Technical Articles and Editorials
- Written by Brian Florian
- Published on 30 November 2007
In 5.1 digital surround sound, the 0.1 channel is called the LFE or Low Frequency Effects channel. While in the commercial system it is referred to as the subwoofer channel, this is not necessarily true for home theater setups. I noticed a little confusion over the 0.1 LFE channel of Dolby Digital cinema sound systems and thought it would be a good idea to write an article that, hopefully, explains the situation. What is the real purpose of this discrete beast? How do we best use it? To understand these and other questions, we must go back to its . . .
The concept of the LFE channel was conceived a long time ago (even before Dolby Surround Sound). Back in the days of 70mm movie prints, an LFE type channel was conceived as a method of achieving higher bass output from an existing magnetic sound system. The large 70mm prints could accommodate 6 channels of magnetic sound: 5 screen channels and 1 surround. Although an excellent system for it's day, it did have headroom limitation and conventional screen speakers were hard pressed to voice useful information below 40 Hz. It was noted that often, sound engineers were only using three of the five screen channels. Dolby gave the two unused tracks a new role, affectionately known in its day as the "boom track". By putting additional bass information for a subwoofer on a separate track, deeper and louder bass could be delivered within the confines of existing hardware installations:
- A discrete channel would not tax the headroom of the existing screen channels with extra information.
- Did not requiring replacement of the front three channel's hardware (with more powerful amps and deeper reaching speakers).
- And no additional crossovers were needed in the signal path.
In short, it was an economical and practical way of achieving extra bass impact in the existing commercial theater system.
Think of the speakers as water faucets, each with its own pipe and water source, but working together to fill a bucket. When all three front 'faucets' are open wide, the only way to get more water flowing is to add a fourth water supply (the LFE channel) with its own pipe and faucet.
To this day, cinemas equipped for 70mm playback are the minority. Going back as far as 20 years, for the Dolby Stereo optical system for 35mm prints, installations which incorporated subwoofer hardware could elect to perform bass redirection (the grandfather of bass management). Here, bass from the main channels, originally under 100 Hz, would be copied from the three screen channels and sent to the subwoofer system. This does not provide any high-pass for the main channels but copies the bass to more capable hardware. In the last 12 years, the cutoff for this system has been dropped to as low as 50 Hz thanks to better hardware for the screen channels.
The LFE track of today's sound systems is a product of the evolution of these two subwoofer systems.
With a digital sound system, and specifically the bass management that we are now accustomed to, it would have been conceivable to create a 5 channel system without the LFE channel. As a measure of maintaining compatibility with existing infrastructure, the LFE channel was included when Dolby Digital was on the drawing table. And while they were at it, they created one killer bass delivery system which, unfortunately, is often misunderstood.
To understand the LFE track, we must first touch on the subject of calibrated loudness levels. In a production facility, pink noise is generated at 20 dB (Figure1) lower than the loudest sound the digital system permits. When this sound is played through each channel, the actual volume heard is adjusted to be 85 dB on a Real Time Analyzer (RTA). In this way, the loudest possible sound in the soundtrack, the peak, will be 105 dB for each channel. When a movie theater sets its playback volume by the same rules, the audience hears the soundtrack at the level and with the impact that the sound engineer intended. Of course, that doesn't mean the audience likes it that way. If you turn around during a crash boom sequence and you can see the teeth of the people sitting behind you, it is probably "too loud" for their tastes. In other words, maybe we don't really want to hear a 357 Magnum as loud as it is in real life, but that may be the engineer's design, and can easily be achieved with today's digital technology.
For optical analogue soundtracks, the CP500 cinema processor can copy bass from any channel to the subwoofer(s). Again, this is a copy and no high-pass (where low frequencies would be eliminated) is generated for the main channels. This information is calibrated to the same 85 dB, within the subwoofer's bandwidth (Figure 2).
Dolby Digital's LFE channel carries additional bass information from 120 Hz on down. This is not a roll-off but a digital brick wall (i.e., no 121 Hz info), so the content is usually rolled off by the sound engineer starting around 80 Hz for a smoother blend. During both soundtrack production and in the movie theaters, the LFE channel, with that same level (-20dB) pink noise (but band limited to the subwoofers range), is calibrated to 95 dB on the RTA within the sub's bandwidth (Figures 2 and 3). This is done so that it can play 10 dB higher than any one of the screen channels. Because of this 10 dB offset, the LFE channel can achieve a balanced output of bass as compared with the total output of bass from the three screen channels (in other words it can single handedly compete with the screen channels in terms of level). The only down side is that we lose a little S/N (signal to noise) performance on that track. Because our hearing is less sensitive to bass to begin with, the system gets away with it just fine.
With the generous bandwidth and headroom in each digital channel, an appreciable amount of bass can reside in the screen channels themselves. This can spread things out nicely or can be used to enhance directional cues. When the sound engineer wants even more, they can elect to put additional bass information in the LFE channel, tapping into those big 18" drivers and what essentially amounts to more bass headroom. In an all out assault, with deep bass in all channels, the LFE channel makes possible an extra 6 dB of headroom over and above what the three screen channels can deliver as a group. In a nutshell, the LFE track is that extra kick in the pants!
If you are doing the math right now, you may have already concluded that in a text-book Dolby Digital theater, one heck of a subwoofer system is called for! Let me explain.
In a movie theater on the island of Utopia, with ideal hardware, any single screen channel should be capable of a clean 105 dB peak with it's own respectable bass. The LFE channel should be capable of a 115 dB peak. Drive all channels to the max and the system should be able to slam you with approximately 120 dB of bass information. Thank you Dolby.
Doing this right means having some powerful hardware. Deep bass at high output necessitates considerable air displacement. This in turn calls for large speaker drivers with appreciable excursion, which further calls for considerable amplifier power. For a modest cinema, two 18" drivers could be considered a practical minimum, with some road-show-sized theaters using as many as eight drivers, backed up by some 4,000 watts of amplifier power to deliver the goods.
DTS's LFE channel is a little different. In fact, in the cinema, DTS is actually 5.0 (as oppose to 5.1) in that there is no discrete LFE channel. The LFE channel from the mixing sessions gets low-passed at 80 Hz and added to the surround channels. In the theater, the surround channels are high-passed at 80 Hz with the reciprocal becoming the LFE channel. The reasons for this multiplexing is one of space. DTS's original encoder only handles full-bandwidth channels so a discrete LFE channel would require a full 20% more space on the medium (the disc) as if it were another full range channel.
LFE in the home
With bass management (the ability to strip bass from one channel and send it to another) as a standard part of all our Dolby Digital home equipment, it would have been conceivable for the home delivery to do without the LFE track. But again, for the sake of being able to transcribe a cinema soundtrack directly to DVD or Laserdisc, the LFE channel remains part of the system. Despite the stigma that in the cinema the LFE channel is 'the subwoofer' channel, most home subwoofer(s) will be asked to voice bass from both the main channels and the LFE track. Two different sources, two different settings. The LFE channel and the subwoofer are not really synonymous or interchangeable from that point of view.
Stop! Before you run home and set your subwoofer 10 dB higher than you've already set it, you need to read on and find out why this is not necessary.
For consumers, we want things to be simple. Up until recently, only serious enthusiasts would use an SPL meter to set the levels of their home theater equipment, let alone ask everyone to remember to set the LFE channel differently. For this reason, home Dolby Digital equipment is pre-set to play LFE data 10 dB higher than a main channel (or 10dB higher than the bass from a main channel). It is only necessary to set the subwoofer relative to a main channel and the LFE level will be correct. Very few processors allow direct manipulation of the LFE level. That is why the menus on most receivers say "Subwoofer Level" rather than "LFE Level".
We home users don't use an RTA to set-up our systems, but if we did, it could look something like Figure 4. Bass which is redirected from a main channel would be balanced with the output of that channel, and the LFE data automatically playing 10 dB higher. Most of us don't have an RTA so an SLP meter (sound pressure level) has to do. There are numerous sources of test tones for setting the subwoofer level (different from LFE level), all optimized for a simple SPL measurement (AVIA, Video Essentials, and Delos Surround Spectacular, to name a few). It is very important to realize that if the subwoofer level has been set correctly using one of these tools to match a screen channel, the LFE data will be at the appropriate setting without any further adjustment. If you were to raise or lower the subwoofer level, the LFE level would rise or fall with it, tracking it at +10 dB. Processors that let you manipulate the LFE level independent of the subwoofer level are few, and most of these only let you reduce the offset to protect a less capable subwoofer from undue stress.
DTS's LFE channel in consumer applications, unlike its cinema counterpart, is discrete but still has a few 'special' considerations. In the early days of mixing 5.1 for DTS CDs, the studios were not being calibrated correctly, with the end result of the LFE channel being set too low. When the material is played back on a correctly calibrated system, the LFE channel is way too high. THX was forced to introduce DTS Music and DTS Movie playback modes which distinguish between a correct LFE setting and a -10 dB setting to compensate for material assembled under mis-calibrated circumstances (though not all THX processors offer this convenience).
In Utopia again, a home theater would have all three screen channels capable of true full range playback, which includes deep bass, and do so with the prescribed 105 dB peak. This is almost never the case in the real world. We need to ask our subwoofer to do more than just voice the LFE channel. To quote Miller & Kreisel, bosses of bass, "never send only the LFE to your subwoofer". No, they're not just trying to sell you a sub. There are very sound reasons for this advice. Some have to do with the software (the movies on the DVDs), some with the hardware.
One reason not to send a subwoofer the LFE channel alone is that there may not be anything on it or there may not be an LFE track at all. When coming out with the DVD format, it was important that consumers be able to always see and hear the program, without extra hardware like 5.1 decoders. This is why there is always an AC-3 or PCM soundtrack: Either can be played by any DVD player. Even a 6-channel AC-3 mix can be played without a 6-channel decoder because of the DVD player's internal downmixer. This takes the soundtrack and downmixes it to a 2-channel Pro Logic output. Important is the fact that the LFE channel is not part of this downmix and gets discarded. Production facilities know that unless they provide a dedicated two channel soundtrack, nothing important should be exclusively put in the LFE channel: Depending on the playback system, it may not be heard. When done right, essential information (which can include deep bass) will be put in the screen channels.
So, on some soundtracks, sending only the LFE to your subwoofer could leave it with nothing to do while your mains struggle. While some people like to feed a full range signal to their main speakers, they should in addition send those channels' bass to the sub so it can fill in the extreme bottom octave information that might be in those tracks. Mains which can reach as low as a dedicated subwoofer are few. Unfortunately this flexibility is not always available on consumer equipment; setting a main speaker to 'large' often excludes the subwoofer from getting a share of that signal.
On the flip side, you don't need a subwoofer to hear the content of the LFE track. If your mains are of substantial mettle, and you don't have a sub, bass management can usually re-route the LFE to your fronts.
Therefore, the subwoofer jack can contain only LFE (not recommended), a mixture of LFE and bass from the main 5 channels, or nothing at all.
The purpose of the LFE remains one of headroom (going back to our water faucet example). By supplying deep bass information on a segregated track, the system effectively permits a higher output level of deep bass information while not imposing on the performance or levels of the rest of the system. This whole system is optimized for a theater's configuration, but thanks to the flexibility of our home processors, we don't need to match that configuration to get good results. In the home, we are in much more intimate settings and at reasonable levels, we can comfortably use one or two subwoofers to produce all the deep bass of the soundtrack, including the LFE channel. There is nothing we need to do as consumers to set up our systems properly beyond the simple procedures provided by AVIA and Video Essentials. But, because of the potentially high output demanded by the presence of a full blown LFE track, judicious management of playback levels is in order. When we attempt to watch a movie at the 'reference' level, that is, with the channels set to play the reference test tones at 85 dB, we run the very real risk of discovering our subwoofer's limits the hard way. Candidly speaking, with bass redirected from other channels in addition to the LFE track, 'reference' level playback could end up asking a sub for something in excess of 115dB (a level that would kill most modest subs). So why haven't we all blown our subs to pieces? It's time to put away the macho image and admit few of us watch a full movie at these levels. And, in my experience, even a 75 dB setup level is a measure louder than most movie enthusiasts enjoy in the home. If you do crave the headroom of the ideal cinema configuration, the most cost effective and practical solution is to simply add a real high performance subwoofer or stack a second one on top of your existing sub.
So before you try to get a cinematic 115 dB LFE peak out of your subwoofer, you should know that subs capable of this so called reference peak level are few and generally the more expensive ones (Note: I think our editor's lab is up to the task with two Velodyne FSR-18s, two Velodyne F-1500s, one Velodyne HGS-18, one M&K MX-5000THX, and one Mirage BPSS-210). Trying to achieve it may be harmful to your hardware and your hearing if you're not careful, and is not necessary for a pleasing home theater experience.
I refer to the LFE channel in relation to the bass of the three screen channels. The surround channels are full range and can contain deep bass ("Saving Private Ryan" comes to mind). But, because of the physical size and cost constraints of multiple wall-hanging speakers in a cinema, more often than not the front three channels and/or the LFE channel are better places to put bass. It's a free county though so cinemas and home users are free to experiment with subwoofers for all channels, finances permitting.
When setting the level of your speakers and sub using AVIA, a setting on the volume knob that yields 85 dB for each of the channels is the so called reference level (the level to use for Video Essentials is somewhat different). All things being equal, if you leave the volume there, the mains should then peak at 105 dB and the LFE information at 115 dB. While this is technically the level for cinemas, there are many important differences between SPL and RTA calibration (which are too complex to discuss here). The close quarters of home theater, not to mention the limitation of the hardware, can make this a very high level at which to watch a two hour movie. Please use common sense when enjoying your system. Anything above 80 dB can permanently deteriorate your hearing with tinnitus as one of the results.
I would like to thank Guy Kuo of Ovation Software and Lonny Jennings of Dolby Labs for their time and assistance in the writing of this article.