Electrical audio signal processing systems and devices – Binaural and stereophonic – Amplifier
Reexamination Certificate
1998-05-26
2001-01-30
Isen, Forester W. (Department: 2747)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Amplifier
C381S027000, C381S120000, C381S089000, C330S199000, C330S12400D
Reexamination Certificate
active
06181796
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to multiple channel power amplifiers and methods for amplifying multiple channel (e.g., stereo) signals. In preferred embodiments, the invention is a system including circuitry for amplifying left and right channel signals, and a set of three transducers driven by the amplification circuitry (e.g., a loudspeaker for each of left, right, and subwoofer channels).
2. Description of the Related Art
FIG. 1
is a diagram of conventional circuitry for amplifying the left and right channel signals (
11
L and
11
R) comprising a stereo audio signal. Typically, the left channel signal
11
L and right channel signal
11
R are highly correlated, in the sense that a substantial amount of the power of each is due to frequency components (typically those having low frequency) for which a frequency component of the left channel is at least substantially “in phase” with a corresponding frequency component of the right channel. Referring to
FIG. 1
, the typical connection of audio loudspeaker
18
L to left channel stereo power amplifier
14
L (as shown in FIG.
1
), and of audio loudspeaker
18
R to right channel stereo power amplifier
14
R (as shown in
FIG. 1
) results in superposition of in-phase power supply current demands by the amplifier circuits for the left and right channels, when highly correlated left
11
L and right
11
R signals are amplified by their respective power amplifiers
14
L,
14
R to drive loudspeakers
18
L,
18
R.
To appreciate this, consider the case that highly correlated voltage signals
11
R and
11
L each have a frequency component (of frequency “f”) and these frequency components are in-phase (so that their positive peaks are aligned when plotted along a common time axis, as are the positive peaks shown at the left side of FIG.
1
). In this case, during amplification of the in-phase positive peaks of the highly correlated voltage signals
11
L,
11
R by amplifiers
14
L and
14
R, the corresponding power supply current demands (ISL and ISR) by the respective power amplifiers (
14
L and
14
R) are in phase. This places a high demand upon the power supply which provides the supply voltage Vs for amplifiers
14
R and
14
L. Specifically, simultaneous positive peaks of voltage signals
11
R and
11
L cause amplifiers
14
R and
14
L to draw simultaneous positive pulses of current (ISL and ISR) from the power supply terminals (zero amplitude or negative amplitude portions of voltage signals
11
R and
11
L cause amplifiers
14
R and
14
L to draw a small, positive DC current from the power supply). Thus, in response to in-phase, sinusoidal input signals
11
R and
11
L, the combined current (ISL+ISR) drawn by both of amplifiers
14
L and
14
R is a sequence of superposed positive pulses of current ISL+ISR (as represented in FIG.
2
). As illustrated, there are high, cumulative current demands by amplifiers
14
R and
14
L during the first half of each cycle (the period of each cycle is 1/f, where “f” is the frequency of each of sinusoidal input signals
11
L and
11
R). Thus, the power supply must be designed to operate with high peak to average current.
Unfortunately, most stereo audio sources have highly correlated left and right signals, particularly at the lower frequencies where power levels tend to be the highest. This is due to the generally non-directional nature of low frequency portions of most audio source material, and is thus generally unavoidable.
Above-referenced U.S. patent application Ser. No. 09/023,095 discloses several embodiments of an improved stereo signal amplifier, each operable with a power supply having reduced requirements (relative to those of power supplies of the type needed to operate conventional stereo amplifiers such as that of FIG.
1
).
FIG. 3
is a schematic diagram of one such embodiment. The system of
FIG. 3
has two signal channels: a left channel including amplifier
114
L and inverting amplifier
120
(which is a phase conversion circuit which inverts the phase of its input); and a right channel including amplifier
114
R. The “right” channel signal
111
R is coupled by input coupling capacitor
112
R, and amplified by power amplifier
114
R. The output of amplifier
114
R is coupled via output coupling capacitor
116
R to drive transducer
118
R (e.g., an audio loudspeaker). The “left” channel voltage signal
111
L is coupled by input coupling capacitor
112
L, and inverted by inverting amplifier
120
. The inverted left channel voltage signal (identified by reference numeral
121
in FIG.
3
) is then amplified by power amplifier
114
L. The amplified current output of amplifier
114
L is coupled via output coupling capacitor
116
L to drive transducer
118
L (e.g., an audio loudspeaker).
As indicated in
FIG. 3
, each of transducers
118
R and
118
L is “polarized” in the sense that it has an inverting terminal (−) and a non-inverting terminal (+). Accordingly, depending upon which terminal (of transducer
118
L or
118
R) is connected to system ground GND and which terminal is driven by the output signal, the resulting transducer output signal (sound wave
119
L or
119
R) will have either the same phase or the opposite phase as the phase of the amplified current signal which drives it (i.e., the same phase or the opposite phase as the current signal output from amplifier
114
L or
114
R). Hence, in the left channel (in which transducer
118
L has an inverted connection relative to the connection of right channel transducer
118
R), the output
119
L of transducer
118
L is opposite in phase to the signal asserted at the output of power amplifier
114
L. However, the phase shift introduced by inverting amplifier
120
cancels that introduced by transducer
118
L, so that output audio wave
119
L is in phase with original input signal
111
L. Hence, the overall “left channel” phase shift between input voltage signal
111
L and audio output signal
119
L is approximately zero, just as the overall “right channel” phase shift between input signal
111
R and audio output signal
119
R is also approximately zero. Accordingly, the
FIG. 3
circuit maintains an overall stereo effect while avoiding simultaneous positive current demands from both channels upon the power supply which provides supply voltage Vs across each of amplifiers
114
R and
114
L.
FIG. 4
shows the peak current demands on the power supply for amplifiers
114
R and
114
L (in response to in-phase frequency components of highly correlated signals
11
R and
11
L which are identical to the frequency components of signals
11
R and
11
L described above with reference to FIG.
2
). Due to the phase inversion of left input signal
111
L, there is no peak positive current ISL drawn (by amplifier
114
L) during the first half of each cycle (amplifier
114
L draws only a small, positive DC current I
DC
from the power supply, and amplifier
114
R draws a peak positive current ISR from the power supply, during the first half of each cycle), and there is no peak positive current ISR drawn (by amplifier
114
R) during the second half of each cycle (amplifier
114
R draws only a small, positive DC current I
DC
, and amplifier
114
L draws a peak positive current ISL, from the power supply during the second half of each cycle). Since the peak current demands by the left channel amplifier
114
L and the right channel amplifier
114
R occur during different (i.e., opposing) half-cycles of the input signals, the power supply for the
FIG. 3
system can have a simpler design than the power supply for the
FIG. 1
system, in that the power supply for the
FIG. 3
system can be designed to operate with lower peak to average current demand than that expected during operation of the
FIG. 1
system (and in that the size requirements for the energy storage capacitors of the power supply for
FIG. 3
are reduced since the peak current demand is reduced). It will be understood that since the phase shift provided by inverting amplifier
120
is done prior to power amplific
Isen Forester W.
Limbach & Limbach L.L.P.
National Semiconductor Corporation
Pendleton Brian Tyrone
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