Getting dual use from a specially recorded height-LFE channel on multichannel SACD
By Michael Bishop, Recording and Mix Engineer, Telarc International Corporation
All essential soundtrack elements of many blockbuster films, these sounds necessitated the creation of the LFE and center channels in today�s 5.1 surround playback systems. The typical home multichannel system is meant to duplicate the movie theater experience and optimize the playback of surround video soundtracks. Such systems do not necessarily serve the needs of music reproduction, especially in an audiophile-quality, multichannel, full-range system. Many music mix engineers have pondered just what to do with the pesky center and LFE tracks, nagged by questions of compatibility across the many possible home surround setups � and wondering whether they are wasting two playback channels on the few sounds assigned to them during the typical surround music mix.
Alternate uses for the center and LFE channels are already being considered and implemented by a few renegade recording engineers and labels. Sometimes causing trouble � always causing discussion � they are trying to find a better way of presenting surround music recordings to the world. Even after years of working with surround recording and mixing techniques, we are still just at the beginning of developing a consumer-friendly surround format.
I accept that we are stuck with the center channel in home surround setups � after all, most consumers bought a home theater surround system to play DVD-Videos, where most of the dialog is assigned only to the center channel. I make surround music mixes that use the center channel, but I don�t rely on it being there. For me, the center channel will always be the center channel � no alternate use.
The LFE track is another story, however. The �.1� or LFE subwoofer track on a typical surround recording is dedicated to reproducing audio in the below 100 Hz or deep-bass region only. Typically there is little audio information on this track above 120 Hz, yet SACD, DTS, and DVD-Audio carriers are capable of containing six full-bandwidth audio tracks! Surround music recordings, especially recordings of acoustic music and orchestral performances, can benefit greatly from the use of a height channel in playback, completing the illusion of actually being in the performance space with the musicians. The height channel can help bridge the gap between the front and surround speakers, especially in difficult playback rooms. The height information in the recording is usually somewhat subtle, so even if this channel is not used in playback, an excellent surround presentation can still be experienced.
Pity the Poor Consumer
Dedicating alternate use of the LFE channel solely to height information can create problems, however. Hardware manufacturers, mostly due to fear of possible confusion of consumers new to surround music or cinema reproduction, discourage alternate use of the LFE track. The concept of fine-tuning or customizing a surround playback system is beyond the capabilities of most people. Any deviation from the standard instructions provided with the home-theater-in-a-box will send many consumers into a trance-like state as they stare at the pile of speakers and hook-up cables.
Another problem of using the LFE track as a height channel is that, in most cases, it will simply be ignored and routed to the system�s subwoofer by default. No damage will result from this signal misdirection, but the subwoofer may not be giving the optimum bass response needed to fill-out the bottom end of the typical satellite surround system. The surround music playback may sound thinner than the movie soundtracks played on the same system. The consumer will be less than satisfied with the audio quality, but won�t know why.
Not For the Faint of Heart
There are some adventurous souls out there, however, that want to get the most from their SACD, DVD-Audio, and home cinema surround playback experience � those that are looking for the realism possible in a well set up and tuned 6-channel (or more!) surround system. For these people there can be an alternative. I am currently recording and mixing a combination height-LFE channel in my orchestral and acoustic music surround recordings for inclusion on future SACD or DVD-A releases. The use of this channel as a height channel is completely optional � the method allows the use of either a subwoofer or height channel, or both where appropriate, from the same audio track. I
n the likelihood that the combination track would be routed to a subwoofer only, the system will reproduce properly balanced low-end � the system�s bass management filtering or subwoofer crossover will virtually eliminate the higher frequency height signal. Adding the height channel to the playback system will reveal the added information provided on the special combination channel. To avoid confusing the average consumer, there will be no instructions provided in the disc�s liner notes on how this combination track is to be employed, only a reference to the record label�s Web site where the curious and adventurous can find setup details on how to optimize their surround systems and include the height channel.
How it works
During the original orchestral recording session I place an omnidirectional microphone quite high above and in front of the orchestra or ensemble. This microphone captures the ambient sounds of the reverberation high in the performance space. The height microphone is adjusted for how intimate or spacious the recording is to be. The signal presented by this height microphone should not be confused with the periphony pickup of an Ambisonics recording.
During postproduction editing of the surround recording, I�ll create the LFE subwoofer channel mix, pulling in the low-end from the main and surround channels as needed and applying a low-pass filter of 160 Hz at 12 dB/octave. I carefully listen as I combine the various channels to the LFE bus of the digital mixing console, making sure there is not any cancellation of the low-end � low-frequency content in acoustic recording can have random, possibly degrading, phase relationships across the channels.
To complete the combination height-LFE track mix, I apply a high-pass filter at 180 Hz, 12 dB/octave, to the original height track recorded at the session. This filtered height signal is combined with the filtered LFE track. The combined height-LFE signal is recorded in place of the usual LFE-only track. Since the height channel signal is mostly reverberation and indirect reflections, there is little meaningful low-frequency or very high-frequency content. Remember that high frequencies diminish rapidly with distance from the sound source. I take advantage of that fact to save data space during lossless data packing used in SACD and DVD-A authoring by applying a high-frequency roll-off to the height channel signal in addition to the above-mentioned high-pass filter. Even with lossless data packing, many longer length high-resolution surround programs will not fit within the space requirements of SACD or DVD-A without some economizing of data, especially if a high-resolution stereo program is included on the disc.
Connection and Placement of the Height Speaker(s)
To implement the combination height-LFE channel, the multichannel SACD or DVD-A player and surround preamplifier need to present a full bandwidth signal at the preamplifier post-volume control LFE channel output. Most players will do this easily. That may not be the case with some surround receivers or preamps, however. One will usually have to dig deep into the bass management menus to �trick� the receiver into outputting a full-bandwidth signal on all channels. Some receivers will not be capable of changing the LFE output at all. Short of purchasing a better preamp or receiver, I have no further solutions for that problem.
The full-bandwidth LFE channel preamplifier output (not the speaker output!) is split with a �Y� and sent to both the subwoofer crossover and to a single-channel high-pass filter for the height speaker amplifier (see fig. 3). The subwoofer crossover feeds the subwoofer amplifier and the subwoofer. The high-pass filter equalizer output goes to an amplifier that powers the height speaker(s). The surround preamp or receiver will control the overall volume, but I suggest using subwoofer and height channel amplifiers that have level trims. Remember: The LFE track should be recorded at a reference level 10 dB below the main channels for proper subwoofer performance. The combined height signal will be at this same level � some adjustment will be needed on playback for the correct height effect.
As the height speakers will not have much demanded of them, relatively small speakers will be satisfactory; that�s pretty important to the person trying to hang these speakers high on the walls or from the ceiling. I have had success using two lightweight Magnapan model MGMC1 planar speakers as height speakers. The Magnapan quasi-ribbon, planar-magnetic drivers act as dipoles. As such, they can contribute to the ambient, diffusive nature of the height channel signal (see fig. 1).
I position the �Maggies� above and just behind the central listening position and angle them downward and toward the center of the room. It�s important to have the positive (front) side of the dipoles facing the front of the room for proper phase relationship.
Alternate height speaker setups could employ small speakers from Paradigm, Theil, Boston Acoustics, Bose, and many others (see fig. 2). You can try mounting them high on the side walls just behind the center of the room or on the ceiling. Again, dipoles will likely work best, and ideally the height speakers should be the same distance from the listener as the five main speakers.
Adjust the combined playback level of the height speakers to be equal to the main channels in the room by using pink noise and measuring from the listening position. The presence of the height signal in the overall mix should not make itself apparent to the listener.
One will only notice its presence if the signal is muted � that extra sense of being in the original recording room will disappear. Too much height channel level will make the listener feel as if he or she is stuck to the ceiling � not a comfortable place to be! A cautionary note: Playing a dedicated LFE-only track through the height speaker will not sound good! Mute the height speaker during playback of those Neanderthal surround recordings.
But Will It Play at (the Average) Home?
You�ll see basically three types of systems in the home, with many variations within each:
1. Audiophile Quality � cost is no object. All main speakers are premium full-range speakers with integral subwoofers for each channel. Equal amplifiers power all speakers, which may be bi- or triamplified. Except for extra very-low frequency �oomph,� there is little need for a separate subwoofer system. Players, decoders, preamplifier, and amplifiers are separate components. Adding a height channel is more easily done with this system as it is already highly customized. The owner is usually knowledgeable and probably likes to tinker and tweak. If the owner has a spouse, they probably don�t see each other for days�.
2. High Quality � reasonable cost. All main speakers are full-range with good bass response. A high-quality, but cost-effective, integral home theater receiver forms the heart of the system. The receiver�s bass management system runs the subwoofer system. One should be able to add a height speaker to this system, but it may take some digging through the receiver menus to get it to output a full-range signal at the receiver LFE output. The system�s owner may be willing to try something different and experiment with alternate setups. Many moderately spouse-acceptable systems available in this group.
3. Home Theater in a Box � low to moderate cost. You�ll see the most variations in features, quality, and versatility of setup in this group. Small satellite speakers of limited frequency response are the main speakers. The system subwoofer is essential to filling out the low-end on these systems and the main speakers are often fed from the processor and amplifier built into the subwoofer cabinet. Bass management, often with little user access to settings, derives the bass from the LFE track when the track is present. Hopefully, when an LFE track is not present at all, the bass information is automatically derived from the main channels. Chances are slim that a height channel can be added without some ingenuity and extreme effort on the owner�s part. It may be best not to tell these system owners about �strange� ways of setting up their systems for surround music playback. Being quite small, these systems are extremely spouse friendly.
It�s only a matter of time before consumers learn they can play music on their home theater systems. One can watch a movie only a limited number of times, but a music title can have a much longer playing life. When the audience at home finds they can play great surround music recordings, there is simply no going back to stereo!
It�s up to us, the audio professionals who are producing surround music titles, to make exciting recordings that turn the consumer on to surround. It�s up to us to help make surround music as consumer-friendly and accessible as possible � with the help of the hi-fi equipment manufacturers. At the professional end of audio, we are maybe four to five years ahead of the consumer market. It can be difficult to not let our enthusiasm for what we can do technologically in the studio get too far ahead of the market we are producing for. At the same time, experimentation builds the future of the market. When we get creative, we�ll certainly make mistakes along the way. When we make mistakes, we learn better ways of doing things.
Go out and make mistakes!