Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
2002-06-24
2004-04-06
JeanGlaude, Jean Bruner (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C381S074000
Reexamination Certificate
active
06717537
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to digital signal processing. More specifically, the present invention is directed to minimization of system latency in signal processing paths including digital control loops.
BACKGROUND OF THE INVENTION
The use of digital signal processing for communication systems, such as cable and satellite transmission systems, has long been known in the art. Presently, these digital communications are in widespread use in establishing links between nearly all types of communication devices in which two or more such devices are in need of high quality communication with one another. As a result, these systems allow for the utilization of sophisticated communication applications in which each member can communicate with other members and other devices. Such digital signal processing devices have been developed in a the intended use. One form of digital signal processing device in use today in communication systems is an active noise cancellation (ANC) device. The ANC-device is most often used in a sound environment where there are one or more disturbance or noise signals that tend to obscure the desired or target signal. The conventional ANC device generally includes a feedback circuit which uses an input transducer such as a microphone to detect ambient noise and an output transducer such as a loudspeaker or receiver to both generate an antinoise signal to cancel the ambient noise and to deliver the desired signal. The particular circuit elements vary from implementation to implementation.
Currently, ANC is achieved in analog form by introducing a canceling antinoise signal. The actual noise is detected through one or more microphones. An antinoise signal of equal amplitude and opposite phase is generated and combined with the actual noise. If done properly, this should result in cancellation of both noises. The amount of noise cancellation depends upon the accuracy of the amplitude and phase of the generated antinoise signal. ANC can be an effective method of attenuating low-frequency noise which can prove to be very difficult and expensive to control using passive noise control techniques.
Turning first to
FIG. 1
, a block diagram of a first prior art feedback active noise cancellation system
10
as disclosed in U.S. Pat. No. 4,455,675 and 4,644,581 is shown. The system
10
has as input a desired signal and a Noise signal and generates an output signal. For discussion purposes, it will be assumed that the desired signal is an input voice (Vin) signal and that the output signal is an output voice (Vout) signal. The Noise signal is considered to be any disturbance signal in the sound environment other than the desired signal. The Vout signal is a combination of the Vin signal, the Noise signal, and an antinoise signal generated by the system
10
. As noted above, in theory the antinoise signal exactly cancels the Noise signal leaving only the Vin signal without attenuation as the Vout signal. In fact, this is not always the result. The system
10
attempts to achieve as high a gain as possible in the overall loop within a predetermined frequency range while maintaining the system stability. The forward path of the system
10
includes a compressor
12
, a compensator
14
, a power amplifier
16
, and a receiver
18
. For example, the receiver
18
could be any output transducer including a loudspeaker. The feedback path of the system
10
includes a microphone
20
as an input transducer and a microphone preamplifier
22
. The Vin signal and the feedback path signal are combined in a first summation node
24
. The forward path signal and the Noise signal are combined in a second summation node
26
.
Turning now to
FIG. 2
, a block diagram of a second prior art feedback active noise cancellation system
30
as disclosed in U.S. Pat. No. 5,182,774 is shown. One will note that the system
30
has similarities with the system
10
of
FIG. 1
except that the forward path includes a high-pass filter
32
, a low-pass filter
34
, and a mid-range filter
36
in combination with the receiver
18
. Further, the feedback path adds a high-pass filter
38
to the microphone
20
and the microphone preamplifier
22
.
Turning now to
FIG. 3
, a block diagram of a third prior art feedback active noise cancellation system
40
as disclosed in U.S. Pat. No. 5,604,813 is shown. In this case, a boost circuit
42
has been added outside of the closed loop, that is, before the first summation node
24
, to equalize the desired signal. The feedback path of the system
40
includes the microphone
20
, a plurality of band-pass filters
44
, and a low-pass filter
46
.
While widely used in the art, the conventional analog approach for reducing noise in a system is not without its problems. ANC systems are theoretically able to null the noise by generating a phase-inverted antinoise signal, however, as a practical concern, the various components of the system such as the input and output transducers will introduce certain undesirable delays. These delays may adversely affect the frequency range over which noise can be cancelled, the degree to which noise can be cancelled, and the stability of the noise-cancellation system. It is therefore desirable to be able to minimize the associated delays in the circuit. Likewise, it is also desirable to be able to adjust the circuit to compensate for component variation and manufacturing tolerances and for usage conditions to maximize the noise-cancellation frequency range and noise-cancellation ratio. Such adjustability is difficult to achieve using analog techniques. Another desirable function that can prove difficult in the analog domain is the equalization of the signal for frequency-dependent attenuation caused by subsequent processing functions.
BRIEF DESCRIPTION OF THE INVENTION
A method and an apparatus for minimizing latency in digital signal processing paths is disclosed. One example is an active noise cancellation device. The system includes a digital closed feedback loop having a forward path and a feedback path. The forward path includes a compensation filter, a digital-to-analog converter, and an output transducer. The feedback path includes an input transducer, a feedback delta-sigma modulator, and a feedback sampling-rate converter. An input signal is processed in one of several ways into a processed digital input signal having a preselected intermediate sampling rate. Through the feedback path, an analog output signal is processed into a digital feedback signal having substantially the same preselected intermediate sampling rate. The processed digital input signal and the digital feedback signal are combined and processed through the forward path to produce an anti disturbance signal that is combined with a disturbance signal to form the analog output signal.
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Davis Keith L.
Fang Xiaoling
Johnson Martin R.
JeanGlaude Jean Bruner
Lauture Joseph
Robbins Steven J.
Sonic Innovations, Inc.
Thelen Reid & Priest LLP
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