use? How are your audio connections made to your receiver? Is BM via the BD player or via the receiver?

How did you determine that you have a 5 dB LFE cut on your LFE signal? A 5 dB cut in LFE is standard precedure with certain BM modes.

LFE+Main has no effect upon the LFE channel. LFE+Main only duplicates and routes bass below the crossover setting from the LARGE front channels to the subwoofer.

Thanks for the info. I'm still learning, and all of this can make one dizzy.

I don't know how your system works, but on mine the crossover setting only affects bass routed to those speakers where the crossover is applied. There is another setting for the LFE channel and it defaults to 120hz, so anything in the LFE channel 120hz and lower gets routed to the subwoofer in addition to anything 80hz and under in the other channels(as that is my crossover point). I would assume your set up is the same and that the crossover doesn't affect the LFE channel, but, as others have said, the only way to know for sure is to get a test disk or maybe there's a note about it in your manual.

What I'll say here is based on what receivers did when DD 5.1 first came out so I don't know if some receivers might be handling it differently now.

Generally, LFE is always sent to the sub unfiltered. The only thing the crossover setting the receiver does is redirect lower frequencies from the other channels to the sub as well to relieve the main channels from having to reproduce the lower freqs. I think the original DVD/DolbyDigital spec says that the LFE can contain content as high as 120Hz. AFAIK most (if not all) receivers don't even have a crossover circuit on the LFE channel so the full signal is sent to the sub. Now modern subs usually have a built in crossover so that should be set to something higher than 120Hz to endure no loss of info. BTW, early days of DVD did see some studios mix higher freq content in the LFE so the spec was not always followed.

Now, setting a crossover too low might still cause you to loose content if your mains can reproduce enough SPL at lower frequencies. Most speakers today have drivers no larger than 7-8" which can't reproduce bass below 40-50Hz. Even if your speakers are rated down to 40Hz. There's likely a 3dB or more roll off starting around 60Hz. My front mains are rated to 40Hz but I've got my xover set to 80Hz. I can probably go as low as 60Hz but I wouldn't go much below that. I've found that the lower I push my mains, the more muddy my mids get.

Best of luck...

No, that is not true, which is why the OP asked the question ifn the first place.

With the majority of processors, the full-range signals of the channels set to SMALL are all combined with the LFE channel and then that summed signal is low-passed at the processor's crossover setting to create the subwoofer output. In this case, the content of the LFE channel that is above the crossover setting is "chopped off". There are some processors that do not do it this way, but this can create phase issues.

I disagree. If the receiver in question does have a LFE "cuttoff" function, then that certainly would do what you say. In the DD spec, the .1 channel is a separate channel but is *not* a full response channel. The bitrate reserved for it would not allow it to be used for a good full freq channel. Also keep in mind that the LFE channel is *not* ALL of the low freq info in a given track. The amount of content sent to LFE vs main channels is up to the source mix. The crossover settings on the receivers redirect *additional* content mastered to go to the mains to be sent to the sub instead. Although it is very clear that LFE channel is not synonymous with sub output, I have never seen a receiver's bass management send parts of LFE to the mains. It's usually all LFE+main signal below crossover to sub if sub exists and mains set to small or LFE only sent to sub if mains set to large or LFE sent to mains if configured for no sub. If there is a receiver that has bass man that slices up LFE, I'd love to know which it is so I can go check it out.

BTW, there should be no phase issue. Signal is still mux'd within the receiver so no variable delays to cause any phase problems.

You're not paying attention to what I wrote.

I am well aware of what LFE is and how it differs from redirected bass. And I am desribing receivers and pre/pros that do NOT have any sort of "LFE cutoff" setting. What I am saying is that when any channels are set to SMALL, the way that the low-pass filter of the processor (the chip) works is that the full-range signal of those channels set to SMALL is first digitally summed with the LFE channel (the full LFE channel), and that then, the processor's crossover setting (really just the low-pass filter portion of the digital "crossover") is applied to that entire summed signal. As I hinted, this is done to prevent phasing issues that can occur if the LFE channel and the full-range signal of those channels set to SMALL are not digitally combined prior to the application of the crossover's low-pass filter. If they are treated independently, phasing issues between the LFE channel and the separately low-passed and redirected bass (which CAN contain overlapping info with the LFE channel), can and do occur. It is also, apparently, from an engineering design standpoint, "easier" for the chip to function this way.

If all channels are set to LARGE, then the LFE channel is not low-passed at all.

Read the OP's initial post. That he knows what I am saying is how many receivers/pre-pros operate is no accident. He has done his homework. There has been extensive discussion of this subject (the truncated LFE issue) here in these as well as other forums of this, unfortunately, inescapable "error". Perhaps this "error" has since been corrected, but I know that it was a major issue with processors, that received considerable attention, a few years back.

http://www.google.com/search?hl=en&q...=Google+Search

http://www.hometheaterhifi.com/volum...rs-9-2002.html

This behavior can be confirmed with an audio frequency sweep that is reserved strictly for the LFE channel. Do you have the AVIA calibration DVD? If so, it has a track called "Low Frequency Sweep, LFE" that is exactly that. It can be used to confirm whether or not your processor behaves the way that I am describing. Observe (listen to) what happens to this sweep when all the speakers are set to SMALL, and the commonly used 80Hz crossover setting is used, versus the behavior of the sweep when all the speakers are set to LARGE.



BTW (and it is totally irrelevant to what we are discussing), although I, myself, consider it to be improper or incorrect bass management, there ARE processors that send LFE to both the sub and the mains when an LFE+Main-type setting is used. This is not how my Denon operates and it is not how most receivers operate. But I know that there are some HKs, SONYs, and (IIRC) Yamahas that operate this way. I have argued/pointed out that this is not true several times in the past and have been proven wrong by the owner's manuals for these particular products.

Seems to be an answer to the content of the LFE channel:

From a Dolby Digital whitepaper on recording:
"Because the LFE channel for cinema applications contains content up to 120 Hz, however, the subwoofer itself should be capable of reproducing up to 120 Hz."

From BlueSky on recording:
"The LFE is a channel and is not the same as the subwoofer. It's actually a 120Hz bandwidth limited channel (80Hz for DTS) that has an additional 10 dB of electrical headroom above the 5 main channels. It was created as part of the film version of Dolby Digital to enhance low frequency sound effects, such as large explosions and other LF content which exceeded the headroom of the main channel. Hence the name LFE: Low Frequency Effects and not low frequency bass channel. So, unless you are recording Tchaikovsky's 1812 Overture with REAL cannons, it doesn't need to be used in most music applications, but can be useful in film and multimedia applications.

An important note about the LFE channel: Never put anything critical to the mix exclusively in the LFE channel. This is because when a Dolby Digital decoder folds down a 5.1 source, it drops the LFE channel completely (poof!)."

I did read and understand what you wrote but I don't think you understand my point. I agree that if a lowpass filter is applied to the LFE channel but that same signal does *not* have a corresponding highpass filter to channel it to the mains that could cause a response gap where the LFE has it's high end filtered out of the sub but not redirect it to the mains. That is clearly broken design and if current bass management does this then that is a problem I have not seen. As I stated in my original post, my experience may be a bit outdated and I'm perfectly willing to admit that. If the process you describe is in fact what most or all bass management processors do today (as you imply) then how do receivers provide a configuration of individual crossover settings for each channel?


It is my understanding that most AV receivers today implement crossover in digital domain, assuming the majority of the source material will be digital rather than analog so no additional A/D conversion would be required. This means that there is no phase sync issue since there is still a single D/A conversion stage for each channel regardless of where the crossover is done.


Yes, I have both AVIA and Video Essentials. I have run the LFE sweep and verified that I get LFE up to 120Hz even though I've got my xovers set to 80Hz. I have not done this recently but I don't expect my receiver to behave differently now vs before. And yes, to answer your next question, I did use an SPL meter.


I have heard of this although my receiver does not have that option. I don't necessarily think it's broken since it is an option that can be disabled. It does allow for more configuration options like an offboard active crossover to be used. Although I agree that in that situation, you probably would configure it as "no sub" and just use the front l/r channels to feed the crossover.

This is ALL true, but it does not at all address the issue at hand; that being, that upon reproduction, processors often truncate the LFE channel material at the processor's "crossover" setting.

It is most definitely an issue, and as I said, it has been discussed here quite a bit in the past.



Same here. But it is with older equipment that this was an issue. Did you read THIS link?



That is a good question and understand what you are implying. I don't know the answer. But I would direct you to these reviews which address this as well as the truncated LFE issue:

Emotive DMC-1
Lexicon MC-12



Yes, and the relevance of this is that because it IS done digitally, that what I am describing (summation of the full-range main channel info with the LFE channel prior to application of a low-pass filter) is even possible in the first place. I pointed out that from an engineering perspective, it is "easier" (cheaper?) to design the processing microchips to function this way.



Then your receiver or pre/pro's processor, fortunately, does not suffer from this issue. But that doesn't mean it is NOT an issue with many processors.

And an SPL meter isn't really necessary, especially if you use a high crossover setting when doing the comparison.




I don't want this to turn into an argument, so I'm not going to say much else regarding this well-known "truncated LFE issue".

You can research this problem online. There is plenty of info out there regarding it. With the proper search term(s), you should be able to find several threads regarding it right here in these forums.

There is a simple way for recording engineers to avoid this and other problems with reproducing LFE, and that is not to use any LFE -- put all the bass content in the front channels. It's not like there was any real point to having LFE bass for home theater. To the extent LFE is avoided, the reproduction issues become academic. I once converted the DD 5.1 sound from some Dolby demo videos into WAV file format, and I was really surprised to find that the .1 channel was entirely silent.

This may seem like a "solution" for bass-managed systems but it is unsatisfactory for non-bass-managed systems (all channels set to LARGE) which is really what the LFE channel was originally designed for. The LFE channel was designed for additional headroom over and above that available in the other 5 full-range channels.

Bass management, which is completely different from the LFE channel (except when no subwoofer is utilized and the LFE channel is rerouted to the LARGE front channels), is something that was created as a compromise for HT systems that are not capable of full-range reproduction of the main channels.

Yes, most theaters' systems cannot truly reproduce the full-range signal from their main channels either, yet strangely, even if they DO include a subwoofer, most theaters do not use bass management. Very rarely can a theater's system reproduce 20Hz (if even 30Hz) at any appreciable level. Imagine if all the theaters in your local metroplex were capable of flat reproduction down to 20Hz! The soundproofing that would be required would be like a bomb shelter.

As to why movies' soundtracks are mixed with low bass and low LFE information in the first place is a mystery to me. No one has ever really said (yet) that engineers are mixing the soundtracks for HT reproduction. Beats me?

No, I missed that. I didn't see the link when I first read your response. Very interesting article... Thanks for posting it. It would definitely seem that my experience has been mis-directed or at least abnormal. I auditioned about 10 units to the point of bring AVIA and VE disks to the stores before I bought my receiver and even took a couple home to test. In all honesty I did not test for this explicitly but I did use the LFE sweep too calibrate each unit before auditioning. In all of that, I never noticed a drop off at the xover freq.


I mentioned that to say that if you are filtering in the digital domain, the main advantage of pre-filtering is not as compelling. The cost of applying a low pass filter for each channel before muxing is low if done in the digital domain. The old audiophile argument is that with an analog source this would require an additional A/D conversion on the input. To resolve this, having the crossover in analog domain allows the analog signal to be effectively passed thru until the crossover. Then it might be more efficient to pre mux all the channels and send it once through the analog crossover right before the output rather than having a digital domain filter in DSP. BTW, I'm just running through some design options and am not claiming this is how an actual receiver is designed.



Agreed. I can only speak about the processors I've had direct experience with. Also, you're correct, SPL meter may not be necessary for this but I did use one to find room harmonics so I used more than my ear to determine that my receiver does not seem to do the type of bass management you have described.


No worries. I don't see it as an argument and always enjoy healthy discussions.


You just reminded me of some of the early discussions going around the internet when DVDs first came out and 5.1 audio was first introduced. It was widely held that the LFE should only be used to enhance the low freq effects and not replace content in the mains. That means that if you set your receiver so that it thinks you have a sub but don't connect one, explosions won't be silent in your movie as long as you have your mains set to large. In a sense, this makes the whole discussion moot if you have mains that can go that low.

Yes, I agree, properly mixed, at least some fraction of the low effects should be "spread" across at least the front channels (if not all the main channels) and the LFE channel, but whether all engineers are (still) mixing this way or not is questionable.

That the soundtrack be mixed this way becomes particularly important for those who simply utilize their DVD player's R/L analog outs in a substantial 2.0 or 2.1 setup, as DVD players (not receivers and pre/pros) completely drop the LFE channel when downmixing 5.1 soundtracks to 2.0. Most 5.1 DVDs do not include a specifically-authored 2.0 stereo track, but when they do, I would presume (hope) that, by the same token and in the same way, the engineer would mix the 2-channel track appropriately so as to include at least some of what would otherwise be included discretely in the LFE channel of the 5.1 track.

Sometimes you'll be missing out on entire effects if you take out the LFE.

If you put in Episode III: ROTS, you'll notice that the entire "force effect" is missing when Anakin holds out his arm to choke Padme at the end, if you don't use a sub or don't have your mains set to large.

Yep. I presume what you saying is that this is an example of something being mixed entirely discretely into the LFE channel. I think this is probably pretty common nowadays.

And because DVD players' processors do not reroute LFE but instead drop it altogether, unless this DVD has a specifically-authored 2-channel track, this "effect" would be totally absent for someone who was utilizing a DVD player's R/L analog outputs (as opposed to a digital connection to a processor), no matter how LARGE their front channel speakers and/or subwoofer. And even if this DVD has a specifically authored 2-channel track, whether the engineer properly included this "effect" in it is questionable.