Bass Management or Redirection

In theater practice, routing of the signals from the source medium into the loudspeakers is simple. The main loudspeaker channels are "full range" and, thus, each of the five main channels on the medium is routed to the respective loudspeakers, and the 0.1 channel is routed to one or more subwoofers.This seemed like such an elegant system,

but it did have a flaw. When I played Laser Discs at home, made from film masters, I noticed that my wide-range home stereo system went lower in the bass than the main channel systems we used in dubbing. So what constitutes a "full-range" channel? Is it essentially flat to 40 Hz and rolled off steeply below there like modern direct-radiator, vented-box main channel theater systems installed in large, flat baffle walls? Or is it the 25 Hz range or below of the best home systems?This was disturbing, because my home system revealed rumbles that were inaudible, or barely audible when you knew where they were, on the dubbing stage system, even though it had the most bass extension of any such professional system.

During this time, I decided to use a technique from satellite-subwoofer home systems, and sum the lowest bass (below 40 Hz) from the five main channels and send it to the subwoofer, along with the 0.1-channel content,Thus, the subwoofer could do double duty, extending the main channels downwards infrequency, as well as adding the extra 0.1 -channel content with high potential headroom. All this so the re-recording mixers could hear all of the bass content being recorded on the master.

This sounds like a small matter at first thought. Does a difference in bass extension from 40 to 25 Hz really count? As it turns out, we have to look back at psychoacoustics, and there we find an answer for why these differences are more audible than expected from just the numbers. Human hearing is more sensitive to changes in the bass than changes in the mid-range.

The equal loudness contour curves are not only rising in the bass, but they are converging as well.This means that a level change of the lowest frequencies is magnified, compared to the same change at mid-range frequencies. Think of it this way: as you go up 10dB at 1kHz, you cross 10dB worth of equal loudness contours, but as you go up 10dB at 25Hz, you cross more of the contour lines—the bass change counts more than the mid-range one. This does not perhaps correspond to everyday experience of operating equalizers, because most signal sources contain more mid-range content compared to very low frequencies, so this point may seem counterintuitive, but it is nonetheless true. So, since extending the bass downwards in frequency by just 15Hz (from 40 to 25Hz) changes the level at the lower frequency by a considerable amount, the difference is audibly great.

Digital Television Comes Along

When the 5.1-channel system was adopted for DigitalTelevision, a question arose from the set manufacturers: What to do with the 0.1 channel?

Surely not all televisions were going to be equipped with subwoofers, yet it was important that the most sophisticated home theaters have the channel available to them. After all, it was going to be available from DVD, so why not from Digital Television? A change in the definition of how the 0.1 channel is thought of occurred. The name "Low-Frequency Enhancement" (LFE) was chosen to describe what had been called the Baby Boom or 0.1 channel up until that time. The LFE name was meant to alert program providers that reproduction of the 0.1-channel content over television is optional on the part of the end user set. In theatrical release, having digital playback in a theater ensures that there will be a subwoofer present to reproduce the channel. Since this condition is not necessarily true for television, the nature of the program content that is to be recorded in the channel changes. To quote from ATSC Standard A/54, "Decoding of the LFE channel is receiver optional. The LFE channel provides non-essential low-frequency effects enhancement, but at levels up to 10dB higher than the other audio channels. Reproduction of this channel is not essential to enjoyment of the program, and can be perilous if the reproduction equipment cannot handle high levels of low-frequency sound energy. Typical receivers may thus only decode and provide five audio channels from the selected main audio service, not six (counting the 0.1 as one)."

This leads to a most important recommendation: Do not record essential story-telling sound content only in the LFE channel for digital television. For existing films to be transferred to DTV, an examination should be made of the content of LFE to be certain that the essential story-telling elements are present in some combination of the five main channels, or else it may be lost to many viewers. If a particular recording exists only in the LFE channel, say the sound effect of the Batmobile, then the master needs remixing for television release. Such a remix has been called a "Consumer 5.1" mix.

Bass management brings with it an overhead on the headroom capacity of subwoofers and associated amplifiers. Since bass management sums together 5 channels with the LFE channel at +10dB gain (which is what gives the LFE channel 10dB more headroom), the sum can reach rather surprisingly high values. For instance, if a room is calibrated for 85 dB (C-weighted, slow) with -20dBFS noise (see level calibration at the end of this chapter), then the subwoofer should be able to produce a sound pressure level of 121 dB SPL in its passband! That is because 5 channels plus the LFE, with the same signal in phase on all the channels, can add to such a high value. Film mixers have understood for a long time that to produce the highest level of bass, the signal is put in phase in all of the channels, and this is what they sometimes do, significantly "raising the bar" for bass managed systems.

Home Reproduction

When it comes to home theater, a version of the satellite-subwoofer system is in widespread use, like what we had been doing in dubbing stages for years, but with higher crossover frequencies. Called bass management, this is a system of high-pass filtering the signals to the five main channels, in parallel with summing the 5 channels together and low-pass filtering the sum to send to the subwoofer. Here, psycho-acoustics is useful too, because it has been shown that the very lowest frequencies have minimally audible stereophonic effect, and thus may be reproduced monophonically with little trouble. Work on this was reported in two important papers: "Perceptibility of Direction and Time Delay Errors in Subwoofer Reproduction," by Juhani Borenius, AES Preprint 2290, and "Loudspeaker Reproduction: Study on the Subwoofer Concept," byChristoph Kuglerand GuntherTheile.AES Preprint 3335.

Many people call the LFE channel the "subwoofer channel."This idea is a carry over from cinema practice, where each channel on the medium gets sent to its associated loudspeaker. Surely many others outside of cinema practice are doing the same thing. They run the risk that their "full-range" main channel loudspeakers are not reproducing the very lowest frequencies, and home theater listeners using a bass management system may hear lower frequencies than the producer! This could include air conditioning rumble, thumps from the conductor stomping on the podium, nearby subways, and analog tape punch-in thumps, to mention just a few of many other undesired noises. Of course desirable low-frequency sound may also be lost in monitoring.

In fact, the LFE channel is the space on the medium for the 0.1 channel, whereas the "subwoofer channel" is the output of the bass management process, after the lowest frequencies from the five main channels have been summed with the 0.1-channel content.

0.1 for Music?

The LFE channel came along with the introduction of AC-3 and Digital Theater System (DTS) to the Laser Disc and DVD media for the purpose of reproducing the channel that had been prepared for the theater at home. Naturally then, it seems logical to supply the function for audio-only media, like multichannel disc formats. While many may question the utility of the added low-frequency headroom for music, multichannel music producers are already using the channel. Among its advantages are not just the added LF headroom, but the accompanying decrease in intermodulation distortion ofthe main channel loudspeakers when handling large amounts of low bass. If the bass required to sound loud were to be put into the main channels, it would cause such intermodulation,

but in a separate channel, it cannot. Whether this is audible or not is certainly debatable, but it is not debatable that having a separate channel reproduced by a subwoofer eliminates the possibility of intermodulation (except in the air of the playback room at really high levels!).

What is most important to the use of LFE for music is the understanding that standards exist for the bandwidth and level of the LFE channel compared to the main channels. Since the bandwidth is controlled by the media encoders, getting the monitor level for the 0.1 channel right is the bottom line.

First, however, it is important to know that what you are monitoring has the correct bandwidth. If you record a bass drum to the LFE channel of a digital multitrack, then play it back in your studio to a subwoofer, you have made a mistake. The problem is that the only bandwidth limiting being done is the high-frequency limit of your subwoofer. You may be very surprised to find that after the tape is mastered, the bass drum has lost all of its "thwack," because the bandwidth I imitation of the LFE channel has come into play. Correct bass management in the studio will allow you to hear what the format encoder is going to do to the LFE channel, and you can mix the higher frequencies of the bass drum into the main channels, as well as its fundamentals into the LFE channel, for best reproduction.

Note that all systems employing the 5.1- or 7.1-channel configurations, whether they are on film or disc, or intended for broadcast, and whether coded by linear PCM, AC-3, DTS, or MPEG, all have two vital specifications that are the same: the sample rate of the LFE channel is 240 Hz for 48kHz sampled systems leading to practically 120 Hz bandwidth (and proportionately lower for 44.1 kHz systems), and the intended playback level is +10dB of "in-band gain" compared to the main channels (Fig. 2-10).

"In-band gain" means that the level in each 1/3-octave band in the main operating range of the subwoofer is 10 dB above the level of each of the 1/3-octave bands of one of the main channels, averaged across its main frequency range. This does not mean that the level measured with a sound level meter will measure 10dB higher, when the LFE channel is compared to a main channel. The reason for this apparent anomaly is that the bandwidth of the main channel is much wider than that of the LFE channel, which leads to the difference—there's more overall energy in a wider bandwidth signal. In an emergency, you could set the LFE level with a sound level meter. It will not read 10dB above the level of broadband pink noise for the reason explained, but instead about 4dB, when measuring with a C-weighting characteristic available even on the simple Radio Shack sound level meter. There are many possible sources of error using just a sound level meter, so this is not a recommended practice, but may have to do in a pinch,

Fig. 2-10 1/3-octave band spectrum analyzer display showing one main channel level in dB SPL versus frequency in Hz.The low-frequency rolloff is typical of a home system; a professional system might roll off starting about an octave lower. The high-frequency rolloff is explained in the section on equalization. Note the average mid-band 1/3-octave level is about 70dB SPL. All of these band together add up to an overall spectrum level of 83dB SPL.

Fig. 2-11 1/3-octave band spectrum analyzer display showing level in dB SPL versus frequency in Hz of a main channel spliced to a subwoofer.This is one of the jobs of bass management—to extend the low-frequency limit on each of the main channels by applying the correct signal to one or more subwoofers.

Fig. 2-12 1/3-octave band spectrum analyzer display showing level versus frequency of a properly aligned LFE channel playing over the same subwoofer as used above.The level of pink noise on the medium is the same as for Fig. 2-11, but the reproduction level is +10dB of in-band gain.

Fig. 2-13 Block schematic diagram of 5.1-channel systems without bass management.This is typical of all motion picture theaters and most film dubbing stages and television mixing rooms. Although the main channels are "wide range," they typically roll off below 40 Hz, so the very lowest frequencies are attenuated in the main channels.This can lead to not hearing certain problems, covered in the text.

An anti-aliasing, low-pass filter is included in media encoders, such as those by Dolby and DTS, in the LFE channel. If you were to listen in the studio to a non-band-limited LFE source over many subwoofer models, you would hear program content out to perhaps 1-2 kHz, which would then subsequently be filtered out by the media encoder. This means you would hear greater subwoofer bandwidth from your source channels than after encoding.Thus it is important in the studio to use a low-pass filter in monitoring the LFE channel when a media encoder is not in use. Characteristics of this filter are a bandwidth of 110 Hz, and a very steep slope.

Typical specs for the filters in pro-audio might be: (1) 50 Hz 2-pole Butterworth (12dB/octave) high pass in each main channel; (2) two 50 Hz 2-pole Butterworth (12dB/octave each) low-pass filters in series in the summed subwoofer path; and (3) 110Hz steep anti-aliasing filter in the LFE feed. Summing the electrical filters and the response of the main channel speakers and subwoofer produces a fourth-order Linkwitz-Riley acoustic response. The slopes (outside the high-pass/ low-pass symbols) and frequencies (given inside the symbols) represent typical professional system use (Fig. 2-14).

Typical specs for the filters for high-quality home theater are: (1) 80 Hz 2-pole Butterworth high pass in each main channel, (2) two 80 Hz 2-pole Butterworth low-pass filters in series in the summed subwoofer path; and (3) anti-aliasing filter for the LFE channel, built into the format decoder.

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