By Michael Bishop, Recording and Mix Engineer

Much has been written and discussed about the use of the center channel in music recordings. 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, not music. Videos have most of the dialog assigned only to the center channel – it has to be present. Mom or Dad is not likely to move speakers around just to listen to music in-between the teenagers watching The Fast & the Furious and Half-Baked. I make surround music mixes that use the center channel, but I don’t rely on it being there. I’ll usually duplicate the center channel assignment to the front left and right channels just in case there is no center channel present on playback. For me, the center channel will always be the center channel – case closed.

The LFE track is another story, however. There is room for improvement in the use of this track and I have tried alternate uses of the LFE track in mixing for several SACD and DVD-Audio releases. 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. By reproducing the height information from the recording space, the sense of "being there" is greatly enhanced. A properly employed height channel combined with the five main channels can complete 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 if this channel is not used in playback an excellent surround presentation can still be experienced.

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. Naturally, they don’t want anything to raise problems in the mind of a person considering investing in a new surround system. I don’t either –

I want to see LOTS of surround systems out there and a demand for surround music titles when the consumer grows tired of watching videos. The fine-tuning or customizing of a surround playback system is beyond the capabilities of the average home listener who has little interest in "tweeking". At this early stage of getting surround music going in the home, we need to help make the process of setup as simple as possible. Any deviation from the standard instructions provided with the home-theater-in-a-box will send many a consumer into a trance-like state as they stare at the pile of speakers and hook-up cables. All they want is to get the system going as quickly as possible to watch Terminator 2 that evening and they’re starting to get a headache… Any thoughts of surround music playback will be an added bonus or an afterthought. We can’t be throwing anything out of the ordinary at these people - yet. Worse is the likely spousal resistance to placing five or more speakers in the family room: "Do you really need ALL of THIS just to watch a movie???" Now try to get the poor sap to put the speakers where they’ll sound best – good luck! The process of getting surround music playback into many homes is going be a slow one. 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 he won’t know why.

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 setup and tuned six-channel (or more!) surround system. For people that don’t mind a departure from the mass merchandiser boxed surround systems and possess at least a minimum of technical know-how, 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 I am using to mix this combination channel allows the use of either a subwoofer or height channel – or BOTH where appropriate, from the same audio track. In the likelihood that the combination track would be routed to a subwoofer only, the system will reproduce properly balanced low-end. Adding the height channel to the playback system will reveal the added information provided on the special combination channel. There will be no instructions provided in the disc’s liner notes on how this combination track is to be employed. Not wanting to add further confusion to the already somewhat complicated task of setting up a surround system, a reference will be made to the record label’s web site where there can be setup details on how to optimize one’s surround system and include the height channel. Curious, adventurous types will no doubt visit the site and implement the instructions provided and probably come up with innovations of their own.

During the original orchestral recording session I place an omni-directional 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 Ambisonics¹ recording.

During post-production 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 125 Hz at 24 dB/octave. I carefully listen as I combine the various channels to the LFE buss 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 phase relationships across the channels and cancellation or diminishment of the low-end can occur if you’re not careful.

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 at the proper level for an LFE-only track, i.e., 10 dB below the main channel reference level.. The result is a track that contains the intended bandwidth-limited low frequency information for the subwoofer and a bandwidth-limited height channel signal. Since the height channel signal is mostly reverberation and

in-direct 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 packing² 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. Classical orchestral pieces are often over 70 minutes in length. 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. Limiting the very high frequency information of the height channel (where it isn’t needed anyway) can make the difference between the program fitting on the disc or not fitting.

To implement the combination height – LFE channel, the multi-channel SACD or DVD-A player and surround preamplifier need to present a full bandwidth signal at the 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. Whatever the receiver, a post-volume control preamplifier output of the LFE signal is required.

The type of surround speaker system one could run into in the home can be quite varied. You’ll see basically three types of systems 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 tri-amplified. Except for extra very-low frequency "oomph", there is little need for a separate

subwoofer system. Players, decoders, preamplifier, and amplifiers are separate components, each

chosen for its individual features, quality and characteristics. Adding a height channel is more easily

done with this system as it is already highly customized. The owner is usually knowledgeable of how

his system works and probably likes to tinker and tweek. 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. Some of these systems border on being glorified boom-boxes. Some actually give pretty good performance considering the price. 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. Maybe, maybe not - it’s a crap shoot… Chances are slim that a height channel can be added without some ingenuity and extreme effort on the owner’s part. The limitations of the system don’t really allow for experimentation. 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.

Regardless of the type of system, should the combination height – LFE track be routed solely to a subwoofer by a playback system default setting or ignorance, the system’s Bass Management filtering or subwoofer crossover will virtually eliminate the higher frequency height signal. What signal may leak through should not interfere with the subwoofer’s performance. The consumer will still be happy.

A note to those that would have a height channel added to their system: 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.

As we’re seeing 5.1 systems proliferate, riding on high volume DVD-Video sales and rentals, there should also be a growing demand for surround music titles. It’s only a matter of time before consumers learn they can play music on their home theater systems. One can watch the same movie 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! If a system can handle a demanding surround music program, it will certainly play movie soundtracks with ease.

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. We’ve got to remember that at the professional end of audio we are may be 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, it builds the future of the market to explore exciting possibilities that stretch the imagination in presenting surround music recordings in new and creative ways. 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!