Electrical audio signal processing systems and devices – Binaural and stereophonic – Pseudo stereophonic
Reexamination Certificate
1998-05-01
2001-12-18
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Pseudo stereophonic
C381S104000, C381S307000
Reexamination Certificate
active
06332026
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The innovation is a setup in home theater or other sound reproduction equipment to be used for handling low frequency audio signals with an audio signal source producing several digital full bandwidth audio channels and possibly in addition to these one or several digital low frequency sub-woofer channels.
2. Description of the Related Art
As laserdisc players and hifi-VCR's have grown popular, also the so called home theater setups have become widespread, especially in USA. Such a setup typically includes a large screen television, laserdisc player and/or VCR and a sound reproduction system consisting of surround sound decoder, multi-channel audio amplifier and speakers. In addition to the usual two main front speakers there is a center speaker between them, one or two surround speakers situated behind or on sides of the listeners and possibly also a separate subwoofer speaker for reproduction of low bass signals.
A simplified block diagram of a home theater setup is presented in FIG.
1
. For the sake of clarity, user and control interfaces, volume control (placed either in digital domain in conjunction with DA-converters or after the bass management) and power amplifiers have been left out of the figure. The setup in
FIG. 1
consists of input stage
100
, into which both several analog inputs
101
,
102
,
103
and several digital inputs
104
,
105
,
106
may be connected. In each case the input signal is selected with signal source selectors
107
and
108
, respectively for analog and digital input sources. After this the analog input signal is AD-converted with AD-converter
109
and digital input signal is handled with digital audio receiver
110
. From these two digital signals one is selected with source selector
111
, followed by the processor unit
112
which will perform the desired processing to the digital audio signal. This processing may include such steps as Dolby Digital (previously Dolby AC-3) decoding, Dolby Pro Logic decoding, THX post processing, room simulations, tone controls etc. After these processing steps these typically six digital audio signals are DA-converted. The number of channels varies between different systems; for instance there may be from 1 up to 4 or 6 surround channels and also there may be more than one center channels. After the DA-conversion there has to be the so called bass management in block
114
, to avoid feeding strong bass signals to speakers unable to handle them. As a result, output signals for left front speaker, center speaker, right front speaker, left surround speaker, right surround speaker and subwoofer are generated. These outputs are labelled
115
-
120
in FIG.
1
.
Let us note, that in this context the word speaker refers to a typically enclosed entity consisting of one or several speaker elements. “Large or full range speaker” refers to a speaker capable of reproducing at least most of the audio bandwidth, ie. both low bass signals and high treble sounds. “Small tweeter speaker” refers to a speaker which is only able to reproduce high frequencies and no low bass signals.
There is also low bass material in movie soundtracks. In many cases it is not practical to have six large speakers in a home theater setup. An example of a more practical solution is to have large front main speakers whereas the other speakers are small. Small speakers can not reproduce low frequency signals; high level low frequency signals may even damage these speakers. One of the functions of a home theater device is therefore to extract low bass signals from small speaker output signals and to redirect them to large speakers capable of reproducing them. This function is called bass management (block
114
in FIG.
1
).
In most home theater devices bass management is realized with analog filters and switches. This approach is very straight forward to build and is directly compatible with eg. the Dolby configuration definitions. Disadvantages of this approach are the limited flexibility resulting from fixed switches and unclarity from the user's point of view. Therefore most home theater devices can only perform configurations
1
and
2
of the Dolby license definitions, some of them configuration
3
as well. The user can not choose freely from all possible combinations of large and small speakers and also the user has to know and understand the meaning of Dolby configurations
1
,
2
and
3
(Dolby AC-3 licensee information manual, version 1.0, figures 9.6, 9.7 and 9.8; all blocks are analog).
The limitations of analog bass management are easy to overcome by carrying out the necessary filtering and summing in digital domain. In this case the user interface for defining the speaker configuration is intuitively: “Front Main: large/small, Center: large/small
one, Surrounds: large/small
one, Subwoofer: yes
o”. Now at its best the bass management for one channel may be performed as described in FIG.
2
. This entity described in
FIG. 2
is unlikely to have been presented before and is placed for each channel in the processing unit
112
in FIG.
1
. The result is that the processing unit is only followed by a six-channel DA-converter
113
while bass management unit
114
is left out as obsolete.
In
FIG. 2
the incoming digital signals for left front speaker, center speaker, right front speaker, left surround speaker, right surround speaker and subwoofer speaker have been marked
201
-
206
. These signals are fed to complementary filters
207
-
211
, which divide the incoming signals to “high” and “low ” output signals, some of which equals the incoming signal except for slight processing delay. These ten output signals as well as the subwoofer output signal are driven through adjustable amplifying stages
212
-
222
to summing stage
223
to generate output signal
224
for one speaker. With summing stages
212
-
222
the output signal for each speaker may be flexibly set as combination of full bandwidth input signals.
Controlled by speaker configuration switches a large speaker gets both “high ” and “low” outputs. A small speaker only gets “high” output signals, which lack the low bass sounds. These will be directed to large speakers. Also leaving a speaker completely off the design is possible. In this case the signal of eg. missing center speaker may be divided between the two main front speakers. Also when bass sounds are divided to several speakers for reproduction (eg. configuration with several large speakers but no subwoofer) the correct level for bass sounds can be set.
The benefits of this approach also include perfect control of processing delays and thus the signal delays experienced by the listener, and the fact that the sound field may be controlled by peculiar ways eg. by shifting it 90 or 180 degrees.
In practice some summing coefficients are always zero eg. right channel signals are never redirected to left channel, and subwoofer never gets any high sounds. To save processing power the system described in
FIG. 2
needs not to be implemented completely. It may also be carried out with appropriate combinations of ordinary non-complementary filters in which the signal is suitably fed, even if the processing block diagram would be substantially different from the one described above.
Even though digital bass management has many benefits it also has its drawbacks especially when using Dolby AC-3 system (also known as Dolby Digital). This system includes a separate subwoofer channel which by definition is to be reproduced ten desibels (10 dB=3.16) louder than the other channels. Since the dynamic range of DA-converters is limited the sound quality may then deteriorate. Let us assume for example a speaker configuration with large main front speakers, small other speakers and no subwoofer speaker in a situation in which the incoming signal has full amplitude bass sound in every channel. If reference level for full amplitude signal is said to be 1 then the summed bass signal level of surround speakers is 2, center channel bass signal level is 1 and subw
Juhola Minna
Kuusama Juha
Laaksonen Juha
Burns Doane Swecker & Mathis L.L.P.
Flextronics Design Finland Oy
Mei Xu
Pendleton Brian Tyrone
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