U.S. patent application number 09/825788 was filed with the patent office on 2001-08-09 for sound enhancement system.
Invention is credited to Klayman, Arnold I., Kraemer, Alan D..
Application Number | 20010012370 09/825788 |
Document ID | / |
Family ID | 25369964 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012370 |
Kind Code |
A1 |
Klayman, Arnold I. ; et
al. |
August 9, 2001 |
Sound enhancement system
Abstract
The present invention provides an audio enhancement apparatus
and method which spectrally shapes the differential information in
a pair of audio signals so as to create an immersive,
three-dimensional effect when the audio signals are acoustically
reproduced. In a preferred embodiment, the invention includes a
two-transistor amplifier which spectrally shapes the differential
information with what are called cross-over networks. The
cross-over networks spectrally shape the differential information
by selectively emphasizing or de-emphasizing certain frequencies in
the differential information. In addition, the preferred embodiment
adjusts the level of the sound which is common to both input
signals to reduce clipping.
Inventors: |
Klayman, Arnold I.;
(Huntington Beach, CA) ; Kraemer, Alan D.;
(Tustin, CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
620 NEWPORT CENTER DRIVE
SIXTEENTH FLOOR
NEWPORT BEACH
CA
92660
US
|
Family ID: |
25369964 |
Appl. No.: |
09/825788 |
Filed: |
April 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09825788 |
Apr 4, 2001 |
|
|
|
08877439 |
Jun 17, 1997 |
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Current U.S.
Class: |
381/98 ;
330/252 |
Current CPC
Class: |
H03F 2203/45544
20130101; H03F 2203/45702 20130101; H03F 2203/45458 20130101; H03F
2203/45594 20130101; H03F 2203/45631 20130101; H03F 3/45098
20130101 |
Class at
Publication: |
381/98 ;
330/252 |
International
Class: |
H03G 005/00; H03F
003/45 |
Claims
What is claimed is:
1. An audio enhancement system comprising: an audio generator
configured to output at least two audio signals; a pair of circuits
coupled to said audio generator, said pair of circuits are
configured to distinguish common-mode information which is common
to said audio signals from differential information which is not
common to said audio signals; and a first cross-over network in
communication with said pair of circuits, said first cross-over
network configured to emphasize a first set of frequencies in said
differential information relative to a second set of frequencies in
said differential information.
2. The audio enhancement system of claim 9 further comprising a
second cross-over network in communication with said pair of
circuits, said second cross-over network configured to modify a
second set of frequencies in said differential information in a
different manner than said first set of frequencies modified by
said first cross-over network.
3. A circuit comprising: at least two inputs, each input configured
to receive an input signal two transistors, each of said
transistors in communication with one of said inputs, said
transistors configured to distinguish common-mode information which
is common to said input signals from differential information which
is not common to said input signals; and spectral shaping circuitry
coupled to said transistors, said spectral shaping circuitry
configured to modify a first range of frequencies in said
differential information in a different manner than a second range
of frequencies in said differential information.
4. The circuit of claim 23 further comprising common-mode
adjustment circuitry coupled to said transistors, said common-mode
adjustment circuitry configured to variably modify the gain of said
common-mode information.
5. The circuit of claim 23 further comprising differential
adjustment circuitry coupled to said spectral shaping circuitry,
said differential adjustment circuitry configured to variably alter
said modification of said differential information.
6. The circuit of claim 23 wherein aid transistors output said
common-mode information and said differential information with at
least two output signals.
7. The circuit of claim 26 further comprising at least two speakers
connected to said output signals.
8. The circuit of claim 26 further comprising sound processing
circuitry connected to said output signals, said sound processing
circuitry configured to further modify said output signals.
9. The circuit of claim 23 further comprising output buffer
circuitry coupled to said transistors, said output buffer circuitry
configured to isolate said transistors from a load placed on either
of said output signals.
10. A circuit comprising: at least two input signals; a means for
distinguishing common-mode information in said input signals from
differential information in said input signals; and a means for
spectrally shaping said differential information.
11. The circuit of claim 30 wherein said means for distinguishing
also combines said common-mode information and said spectrally
shaped differential information to generate at least two output
signals.
12. The circuit of claim 31 further comprising a means for
buffering said output signals.
13. The circuit of claim 30 further comprising a means for variably
adjusting said spectral shaping of said differential signal.
14. The circuit of claim 30 wherein said means for distinguishing
also modifies said common-mode information.
15. The circuit of claim 34 further comprising a means for variably
adjusting said modification of said common-mode information.
16. A method of enhancing a pair of signals comprising the steps
of: receiving a pair of signals; discerning with a pair of
transistors common-mode information which is common to said audio
signals and differential information which is not common to said
audio signals; spectrally shaping said differential information;
and combining with said transistors said spectrally shaped
differential information with said common-mode information to
generate at least two output signals.
17. A method of enhancing audio information comprising the steps
of: directing at least two audio signals to a pair of transistors
which distinguish common-mode information which is common to said
audio signals from differential information which is not common to
said audio signals; and spectrally shaping said differential
information with multiple cross-over networks which are in
communication with said transistors.
18. A circuit comprising: at least two input signals; a means for
distinguishing common-mode information in said input signals from
differential information in said input signals; and a means for
spectrally shaping said differential information, wherein said
means for distinguishing also combines said common-mode information
and said spectrally shaped differential information to generate at
least two output signals.
19. The circuit of claim 50 further comprising a means for
buffering said output signals.
20. A circuit comprising: at least two input signals; a means for
distinguishing common-mode information in said input signals from
differential information in said input signals; and a means for
spectrally shaping said differential information, wherein said
means for distinguishing also modifies said common-mode
information.
21. The circuit of claim 52 further comprising a means for variably
adjusting said modification of said common-mode information.
22. An audio enhancement system comprising: a pair of circuits that
receive at least a pair of audio signals, said pair of circuits are
configured to distinguish common-mode information which is common
to said audio signals from differential information which is not
common to said audio signals; and a first cross-over network in
communication with said pair of circuits, said first cross-over
network configured to emphasize a first set of frequencies in said
differential information relative to a second set of frequencies in
said differential information.
23. A method of enhancing audio information comprising the steps
of: receiving at least two audio signals with a pair of circuits
which distinguish common-mode information which is common to said
audio signals from differential information which is not common to
said audio signals; and spectrally shaping said differential
information with multiple cross-over networks which are in
communication with said pair of circuits.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to audio enhancement
systems and methods for improving the realism of sound
reproduction. More particularly, this invention relates to
apparatus and methods for enhancing a pair of audio signals to
create an immersive, three-dimensional effect when the audio
signals are acoustically reproduced.
BACKGROUND
[0002] The audio and multimedia industries have continually strived
to overcome the imperfections of reproduced sound. For example, a
common imperfection is that the recording of sounds emanating from
multiple locations is not properly reproduced in an audio system.
One approach directed to improving the reproduction of sound
includes surround sound systems which have multiple recording
tracks. The multiple recording tracks are used to record the
spatial information associated with sounds which emanate from
multiple locations.
[0003] For example, in a surround sound system, some of the
recording tracks contain sounds which are intended to originate
from in front of the listener, while other recording tracks contain
sounds which are intended to originate from behind the listener.
When multiple speakers are placed around the listener at positions
representing the intended origin of the recorded sounds, the audio
information contained in the recording tracks makes the produced
sounds appear more realistic to the listener. Such systems,
however, are typically more expensive than systems which do not use
multiple recording tracks and multiple speaker arrangements.
[0004] To conserve costs, many conventional two-speaker systems
attempt to simulate a surround sound experience by introducing
unnatural time-delays or phase-shifts between left and right signal
sources. Unfortunately, such systems often suffer from unrealistic
effects in the reproduced sound.
[0005] Other known sound enhancement techniques operate on what are
called "sum" and "difference" signals. The sum signal, which is
also called the monophonic signal, is the sum of the left and right
signals. This can be conceptionalized as adding or combining the
left and right signals (L+R).
[0006] The difference signal, on the other hand, represents the
difference between the left and right audio signals. This is best
conceptionalized as subtracting the right signal from the left
signal (L-R). The difference signal is also often called the
ambient signal.
[0007] It is known that modifying certain frequencies in the
difference signal can widen the perceived sound projected from the
left and right speakers. The widened sound image typically results
from altering the reverberant sounds which are present in the
difference signal. Sound enhancement techniques which process sum
and difference signals is disclosed in U.S. Pat. Nos. 4,748,669 and
4,866,774, both issued to Arnold Klayman, one of the inventors for
the invention disclosed in the present application.
[0008] In existing sound enhancement systems which process the sum
and difference signals, the sum and difference signals are
generated from circuitry which combines the left and right input
signals. In some systems, once the circuitry generates the sum and
difference signals, additional circuitry then separately processes
and recombines the sum and difference signals in order to produce
an enhanced sound effect. For example, the sound system disclosed
in U.S. Pat. No. 4,748,669 has a sum and difference signal
generator. Also, the sound system disclosed in U.S. Pat. No.
4,308,423 has a difference signal generator.
[0009] Typically, the creation and processing of the sum and
difference signals are accomplished with digital signal processors,
operational amplifiers and the like. Such implementations usually
require complicated circuitry which increases the cost of such
systems. Thus, despite the contributions from the prior art, there
exists a need for a simplified audio enhancement system which
reduces costs associated with producing an enhanced listening
experience.
SUMMARY OF THE INVENTION
[0010] The present invention provides a unique apparatus and method
which simplifies the enhancement of audio information and provides
a widened stereo image. This is accomplished by correcting the
perspective of audio information through the modification of the
differential information existing in the audio information.
Advantageously, the preferred sound enhancement system uses fewer
components than the prior art sound enhancement systems and thus
reduces the costs of enhancing audio signals. As a result, the
preferred sound enhancement apparatus is easy to manufacture and
costs less than many other sound enhancement devices.
[0011] In addition, the preferred sound enhancement apparatus
enhances audio information without generating discrete sum and
difference signals. As a result, the preferred embodiment does not
need the complex circuitry which processes audio input signals to
generate intermediate signals which are then separately processed
and recombined to generate enhanced output signals. Advantageously,
the preferred embodiment can be used to enhance sound in a wide
range of low-cost audio and audio-visual devices which by way of
example can include radios, mobile audio systems, computer games,
multimedia presentation devices and the like.
[0012] Broadly speaking, the sound enhancement apparatus receives
at least two input signals, from a host system and in turn,
generates two enhanced output signals. In particular, the two input
signals are processed collectively to provide a pair of spatially
corrected output signals. In addition, the preferred embodiment
modifies the audio information which is common to both input
signals in a different manner than the audio information which is
not common to both input signals.
[0013] The audio information which is common to both input signals
is referred to as the common-mode information, or the common-mode
signal. The common-mode audio information differs from a sum signal
in that rather than containing the sum of the input signals, it
contains only that audio information which exists in both input
signals at any given instant in time. In addition, the preferred
embodiment reduces the amplitude of the frequencies in the
common-mode signal in order to reduce the clipping which may result
from high-amplitude input signals.
[0014] In contrast, the audio information which is not common to
both input signals is referred to as the differential information
or the differential signal. Although the differential information
is processed in a different manner than the common-mode
information, the differential information is not processed to form
a discrete signal. Rather, the common-mode and differential
information are processed together.
[0015] As discussed in more detail below, the sound enhancement
system spectrally shapes the differential signal with a variety of
filters to create an equalized differential signal. By equalizing
selected frequency bands within the differential signal, the sound
enhancement apparatus widens a perceived sound image projected from
a pair of loudspeakers placed in front of a listener.
[0016] As discussed in more detail below, the preferred sound
enhancement apparatus includes two transistors which are
interconnected at their emitters and their collectors with multiple
cross-over networks. Preferably, the cross-over networks act as
filters which equalize desired frequency ranges in the differential
input. Thus, the differential gain varies based on the frequency of
the input signals.
[0017] Because the cross-over networks equalize the frequency
ranges in the differential input, the frequencies in the
differential signal can be altered without affecting the
frequencies in the common-mode signal. As a result, the preferred
embodiment can create enhanced audio sound in an entirely unique
and novel manner.
[0018] The preferred sound enhancement apparatus is in turn,
connected to one or more output buffers. The output buffers isolate
the sound enhancement apparatus from other components connected to
the first and second output signals. For example, the output
signals can be directed to other audio devices such as a recording
device, a power amplifier, a pair of loudspeakers and the like
without affecting the operation of the preferred sound enhancement
apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] These and other aspects, advantages, and novel features of
the invention will become apparent upon reading the following
detailed description and upon reference to the accompanying
drawings in which:
[0020] FIG. 1 is a block diagram of an audio system appropriate for
use with the preferred embodiment of the present invention;
[0021] FIG. 2 is a schematic diagram of a common-emitter
differential amplifier;
[0022] FIG. 3 is a schematic diagram of a preferred sound
enhancement apparatus;
[0023] FIG. 4 illustrates a graphical representation of the
common-mode gain of the preferred sound enhancement apparatus;
[0024] FIG. 5 is a schematic diagram of an embodiment of the sound
enhancement apparatus with a first cross-over network;
[0025] FIG. 6 illustrates a graphical representation of the
differential signal equalization curve associated with a first
cross-over network;
[0026] FIG. 7 is a schematic diagram of an embodiment of the
preferred sound enhancement apparatus with two cross-over
networks;
[0027] FIG. 8 illustrates a graphical representation of the
differential signal equalization curve associated with a second
cross-over network;
[0028] FIG. 9 illustrates a graphical representation of the
differential signal equalization curve associated with a third
cross-over network;
[0029] FIG. 10 illustrates a graphical representation of the
overall differential signal equalization curve of the preferred
sound enhancement apparatus;
[0030] FIG. 11 illustrates another embodiment of the sound
enhancement apparatus;
[0031] FIG. 12 illustrates yet another embodiment of the sound
enhancement apparatus;
[0032] FIG. 13 illustrates a further alternative embodiment of the
sound enhancement apparatus;
[0033] FIG. 14 illustrates a graphical representation of the
overall differential signal equalization curve of an alternative
embodiment of the sound enhancement apparatus; and
[0034] FIG. 15 is a schematic diagram of the preferred sound
enhancement apparatus connected to a set of output buffers.
[0035] In the drawings, the first digit of any three-digit number
indicates the number of the figure in which the element first
appears. For example, an element with the reference number 302
first appears in FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0036] The present invention provides a method and system for
enhancing audio signals. In a preferred embodiment, the sound
enhancement system improves the realism of sound with a unique
sound enhancement apparatus. Generally speaking, the sound
enhancement apparatus receives two input signals, a left input
signal and a right input signal, and in turn, generates two
enhanced output signals, a left output signal and a right output
signal.
[0037] The left and right input signals are processed collectively
to provide a pair of spatially corrected left and right output
signals. In particular, the preferred embodiment equalizes the
differences which exist between the two input signals in a manner
which broadens and enhances the sound perceived by the listener. In
addition, the preferred embodiment adjusts the level of the sound
which is common to both input signals so as to reduce clipping.
Advantageously, the preferred embodiment achieves sound enhancement
with a simplified, low-cost, and easy-to-manufacture circuit which
does not require separate circuits to process the common and
differential signals.
[0038] Although the preferred embodiment is described herein with
reference to a preferred sound enhancement system, the invention is
not so limited, and can be used in a variety of other contexts in
which it is desirable to adapt different embodiments of the sound
enhancement system to different situations. To facilitate a
complete understanding of the invention, the remainder of the
detailed description is organized into the following sections and
subsections:
[0039] I. Overview Of A Sound Enhancement System Appropriate For
Use With The Preferred Sound Enhancement Apparatus
[0040] II. Overview Of A Differential Amplifier
[0041] III. Implementation Of The Preferred Differential
Perspective Correction Apparatus
[0042] IV. Operation Of the Preferred Differential Perspective
Correction Apparatus
[0043] A. The Common-Mode Gain
[0044] B. The Differential Gain
[0045] C. Other Embodiments
[0046] D. The Output Buffers
[0047] V. Conclusion
[0048] I. Overview Of A Sound Enhancement System Appropriate For
Use With The Preferred Sound Enhancement Apparatus
[0049] FIG. 1 illustrates a block diagram of a sound enhancement
system 100 appropriate for use with the preferred sound enhancement
apparatus 102. The preferred sound enhancement system 100 includes
a host system 104, the sound enhancement apparatus 102 and a set of
output buffers 106. In the preferred embodiment, the host system
104 is an audio generator which generates two audio signals, a
first input signal 110 and a second input signal 112.
[0050] The host system 104 can include, by way of example, a stereo
receiver, a radio, a compact disc player, a video cassette recorder
(VCR), audio amplifiers, theater systems, televisions, laser disc
players, digital versatile disk (DVD) players, devices for
recording and playback of prerecorded audio, multimedia devices,
computer games and the like. While the host system generates a set
of stereo signals; it should be understood that the host system 104
is not limited to stereo signals. Thus, in other embodiments, the
host system 104 can generate a wide variety of audio signals such
as audio systems which generate multi-channel signals.
[0051] The host system 104 transmits the first and second input
signals 110 and 112 to the sound enhancement system 102. Because
the preferred sound enhancement system 102 corrects the perspective
of audio information through modification of the differential
signal, the sound enhancement apparatus 102 is also referred to as
the differential perspective correction apparatus 102. In the
preferred embodiment the first and second input signals 110 and 112
are stereo signals; however, the first and second input signals 110
and 112 need not be stereo signals and can include a wide range of
audio signals such as Dolby Laboratories Pro-Logic system which
uses a matrixing scheme to store four or more separate audio
channels on just two audio recording tracks.
[0052] The audio signals could also include surround sound systems
which can deliver completely separate forward and rear audio
channels. One such system is Dolby Laboratories five-channel
digital system dubbed "AC-3." As explained in more detail below,
the preferred differential perspective correction apparatus 102
modifies the audio sound information which is common to both the
first and second input signals 110 and 112 in a different manner
than the audio sound information which is not common to both the
first and second input signals 110 and 112.
[0053] The audio information which is common to both the first and
second input signals 110 and 112 is referred to as the common-mode
information, or the common-mode signal (not shown). In the
preferred embodiment, the common-mode signal does not exist as a
discrete signal. Accordingly, the term common-mode signal is used
throughout this detailed description to conceptionally refer to the
audio information which exists equally in both the first and second
input signals 110 and 112 at any instant in time. For example, if a
one-volt signal is applied to both the first and second input
signals 110 and 112, the common-mode signal consists of one
volt.
[0054] The adjustment of the common-mode signal is shown
conceptionally in the common-mode behavior block 120. The
common-mode behavior block 120 represents the alteration of the
common-mode signal. The preferred embodiment reduces the amplitude
of the frequencies in the common-mode signal in order to reduce the
clipping which may result from high-amplitude input signals.
[0055] In contrast, the audio information which is not common to
both the first and second input signals 110 and 112 is referred to
as the differential information or the differential signal (not
shown). In the preferred embodiment, the differential signal is not
a discrete signal. Rather, throughout this detailed description,
the differential signal refers to the audio information which
represents the difference between the first and second input
signals 110 and 112 over time. For example, if, at a given point in
time, the first input signal 110 is zero volts and the second input
signal 112 is two volts, the differential signal at that point in
time is two volts (the difference between the two input signals 110
and 112). The differential signal will thus vary as a function of
time depending upon the signals received at the inputs. The rate at
which the differential signal varies over time can be described in
terms of frequency. Thus, the differential signal may have
individual components existing throughout the entire frequency
spectrum, depending upon the rate of fluctuation between the
corresponding input signals.
[0056] The modification of the differential signal is shown
conceptionally in the differential-mode behavior block 122. As
discussed in more detail below, the differential perspective
correction apparatus 102 equalizes selected frequency bands in the
differential signal. That is, the preferred embodiment equalizes
the audio information in the differential signal in a different
manner than the audio information in the common-mode signal.
[0057] The preferred differential perspective correction apparatus
102 spectrally shapes the differential signal in the
differential-mode behavior block 122 with a variety of filters to
create an equalized differential signal. By equalizing selected
frequency bands within the differential signal, the differential
perspective correction apparatus 102 widens a perceived sound image
projected from a pair of loudspeakers placed in front of a
listener.
[0058] Furthermore, while the common-mode behavior block 120 and
the differential-mode behavior block 122 are represented
conceptionally as separate blocks, the preferred embodiment
performs these functions with a single, uniquely adapted circuit.
Thus, the preferred embodiment processes both the common-mode and
differential audio information simultaneously. Advantageously, the
preferred embodiment does not require the complicated circuitry to
separate the audio input signals into discrete common-mode and
differential signals. In addition, the preferred embodiment doesn't
require a mixer which then recombines the processed common-mode
signals and the processed differential signals to generate a set of
enhanced output signals.
[0059] The preferred differential perspective correction apparatus
102 is in turn, connected to one or more output buffers 106. The
output buffers 106 output the enhanced first output signal 130 and
second output signal 132. As discussed in more detail below, the
output buffers 106 isolate the differential perspective correction
apparatus 102 from other components connected to the first and
second output signals 130 and 132. For example, the first and
second output signals 130 and 132 can be directed to other audio
devices such as a recording device, a power amplifier, a pair of
loudspeakers and the like without altering the operation of the
preferred differential perspective correction apparatus 102.
[0060] II. Overview Of A Differential Amplifier
[0061] FIG. 2 illustrates a prior-art, common-emitter differential
amplifier 200. The common-emitter differential amplifier 200
(hereinafter referred to simply as a differential amplifier)
responds to the difference in the amplitude between two input
signals applied at a first input terminal 202 and a second input
terminal 204. The differential amplifier 200 also has two output
terminals illustrated as a first output terminal 206 and a second
output terminal 208.
[0062] The differential amplifier 200 responds to the difference in
the amplitude between the input signals applied at the first and
second input terminals 202 and 204. In general, when two identical
signals are applied to the first and second input terminals 202 and
204, the output at the first and second output terminals 206 and
208 is zero. On the other hand, when two different signals are
applied to the first and second input terminals 202 and 204, the
greater the difference in the amplitudes for a given frequency, the
greater the amplitude generated at the output terminals 206 and 208
for that frequency.
[0063] The differential amplifier also includes two transistors 210
and 212, a plurality of capacitors 214 and 216, and a plurality of
resistors, 220, 222, 224, 226, 228, 230, 232, 234 and 236. The
transistors, capacitors and resistors are arranged such that the
first input terminal 202 transmits a first input signal (not shown)
from the first input terminal 202 to the base of the transistor 210
through the capacitor 214 and the resistor 220. A power supply
V.sub.cc 240 is connected to the base of transistor 210 through the
resistor 222 and to the collector of transistor 210 through the
resistor 226. The base of transistor 210 is also connected to
ground 242 through the resistor 224 while the emitter of transistor
210 is connected to ground 242 through the resistor 228.
[0064] Focusing now on the second input terminal 204, the second
input terminal transmits a second input signal (not shown) to the
base of the transistor 212 through the capacitor 216 and the
resistor 230. The power supply V.sub.cc 240 is connected to the
base of transistor 212 through the resistor 232. In addition, the
power supply V.sub.cc 240 is connected to the collector of
transistor 212 through the resistor 236. The base of transistor 212
is also connected to ground 242 through the resistor 234. In
addition, the emitter of transistor 212 is connected to ground 242
through the resistor 228.
[0065] This conventional differential amplifier 200 creates two
types of voltage gains (not shown), a common-mode voltage gain and
a differential voltage gain. The common-mode gain is defined as the
change between the input and the output of the common-mode signal.
The common-mode gain is typically expressed as the ratio of the
common-mode output voltage divided by the common-mode input
voltage.
[0066] For example, assume that at a particular instant in time,
the identical signal having a two-volt amplitude is applied to both
the first input terminal 202 and the second input terminal 204. In
this example, the common-mode input voltage is two volts because a
two volt value exists on both the first input terminal 202 and the
second input terminal 204. If, in this example, the common-mode
output voltage is 3 volts, the common-mode gain will equal 1.5 (3
common-mode output volts/2 common-mode input volts).
[0067] Focusing now on the differential gain, the differential gain
is defined as the ratio of the differential input voltage and the
differential output voltage. For example, assume that a particular
instant in time, the voltage of the first input terminal 202 is one
volt and the voltage at the second input terminal 204 is zero
volts. In this example, the differential signal is one volt (1-0).
If the differential voltage at the first output terminal 206 is two
volts, the differential gain equals 2, the 2 volt differential
voltage output divided by the one volt differential input
voltage.
[0068] III. Implementation Of The Preferred Differential
Perspective Correction Apparatus
[0069] FIG. 3 illustrates a schematic diagram of the preferred
differential perspective correction apparatus 102. The preferred
differential perspective correction apparatus 102 includes a unique
differential amplifier which adjusts the level of the common-mode
signal while equalizing select frequency bands in the differential
signal.
[0070] The preferred differential perspective correction apparatus
102 includes two transistors 310 and 312; multiple capacitors 320,
322, 324, 326 and 328; and multiple resistors 340, 342, 344, 346,
348, 350, 352, 354, 356, 358, 360, 362 and 364. Located between the
transistors 310 and 312 are three cross-over networks 370, 372 and
374. The first cross-over network 370 includes the resistor 360 and
the capacitor 324. The second cross-over network 372 includes the
resistor 362 and the capacitor 326, and the third cross-over
network 374 includes the resistor 364 and the capacitor 328.
[0071] A left input terminal 300 (LEFT IN) transmits the left input
signal (not shown) to the base of transistor 310 through the
capacitor 320 and the resistor 340. A power supply V.sub.cc 240 is
connected to the base of transistor 310 through the resistor 342.
The power supply V.sub.cc 240 is also connected to the collector of
transistor 310 through resistor 346. The base of transistor 310 is
also connected to ground 242 through the resistor 344 while the
emitter of transistor 310 is connected to ground 242 through the
resistor 348.
[0072] The capacitor 320 is a decoupling capacitor which provides
direct current (DC) isolation of the input signal at the left input
terminal 300. The resistors 342, 344, 346 and 348, on the other
hand, create a bias circuit which ensures stable operation of the
transistor 310. In particular, the resistors 342 and 344 set the
base voltage of transistor 310. The resistor 346 in combination
with the third cross-over network 374 set the DC value of the
collector-to-emitter voltage of the transistor 310. The resistor
348 in combination with the first and second cross-over networks
370 and 372 set the DC current of the emitter of the transistor
310.
[0073] In the preferred embodiment, the transistor 310 is an npn
2N2222A transistor which is commonly available from a wide variety
of transistor manufacturers. The capacitor 320 is 0.22 microfarads.
The resistor 340 is 22 kilohms (kohm), the resistor 342 is 41.2
kohm, the resistor 346 is 10 kohm, and the resistor 348 is 6.8
kohm. One of ordinary skill in the art will recognize, however,
that a variety of transistors, capacitors and resistors with
different values can be used to implement the differential
perspective correction apparatus 102.
[0074] Focusing now on the right input terminal 302, the right
input terminal 302 transmits a right input signal (not shown) to
the base of transistor 312 through the capacitor 322 and the
resistor 350. The power supply V.sub.cc 240 is connected to the
base of transistor 312 through the resistor 352. The power supply
V.sub.cc 240 is also connected to the collector of transistor 312
through the resistor 356. The base of transistor 312 is also
connected to ground 242 through the resistor 354 while the emitter
of transistor 312 is connected to ground 242 through the resistor
358.
[0075] The capacitor 322 is a decoupling capacitor which provides
direct current (DC) isolation of the input signal at the right
input terminal 302. The resistors 352, 354, 356 and 358, on the
other hand, create a bias circuit which ensures stable operation of
the transistor 312. In particular, the resistors 352 and 354 set
the base voltage of transistor 312. The resistor 356 in combination
with the third cross-over network 374 set the DC value of the
collector-to-emitter voltage of the transistor 312. The resistor
358 in combination with the first and second cross-over networks
370 and 372 set the DC current of the emitter of the transistor
312.
[0076] In the preferred embodiment, the transistor 312 is an npn
2N2222A transistor which is commonly available from a wide variety
of transistor manufacturers. The capacitor 322 is 0.22 microfarads.
The resistors 350 is 22 kilohms (kohm), the resistor 352 is 41.2
kohm, the resistor 356 is 10 kohm, and the resistor 358 is 6.8
kohm. One of ordinary skill in the art will recognize however, that
a variety of transistors, capacitors and resistors with different
values can be used to implement the differential perspective
correction apparatus 102.
[0077] IV. Operation Of the Preferred Differential Perspective
Correction Apparatus
[0078] The unique differential perspective correction apparatus 102
creates two types of voltage gains, a common-mode voltage gain and
a differential voltage gain. As explained above, the common-mode
voltage gain is the change in the voltage which is common to both
the left and right input terminals 300 and 302. The differential
gain is the change in the output voltage due to the difference of
the voltages applied to the left and right input terminals 300 and
302.
[0079] A. The Common-Mode Gain
[0080] In the differential perspective correction apparatus 102,
the common-mode gain is designed to reduce the clipping which may
result from high-amplitude input signals. In the preferred
embodiment, the common-mode gain at the left output terminal 304 is
primarily defined by the resistors 340, 342, 344, 346 and 348. In
the preferred embodiment, the common-mode gain is approximately a
negative six decibels.
[0081] FIG. 4 is an amplitude-versus-frequency chart which
illustrates the preferred common-mode gain at both the left and
right output terminals 304 and 306. The common-mode gain is
represented with a first common-mode gain curve 400. As shown in
the common-mode gain curve 400, the frequencies below approximately
30 hertz (Hz) are de-emphasized more than the frequencies above
approximately 30 Hz. For frequencies above approximately 30 Hz, the
frequencies are uniformly reduced by approximately 6 decibels.
[0082] The common-mode gain, however, may vary for or a given
implementation by varying the values of the resistors 340, 342,
344, 350, 352 and 354.
[0083] B. The Differential Gain
[0084] Focusing now on the differential gain, the differential gain
at the left and right output terminals 304 and 306 is defined
primarily by the ratio of the resistors 346 and 348, the ratio of
the resistors 356 and 358, and the three cross-over networks 370,
372 and 374. As discussed in more detail below, the preferred
embodiment equalizes certain frequency ranges in the differential
input. Thus, the differential gain varies based on the frequency of
the left and right input signals.
[0085] Because the cross-over networks 370, 372 and 374 equalize
the frequency ranges in the differential input, the frequencies in
the differential signal can be altered without affecting the
frequencies in the common-mode signal. As a result, the preferred
embodiment can create enhanced audio sound in an entirely unique
and novel manner. Furthermore, the differential perspective
correction apparatus 102 is much simpler and cost-effective to
implement than many other audio enhancement systems.
[0086] Focusing now on the three cross-over networks 370, 372 and
374, the preferred cross-over networks 370, 372 and 374 act as
filters which spectrally shape the differential signal. A filter is
usually characterized as having a cut-off frequency which separates
a passband of frequencies from a stopband of frequencies. The
cut-off frequency is the frequency which marks the edge of the
passband and the beginning of the transition to the stopband.
Typically, the cut-off frequency is the frequency which is
de-emphasized by three decibels relative to other frequencies in
the passband. The passband of frequencies are those frequencies
which pass through a filter with essentially no equalization or
attenuation. The stopband of frequencies, on the other hand, are
those frequencies which the filter equalizes or attenuates.
[0087] FIG. 5 shows one embodiment of the present invention
incorporating only the first cross-over network 370. The first
cross-over network 370 comprises the resistor 360 and the capacitor
324 which interconnect the emitters of transistors 310 and 312.
Because the first cross-over network 370 equalizes frequencies in
the lower portion of the frequency spectrum, it is thus called a
high-pass filter. In the preferred embodiment, the value of the
resistor 360 is approximately 3 kohm and the value of the capacitor
324 is approximately 0.68 microfarads.
[0088] The values of the resistor 360 and the capacitor 324 are
selected to define a cut-off frequency in a low range of
frequencies. In the preferred embodiment, the cut-off frequency is
approximately 78 Hz, a stopband below approximately 78 Hz and a
passband above approximately 78 Hz. FIG. 6 illustrates the
correction provided by the first cross-over network 370 as a first
correction curve 600 having different amplitude-versus-frequency
characteristics. The amplitude-versus-frequenc- y characteristics
are illustrated as a function of gain, measured in decibels,
against audible frequencies displayed in a log format.
[0089] The first correction curve 600 illustrates that the
frequencies below approximately 78 Hz are de-emphasized relative to
frequencies above approximately 78 Hz. However, because the first
cross-over network 370 is only a first-order filter, frequencies
defining the cut-off frequency are design goals. The exact
characteristic frequencies may vary for a given implementation.
Furthermore, other values for the resistor 360 and the capacitor
324 can be chosen to vary the cut-off frequency in order to
de-emphasize other desired frequencies.
[0090] FIG. 7 illustrates a schematic diagram of the differential
perspective correction apparatus 102 with both the second and third
cross-over networks 370 and 372. Like the first cross-over network
370, the second cross-over network 372 is also preferably a filter
which equalizes certain frequencies in the differential signal.
Unlike the first cross-over network 370, however, the second
cross-over network 372 is a high-pass filter which also
de-emphasizes lower frequencies in the differential signal relative
to the higher frequencies in the differential signal.
[0091] As shown in FIG. 7, the second cross-over network 372
interconnects the emitters of transistors 310 and 312. In addition,
the second cross-over network 372 comprises the resistor 362 and
the capacitor 326. Preferably, the value of the resistor 362 is
approximately 1 kohm and the value of the capacitor 326 is
approximately 0.01 microfarads.
[0092] The values of the resistor 362 and capacitor 326 are
selected to define a cutoff frequency in a high range of
frequencies. In the preferred embodiment, the cutoff frequency of
the cross-over network 372 is approximately 15.9 kilohertz (kHz).
FIG. 8 illustrates the second correction curve 800 provided by the
second crossover network 372. The second correction curve 800 is
also illustrated as a function of gain, measured in decibels,
against audible frequencies displayed in a log format. The second
correction curve 800 illustrates that the frequencies in the
stopband below approximately 15.9 kHz are de-emphasized relative to
frequencies in the passband above 15.9 kHz.
[0093] However, because the second cross-over network 372, like the
first crossover network 370, is a first-order filter, frequencies
defining the passband are design goals. The exact characteristic
frequencies may vary for a given implementation. Furthermore, other
values for the resistor 362 and capacitor 326 can be chosen to vary
the cut-off frequency so as to de-emphasize other desired
frequencies.
[0094] Referring now to FIG. 3, the third cross-over network 374
interconnects the collectors of transistors 310 and 312. The third
cross-over network 374 includes the resistor 364 and the capacitor
328 which are selected to create a low-pass filter which
de-emphasizes frequencies above a mid-range of frequencies. In the
preferred embodiment, the cut-off frequency of the low-pass filter
is approximately 795 Hz. Preferably, the value of resistor 364 is
approximately 9.09 kohm and the value of the capacitor 328 is
approximately 0.022 microfarads.
[0095] FIG. 9 illustrates a third correction curve 900 generated by
the third crossover network 374. The third correction curve 900
illustrates that the frequencies in the stopband above
approximately 795 Hz are de-emphasized relative to frequencies in
the passband below approximately 795 Hz. As discussed above,
because the third cross-over network 374 is only a first-order
filter, frequencies defining the low-pass filter in the third
cross-over network 374 are design goals. The frequencies may vary
for or given implementation. Furthermore, other values for resistor
364 and capacitor 328 can be chosen to vary the cut-off frequency
so as to de-emphasize other desired frequencies.
[0096] In operation, the first, second and third cross-over
networks 370, 372 and 374 work in combination to spectrally shape
the differential signal. FIG. 10 illustrates the overall correction
curve 800 generated by the combination of the first, second and
third cross-over networks 370, 372 and 374. The approximate
relative gain values of the various frequencies within the overall
correction curve 800 can be measured against a zero (0) dB
reference.
[0097] With such a reference, the overall correction curve 800 is
defined by two turning points labelled as point A and point B. At
point A, which in the preferred embodiment is approximately 125 Hz,
the slope of the correction curve changes from a positive value to
a negative value for increasing frequency levels. At point B, which
in the preferred embodiment is approximately 1.8 kHz, the slope of
the correction curve changes from a negative value to a positive
value.
[0098] Thus, the frequencies below approximately 125 Hz are
de-emphasized relative to the frequencies near 125 Hz. In
particular, below 125 Hz, the gain of the overall correction curve
800 decreases at a rate of approximately 6 dB per octave. This
de-emphasis of signal frequencies below 125 Hz prevents the
overemphasis of very low, i.e., bass, frequencies. With many audio
reproduction systems, over emphasizing audio signals in this
low-frequency range relative to the higher frequencies can create
an unpleasurable and unrealistic sound image having too much bass
response. Furthermore, over emphasizing these frequencies may
damage a variety of audio components including the
loudspeakers.
[0099] Between point A and point B, the slope of the preferred
overall correction curve is negative. That is, the frequencies
between approximately 125 Hz and approximately 1.8 kHz are
de-emphasized relative to the frequencies near 125 Hz. Thus, the
gain associated with the frequencies between point A and point B
decrease at variable rates towards the maximum-equalization point
of -8 dB at approximately 1.8 kHz.
[0100] Above 1.8 kHz the gain increases, at variable rates, up to
approximately 20 kHz, i.e., approximately the highest frequency
audible to the human ear. That is, the frequencies above
approximately 1.8 kHz are emphasized relative to the frequencies
near 1.8 kHz. Thus, the gain associated with the frequencies above
point B increases at variable rates towards 20 kHz.
[0101] These relative gain and frequency values are merely design
objectives and the actual figures will likely vary from circuit to
circuit depending on the actual value of components used.
Furthermore, the gain and frequency values may be varied based on
the type of sound or upon user preferences without departing from
the spirit of the invention. For example, varying the number of the
cross-over networks and varying the resister and capacitor values
within each cross-over network allows the overall perspective
correction curve 800 be tailored to the type of sound
reproduced.
[0102] The selective equalization of the differential signal
enhances ambient or reverberant sound effects present in the
differential signal. As discussed above, the frequencies in the
differential signal are readily perceived in a live sound stage at
the appropriate level. Unfortunately, in the playback of a recorded
performance the sound image does not provide the same 360 degree
effect of a live performance. However, by equalizing the
frequencies of the differential signal with the preferred
differential perspective correction apparatus 102, a projected
sound image can be broadened significantly so as to reproduce the
live performance experience with a pair of loudspeakers placed in
front of the listener.
[0103] Equalization of the differential signal in accordance with
the overall correction curve 800 is intended to de-emphasize the
signal components of statistically lower intensity relative to the
higher-intensity signal components. The higher-intensity
differential signal components of a typical audio signal are found
in a mid-range of frequencies between approximately 1 to 4 kHz. In
this range of frequencies, the human ear has a heightened
sensitivity. Thus, frequencies in the differential signal
information are modified by perspective correction. Accordingly,
the enhanced left and right output signals produce a much improved
audio effect.
[0104] C. Other Embodiments
[0105] The number of cross-over networks and the components within
the crossover networks can be varied in other embodiments to
simulate what are called head related transfer functions (HRTF).
Head related transfer functions describe different signal
equalizing techniques for adjusting the sound produced by a pair of
loudspeakers so as to account for the time it takes for the sound
to be perceived by the left and right ears. Advantageously, an
immersive sound effect can be positioned by applying HRTF-based
transfer functions to the differential signal so as to create a
fully immersive positional sound field.
[0106] Examples of HRTF transfer functions which can be used to
achieve a certain perceived azimuth are described in the article by
E. A. B. Shaw entitled "Transformation of Sound Pressure Level From
the Free Field to the Eardrum in the Horizontal Plane", J. Acoust.
Soc. Am., Vol. 56, No. 6, December 1974, and in the article by S.
Mehrgardt and V. Mellert entitled "Transformation Characteristics
of the External Human Ear", J. Acoust. Soc. Am., Vol. 61, No. 6,
June 1977, both of which are incorporated herein by reference as
though fully set forth.
[0107] FIG. 11 illustrates another embodiment of the differential
perspective correction apparatus 102 which allows a user to vary
the amount of overall differential gain. In this embodiment, a
fourth cross-over network 1100 interconnects the emitters of
transistors 310 and 312. In this embodiment, the fourth cross-over
network 1100 comprises a variable resister 1102.
[0108] The variable resister 1102 acts as a level-adjusting device
and is ideally a potentiometer or similar variable-resistance
device. Varying the resistance of the variable resister 1102 raises
and lowers the relative equalization of the perspective correction
circuit. Adjustment of the variable resistor is typically performed
manually so that a user can tailor the level and aspect of the
differential gain according to the type of sound reproduced, and
based on the user's personal preferences. Typically, a decrease in
the overall level of the differential signal reduces the ambient
sound information creating the perception of a narrower sound
image.
[0109] FIG. 12 illustrates an additional embodiment which allows a
user to vary the amount of common-mode gain. In this embodiment,
the differential perspective correction apparatus 102 contains a
fourth cross-over network 1200. The fourth cross-over network 1200
includes a resistor 1202, a resistor 1204, a capacitor 1206 and a
variable resistor 1208. The capacitor 1206 removes the differential
information and allows the variable resistor and resistors 1202 and
1204 to vary the common-mode gain.
[0110] The resistors 1202 and 1204 can be a wide variety of values
depending on the desired range of common-mode gain. The variable
resistor 1208, on the other hand, acts as a level-adjusting device
which adjusts the common-mode gain within the desired range.
Ideally, the variable resistor 1208 is a potentiometer or similar
variable-resistance device. Varying the resistance of the variable
resistor 1208 affects both transistors 310 and 312 equally and
thereby raises and lowers the relative equalization of the overall
common-mode gain.
[0111] Adjustment of the variable resistor is typically performed
manually so that a user can tailor the level and aspect of the
common-mode gain. An increase in the common-mode gain emphasizes
the audio information which is common to both input signals 302 and
304. For example, increasing the common-mode gain in a sound system
will emphasize the audio information at the center stage positioned
between a pair of loudspeakers.
[0112] FIG. 13 illustrates another alternative embodiment of the
differential perspective correction apparatus 102. In this
embodiment, the differential perspective correction apparatus 102
has a first cross-over network 1300 located between the emitters of
transistors 310 and 312 and a second cross-over network 1302
located between the collectors of transistors 310 and 312.
[0113] The first cross-over network 1300 is a high-pass filter
which de-emphasizes frequencies in the lower portion of the
frequency spectrum. In this embodiment, the first cross-over
network 1300 comprises a resistor 1310 and a capacitor 1312. The
values of the resistor 1310 and the capacitor 1312 are selected to
define a high-pass filter with a cut-off frequency of approximately
350 Hz. Accordingly, the value of resistor 1310 is approximately 3
kohm and the value of the capacitor 1312 is approximately 0.15
microfarads. In operation, the frequencies below 350 Hz are
de-emphasized relative to the frequencies above 350 Hz.
[0114] The second cross-over network 1302 interconnects the
collectors of transistors 310 and 312. The second cross-over
network 1302 is a low-pass filter which de-emphasizes frequencies
in the lower portion of the frequency spectrum. In this embodiment,
the second cross-over network 1302 comprises a resistor 1320 and a
capacitor 1322.
[0115] The values of the resistor 1320 and the capacitor 1322 are
selected to define a low-pass filter with a cut-off frequency of
approximately 2.3 kHz. Accordingly, the value of the resistor 1320
is approximately 9.09 kohm and the value of the capacitor 1322 is
approximately 0.0075 microfarads. In operation, the frequencies
above 2.3 kHz are de-emphasized relative to the frequencies below
2.3 kHz.
[0116] The first and second cross-over networks 1300 and 1302 work
in combination to spectrally shape the differential signal. FIG. 14
illustrates the overall correction curve 1400 generated by the
combination of the first and second cross-over networks 1300 and
1302 in this alternative embodiment. In FIG. 14, the overall
correction curve 1400 is represented as a function of gain,
measured in decibels, against audible frequencies displayed in a
log format. In addition, the overall correction curve is measured
against a zero (0) dB reference.
[0117] With a zero dB reference, the overall correction curve 1400
is defined by a single turning point labelled as point C. At point
C, which in the preferred embodiment is approximately 500 Hz, the
slope of the overall correction curve 1400 changes from a positive
value to a negative value. Thus, the frequencies below
approximately 500 Hz are de-emphasized relative to the frequencies
near 500 Hz. Furthermore, above 500 Hz, the gain of the overall
correction curve 1400 also decreases relative to the frequencies
near 500 Hz.
[0118] D. The Output Buffers
[0119] The sound enhancement system of the preferred embodiment
also includes the output buffers 106 as illustrated in FIG. 15. The
output buffers 106 are designed to isolate the perspective
correction differential apparatus 102 from variations in the load
presented by a circuit connected to the left and right output
terminals 304 and 306. For example, when the left and right output
terminals 304 and 306 are connected to a pair of loudspeakers, the
impedance load of the loudspeakers will not alter the manner in
which the differential perspective correction apparatus 102
equalizes the differential signal. Accordingly, without the output
buffers 106, circuits, loudspeakers and other components will
affect the manner in which the differential perspective correction
apparatus 102 equalizes the differential signal.
[0120] In the preferred embodiment, the left output buffer 106A
includes a left output transistor 1500, a resistor 1504 and a
capacitor 1504. The power supply V.sub.cc 240 is connected directly
to the collector of transistor 1500. The emitter of transistor 1500
is connected to ground 242 through the resistor 1504 and to the
left output terminal 304 through the capacitor 1502. In addition,
the base of transistor 1500 is connected to the collector of the
transistor 310.
[0121] In the preferred embodiment, the transistor 1500 is an npn
2N2222A transistor, the resistor 1504 is 1 kohms and the capacitor
1502 is 0.22 microfarads. The resistor 1504, the capacitor 1502 and
the transistor 1500 create a unity gain. That is, the left output
buffer 106A primarily passes the enhanced sound signals to the left
output terminal 304 without further equalizing the enhanced sound
signals.
[0122] Likewise, the preferred right output buffer 106B includes a
right output transistor 1510, a resistor 1512 and a capacitor 1514.
The power supply V.sub.cc 240 is connected directly to the
collector of the transistor 1510. The emitter of transistor 1510 is
connected to ground 242 through the resistor 1512 and to the right
output terminal through the capacitor 1514. In addition, the base
of transistor 1510 is connected to the collector of transistor
312.
[0123] In the presently preferred embodiment, the transistor 1510
is an npn 2N2222A transistor, the resistor 1512 is 1 kohm and the
capacitor 1514 is 0.22 microfarads. The resistor 1512, the
capacitor 1514 and the transistor 1510 create a unity gain. That
is, the right output buffer 106B primarily passes the enhanced
sound signals to the right output terminal 306 without further
equalizing the enhanced sound signals.
[0124] V. Conclusion
[0125] While certain preferred embodiments of the invention have
been described, these embodiments have been presented by way of
example only, and are not intended to limit the scope of the
present invention. For example, although the invention described
herein uses multiple cross-over networks 170, 172 and 174 with a
variety of resistor and capacitor values, a circuit designer can
utilize fewer cross-over networks or more cross-over networks or
additional components within the cross-over networks to customize
the overall differential and common-mode signals. Accordingly, the
breadth and scope of the present invention should be defined only
in accordance with the following claims and their equivalents.
* * * * *