previous paper

next paper

print

authors

contents

Reproducing Commercial Multichannel
Formats Through a Single
Monitoring System
A. P. Goldberg
Genelec, Finland

Abstract
A modern audio production facility must be able to supply productions in a large
number of different formats. The change from mono & stereo to multi-channel
reproduction has created many problems, both in converting existing production
facilities to multichannel format and when building new installations. This paper
examines the LFE channel across different encoding formats and presents a method toreproduce it, using a bass management system, through a single monitoring system.

Introduction
Today, a modern audio production facility must be able to serve productions ina large number of different formats. The change from mono and stereo tomultichannel reproduction has produced many problems, both in converting
existing production facilities to multichannel format and in new installations. The
audio formats that must be handled by a modern production facility currently
include mono, stereo, matrix four channel formats, five 5.0 systems, 5 channel
systems with a separate Low Frequency Enhancement channel (5.1 systems) andadvanced multichannel formats such as 6.1 and 7.1. This paper presents a brief
overview of the current multichannel formats and a hardware solution to reproduce
all these formats using a single loudspeaker system.

Loudspeaker set-up for multichannel audio
The ITU that issues many recommendations for various application areas, and offers
two recommendations that are of particular interest for surround soundreproduction: ITU–R BS.775–1 [1] and ITU–R BS 1116–1 [2]. The following sections
highlight the most important points of these recommendations with regard to the
LFE channel and subwoofer.

Placement of the subwoofer in the room
Psychoacoustics research work has shown over the years that the very lowest
frequencies have minimal audible stereophonic effect and hence may be reproduced
by a single source [3, 4]. However, in some relatively recent, but uncorroborated,research [5], it is suggested that for best psychoacoustic perception results

Conference Publication 2003

365

Å Æ

previous paper

next paper

print

authors

contents(spaciousness and envelopment), there should be two subwoofers placed on eitherside of the room and driven with the same signal but with a 90° phase difference
between them.
A commonly recommended place for the subwoofer is at the front of the room, on
the floor and in the middle, equidistant from the sidewalls. This position can bea serious compromise since the subwoofer sits in the pressure minimum of the odd
numbered lateral standing waves. To overcome this, the subwoofer can be slightly
offset from the middle of the room or placed in one of the front corners. The second
option maximises the system efficiency due to corner loading but risks the increased
probability of localisation. Gain (input sensitivity) and frequency response (bass rolloff) adjustment of the subwoofer is necessary since the acoustical loading has changed
relative to calibrated anechoic conditions. The subwoofer can also be flush mounted
in the front wall but the discussion of the position of the source relative to the roomremains valid. The phase adjustment on the subwoofer at the crossover frequency
is also important to achieve a flat frequency response around the crossover region.
Sometimes the rear loudspeakers also need additional low frequency extension.
There are two options. Firstly, a bass management system can include the rearchannels as inputs and so the low frequency content will be reproduced through thesubwoofer(s). In this case, it is important that all satellite loudspeakers are placed on
the circle radius so that the arrival time difference between each loudspeaker and thesubwoofer is the same. It must be also noted that the acoustical behaviour of the room
at low frequency will greatly affect the way the low frequencies are summed andreplayed by a single subwoofer. In this case, the low frequency acoustical characteristicof the room should be as identical as possible in the front and in the rear part of theroom. Alternatively, a separate subwoofer can be used for the rear loudspeakers. The
location of this subwoofer should be to the rear of the room and positioned in asimilar fashion to the subwoofer for the front channels. If the room has very different
low frequency acoustical characteristics from the front to the back, the option of dual
subwoofers might provide better results than using only a single subwoofer.

The LFE channel
What is the LFE (.1) channel?
Often the LFE channel is referred to as the ‘subwoofer channel.’ In fact, the LFE channel
is the space on the recording medium for the .1 encoded, band limited audio channel.
The ‘subwoofer channel’ is not a ‘channel’ as such, as the subwoofer, together with thebass management system [6, 7], replays a specific low frequency bandwidth. The
subwoofer signal may consist of some, or all, of the LFE channel and possibly the low
frequencies of the main channels if bass management is used and depending on how
the system is set-up.
In a 5.1 surround sound system, the five main channels (Left, Centre, Right, Rear
Left and Rear Right) are all full bandwidth, i.e. <20 Hz to >20 kHz. Before encoding,the LFE channel is not band limited, i.e. it is just another full bandwidth channel.
Once encoded, the LFE channel has a limited bandwidth (hence the slightly
misleading label ‘.1’) from <20 Hz up to various upper cut-off frequencies dependingon the encoding format (Figure 1 and Table 1).

366International Broadcasting ConventionÅ Æprevious paper

next paper

print

authors

Encoding Formats

LFE Upper Cut-off Specifications

Dolby Digital (AC-3, 5.1)

120Hz, fixed cut-off [9]Dolby EX (AC-3, 6.1)

120Hz, fixed cut-off [9]DTS (Coherent Acoustics, 5.1)

120Hz, nominal [10]DTS-ES (Coherent Acoustics, 6.1)

120Hz, nominal [10]DVD-Audio (MLP)

Full bandwidth channel [11]Super Audio CD (Sony/Philips)

Full bandwidth channel [12]MPEG-2 BC (ISO/IEC 13818-3)

Most likely variable? [13]AAC MPEG-2 (ISO/IEC 13818-7)

Variable, up to 1kHz [14]Sony Dynamic Digital Sound (7.1)

Variable up to 330Hz [15]contents

Table 1: LFE channel bandwidths

The use of the LFE channel is not consistent throughout the audio industry, as theneeds of movie theatres (cinemas), home theatres and digital broadcasting (DVB &DAB) are all different. Also, sound engineers new to multichannel surround soundmixing are still experimenting with new ideas and techniques. In general, regardlessof the obvious benefits of the low frequency extension, the use of subwoofer(s) andreproduction of the LFE channel is currently causing many problems. These shouldbe overcome with proper bass management circuitry and more clearly defined
standard practices of the use of the LFE channel [8].
20Hz

20kHz

L
C
6 Channel Recording
Full Bandwidth

5.1
Encoding

R
RL
RR
LFE

Figure 1: Audio bandwidth of encoded 5.1 Materials

Headroom in the LFE channel [6, 7]
If the LFE channel is recorded at the same nominal level as the all the main channels
then a high SPL effect could easily overload the recording medium. This would
defeat the object of having a special channel for loud sound effects as the main
channels would have to be recorded at a lower level to accommodate the ‘big bangs’
on the LFE channel. To overcome this problem some encoding systems require the
LFE channel to be monitored with an in-band gain of +10 dB. In doing so, the audio
level recorded to tape will have an additional 10 dB of available headroom as theengineer will naturally reduce the level by 10 dB on the mixing consol to maintainthe sound level balance between the channels. Note that this increase in headroom
is at expense of 10 dB of signal-to-noise ratio but as the LFE channel is eventuallyband limited in the encoder, it is considered to be a price worth paying.

Conference Publication 2003

367

Å Æ

previous paper

next paper

print

authors

LFE

Output to
Monitoring systemcontents

LFE

Mixing Console Monitoring Section
+10dB
L

C

R

SL SC SR

Output to
Recording media

LFE

Figure 2: +10dB Level change in the LFE channel

To monitor the LFE channel correctly, there must be a +10 dB level change somewhere
between the LFE channel fader and the acoustic output of the monitoring system(Figure 2). This level change can happen at various places in the audio chain, but it
must be implemented during the production stage. Note that there are no levelchanges on any of the outputs to the tape or hard disk recorder. The net effect is that
there is 10 dB additional headroom on the LFE channel on the tape compared to themain channels. All the tracks are then encoded using various encoding schemes,
transported to the end user and replayed accordingly using a +10 dB gain stage on
the LFE channel only to restore the channel balance.

Implications for monitoring of the LFE channel audio content
As stated in various standards, the LFE channel was originally intended to carrysignals that are not included in the main channels [8, 16]. In the movie industry,engineers use the LFE channel to reproduce special low frequency effects, however,the practical use of the LFE channel is open to many possibilities.
As there are six (or more) channels to be replayed, it may seem logical to connect
each output of the multichannel source directly to the appropriately positioned
loudspeakers and to the subwoofer(s). However, without taking care that the entire
bandwidth of each channel is properly monitored with bass management or
redirecting some of the LFE channel correctly, important parts of the audio
bandwidth are not reproduced or serious cancellation effects can be induced, bothelectrically and acoustically.
Firstly, unless they are full bandwidth, each loudspeaker replaying the five or six
main channels will have a –3 dB LF cut-off that is higher than 20 Hz. Therefore, thelowest frequencies of the main channels will not be reproduced and monitored. Thisis a compromise as all the main channels are full bandwidth even after an encode/
decode cycle.
Secondly, the subwoofers considered later in this paper have a crossover point fixedat 85 Hz. Research has found that, after many investigations and subjective listening
tests [3, 4], a fixed 85 Hz frequency has been chosen for best acoustical results. If theencoded LFE channel is connected to the subwoofer and the main loudspeakers are
368

International Broadcasting ConventionÅ Æprevious paper

next paper

print

authors

contents

not connected via the subwoofer, audio material up to 85 Hz only will be monitored
via the subwoofer. No information above that frequency will be heard, even though
the LFE upper cut off can be higher.

Bass management
What is bass management?
In stereo reproduction, signals from 20 Hz to 20 kHz need to be replayed and goodquality multi-way loudspeaker systems can reproduce that bandwidth fairly evenly.
In multichannel audio, professional and consumer applications must be able toreproduce all frequencies from each channel. To achieve this, the main loudspeakers,
subwoofers and crossovers should work together to give a flat response foreach channel.
In small rooms with basic acoustic treatment and geometry, the frequency
response of a loudspeaker system below 100 Hz is dominated by the modal responseof the room, especially if it has parallel walls. Therefore, it is very difficult to achievea consistent low frequency response from multiple full-range loudspeakers in such acompromised space. One solution to this basic acoustical problem is to employ asystem called ‘Bass Management’ [6, 7]. Using active electronic filtering, the low
frequency information from the main channels can be extracted and routed to asingle subwoofer feed. The low frequencies now originate from a single source thatcan be placed in an optimum position in the room. Furthermore, the LFE channel canalso be monitored using the subwoofer by adding it to the low frequencies of theother main channels. Therefore, the Bass Management’s goal can be summarised as,‘To ensure that the entire audio bandwidth of all channels is accurately monitored.’In their recommendations [8], Dolby Laboratories say that, ‘Bass Management
allows the user to redirect low-frequency information from any of the main
loudspeakers to the subwoofer’. In addition [6, 7], Tomlinson Holman of TMH
Laboratories says that, ‘Bass Management in monitoring can be used to reproduce
both the very low frequency content of the main channels, as well as the LFE channel,
over one or more subwoofers.’
If the multichannel set-up uses small or mid sized monitors, then a subwoofer
should be included to extend the frequency response of all of the main channels andto correctly monitor the LFE channel. The low frequency cut-off of the subwoofer inthe room should be <20 Hz and the low pass filter of the subwoofer crossover shouldbe very steep so that midrange information is sufficiently attenuated. For the
subwoofer bandwidth, a fixed crossover point has been carefully chosen to be 85 Hz
to minimise the possibility of localisation of the subwoofer. This means that for thebass management of the six main channels, the crossover point between the
subwoofer and the main channels is set at 85 Hz.
Looking at the LFE channel after encoding, it can be seen that the upper cut-off
frequency can vary quite significantly. For that reason, the Bass Management should
provide three different setting possibilities for reproducing the LFE channel. The
setting of the LFE channel bandwidth on the subwoofer should not at all affect theencoding of the LFE channel in the various formats but should provide different
replay bandwidths.
Conference Publication 2003

369

Å Æ

previous paper

next paper

print

authors

contents

Example of a 6.1 bass management system
An example of a bass management system can be seen in the 7000 Series Subwoofers
manufactured by Genelec Oy [17]. The active electronics of the subwoofer are
combined with the bass management section thereby offering a versatile system. The
integrated 6.1 bass management system features:• Six main channels of inputs & outputs are provided for low frequency redirection.
Each channel is identical and has a fixed crossover frequency set at 85 Hz.
• Bass management bypass function for the six main channels. The LFE channel is
not affected by the bypass function, regardless of the LFE bandwidth andredirection settings.
• Discrete LFE input with user-selectable reproduction bandwidth with an upper
cut-off at 85 Hz or 120 Hz.
• Possibility to redirect the LFE information above 85 Hz to the Centre channel
when the bandwidth is set to 85 Hz.
• Selectable +10 dB LFE gain for the LFE channel.
• Possibility to add more subwoofers to the system for increased SPL and headroom.
• Indication of clipping and driver protection activation.
• 90° step phase adjustment at 85 Hz crossover frequency between the subwoofer
and the main loudspeakers. Built-in 85 Hz test tone generator tool for easy on-site
adjustment.
• Frequency response adjustment, Bass Roll-off: 0 dB, –2 dB, –4 dB and –6 dB.
• Variable input sensitivity from –6 dBu to +12 dBu (=100 dB SPL at 1 m).
The correct connection of the system is shown in Figure 3. All channels, including
the LFE channel, are connected through the bass management system with the six
main channels connected to each of the main loudspeakers. A single output from themaster subwoofer (Sum Out) is fed into the optional slave subwoofer unit (Sum In).
OVL
ONGENELEC

OVL
ON

OVL
ON

GENELECGENELEC

Remote Controls

L

OVL
ON

GENELEC

R SL SC SR LFE

OVL
ON

OVL
ON

GENELECC

GENELEC

Figure 3: Connection to the 6.1 bass management system

370

International Broadcasting ConventionÅ Æprevious paper

next paper

print

authors

contents

Full range LFE input
The first possibility for the LFE channel bandwidth is the reproduction of the LFE
channel by the subwoofer up to 85 Hz and then the redirection of the energy above
85 Hz to the Centre channel. This means that all information present on the LFE
channel will be reproduced through the loudspeaker system, whatever the upper
cut-off frequency of the LFE channel (Figure 4). Any noise, distortion artefacts andother unwanted sounds on the LFE channel can be monitored in addition to the LFE
signal itself.
In addition, any type of LFE content (coherent or non-coherent audio material) will
be properly monitored without unpredictable summing of the low frequencies
between the different channels in the room. This should be the default setting
for the bass management system as it gives full range monitoring of the
LFE channel.
20Hz

85Hz

20kHz

L
C
Mixing Desk Multichannel
Monitoring Section

R

OR

SL

Surround Sound
Decoder Outputs

SC
SR
Routed toCentre Channel

LFE
Subwoofer

Figure 4: Redirection of LFE channel above 85Hz to centre channel

LFE input band-limited to 85 Hz
The second possibility for the LFE channel bandwidth is the reproduction of the LFE
channel by the subwoofer up to 85 Hz, without any redirection above 85 Hz to theCentre channel. This setting is comparable to pre-filtering the LFE channel in the
mixing console using an 80 Hz LP filter [16]. No information above 85 Hz present on
the LFE channel will be heard through the loudspeaker system (Figure 5). Thissetting is not recommend for normal everyday use as the LFE channel has afrequency response up to 120 Hz in Dolby Digital & DTS and a higher frequency
response still on some of the other formats. A good use for this setting is to simulate
the side effect of some (usually cheaper) consumer decoders that, when the bass
management is used, do not replay the information on the LFE channel that is above
80 Hz. It is important for the mixing engineer to be aware of such a limitation so thatthe multichannel mix translates well in the home environment. This configuration
provides a replica of what will happen in many home surround system situations
and yields a consistent reproduction of the low frequencies below 85 Hz.

Conference Publication 2003

371

Å Æ

previous paper

next paper

print

20Hz

85Hz

authors

contents

20kHz

L
C
Mixing Desk Multichannel
Monitoring Section

R

OR

SL

Surround Sound
Decoder Outputs

SC
SR
LFE
Subwoofer

85 Hz cut-off
NO Routing toCentre Channel

Figure 5: LFE input band-limited to 85Hz

LFE input band-limited to 120 Hz
The third possibility for the LFE channel bandwidth is the reproduction of the LFE
channel by the subwoofer up to 120 Hz without any redirection above that frequency.
This means that no information above 120 Hz on the LFE channel will be heard
through the loudspeaker system (Figure 6). This setting has been provided so thatthe production facility can emulate the replay systems that exist in movie theatresand cinemas. In this application, there are strict rules on how the replay systemshould perform [16]. Dedicated subwoofers with a frequency response from
20–120 Hz reproduce the low frequency content for the 35 mm movie soundtracks.
However, it must be noted that when mixing music and sound effects for film release,engineers always use non-coherent low frequency information in the LFE channel
compared to the main channels. In other words, the low frequency LFE channel
content is different from any other main channel low frequency audio content so that
potentially unpredictable acoustic summing in the room is avoided.
20Hz

85Hz

20kHz

L
C
Mixing Desk Multichannel
Monitoring Section

R

OR

SL

Surround Sound
Decoder Outputs

SC
SR
LFE
Subwoofer

120 Hz cut-off
NO Redirection toCentre Channel

Figure 6: LFE input band-limited to 120 Hz

Bass management bypass function
The LFE channel is unaffected by the bypass function as it affects the main channels
only. This means that the low frequency extension of all main channels provided by
the subwoofer is bypassed and the main loudspeakers monitor the full bandwidth ofeach of the main channels. The low frequency reproduction of the main channels
will extend as low as the main loudspeaker low frequency extension. The LFE input
is not changed in any way and hence LFE bandwidth and routing settings are notaffected (Figure 7).
372

International Broadcasting ConventionÅ Æprevious paper

next paper

print

20Hz

authors

contents

20kHz

L
C
R

OR

SL

Surround Sound
Decoder Outputs

SC

Bypassed

Console Multichannel
Monitoring SectionSR– Routed to Center– 85 Hz fixed– 120 Hz fixed

LFE
Subwoofer

Figure 7: Main channels bypassed, LFE channel unchanged

Monitoring the LFE channel with full range loudspeakers
Some music recording engineers would rather not use subwoofers as they prefer touse ‘full bandwidth’ loudspeakers only. The argument is, for example, that in a realorchestra performance all the low frequency information does not come from one
physical point in space and that the harmonic content and various reflections in thehall guide our spatial cues towards different places for the instruments in theorchestra. Other engineers in the audio post-production and movie industry sectors
want to use subwoofers to reproduce the LFE channel only. In doing so, they emulate
the replay conditions found in movie theatres. In that situation, the audio content of
the LFE channel has to be carefully considered as cancellations may occur between
the audio replayed by the main channels and by the subwoofer.
20Hz

20kHz

L
C
Mixing Desk Multichannel
Monitoring Section

R

OR

LFE

Surround Sound
Decoder Outputs

SL

Routed to :C, or L/R, or L/C/RSCSR

Figure 8: Re-routing the LFE to full range loudspeakers

Whatever the opinions are, it is possible to monitor accurately all the channels
without a subwoofer when using multiple large full range loudspeakers. The
important point is to be able to monitor each channel in its full bandwidth. Therefore,
in this case, the five or six full range loudspeakers will reproduce the five or six main
channels and the LFE channel should be monitored via one, or more, of the full range
loudspeakers. Re-routing is required and can be done inside the mixing desk’s
multichannel monitoring section or in a separate multichannel loudspeakercontroller (Figure 8).
The most widely used and simplest routing for redirecting the LFE channel is tosend it to the Centre channel. There are other set-ups where the LFE channel isredirected to Front Left and Front Right loudspeakers (note that consumer andConference Publication 2003

373

Å Æ

previous paper

next paper

print

authors

contents

professional decoders do this if there is no subwoofer in the system) or even to all
three front loudspeakers (remembering the +10 dB LFE nominal level relative tomain channels, this is acoustically the best solution for SPL reasons). Note that whenrouting the LFE channel to more than one loudspeaker, the total SPL reproductionlevel of that channel increases. For example, routing the LFE to the Front Left andFront Right loudspeakers will add 6 dB SPL to the overall LFE channel level,
therefore, the output level of the LFE channel should be reduced accordingly in the
console monitoring matrix output. The LFE channel +10 dB gain also should besummed into this equation.

Conclusions
This paper has described the LFE channel, especially its gain relative to the main
channels and its upper cut off frequency in different multichannel formats.
The constraints of physical subwoofers and the LFE channel were discussed anda method that overcomes these constraints, called bass management, was described.
The handling of the LFE channel was given particular focus with reference todifferent operating methods in different industries and redirection of some or all of
the LFE content to different loudspeakers. The system presented is compatible withmono, two channel stereo, all matrix formats and discrete multichannel formats
up to 6.1.

References
1

ITU-R BS.775-1 ‘Multichannel stereophonic sound system with and without accompanying picture’,
International Telecommunications Union, Geneva 1992-94

2

ITU-R BS.1116-1 ‘Methods for the Assessment of Small Impairments in Audio Systems Including
Multichannel Sound Systems’, International Telecommunications Union, Geneva 1994–97

3

Borenius, J., ‘Perceptibility of Direction and Time Delay Errors in Subwoofer Reproduction’, Presented at
79th Conv. Audio Eng. Soc., preprint 2290. Oct. 1985

4

Kügler, C. and Theile, G., ‘Loudspeaker Reproduction: Study on the Subwoofer Concept’, Presented at
92nd Conv. Audio Eng. Soc., preprint 3335. Mar. 1992

5

Griesinger, D., ‘Speaker Placement, Externalisation, and Envelopment in Home Listening Rooms’,
Presented at 105th Conv. Audio Eng. Soc., preprint 4860. Sep. 1998

6

Surround Professional, Issue 1; ‘Bass Management Basics.’, Tomlinson Holman

7

Holman, T., ‘5.1 Surround Sound: Up and Running’, Focal Press, 2000

8

Dolby Laboratories Inc., ‘5.1 Channel Production Guidelines, Issue 1’

9Dolby Laboratories Inc., www.dolby.com, Mar. 2003

10

Digital Theatre Systems DTS, www.dtsonline.com, Mar. 2003

11

Meridian Audio Ltd., www.meridian-audio.com/, Mar 2003

12

Sony Technical Information, ‘SACD DSD Recording & Format’, 2001

13

MPEG Audio, www.mpeg.org/MPEG/audio.html, Mar. 2003

14

MPEG AAC, www.mpeg.org/MPEG/aac.html, Mar. 2003

15

SDDS, www.sdds.com, Mar. 2003

16

ANSI/SMPTE, ‘202M-1998: For Motion Pictures – Dubbing Theatres, Review Rooms and Indoor Theatres– B-Chain Electroacoustic Response’, 1998

17

Genelec Oy, www.genelec.com, Mar. 2003

374

International Broadcasting ConventionÅ Æ