U.S. patent application number 12/356256 was filed with the patent office on 2009-08-20 for digital automatic gain control.
This patent application is currently assigned to AMI Semiconductor, Inc.. Invention is credited to Jon Schmidt Kindred, Bryant Sorensen, Jerry Wahl.
Application Number | 20090208033 12/356256 |
Document ID | / |
Family ID | 24934362 |
Filed Date | 2009-08-20 |
United States Patent
Application |
20090208033 |
Kind Code |
A1 |
Kindred; Jon Schmidt ; et
al. |
August 20, 2009 |
DIGITAL AUTOMATIC GAIN CONTROL
Abstract
Systems, devices, and methods are provided to inhibit apparent
amplitude modulation in non-linear processing that causes
distortion in a processed signal. One aspect of the invention
includes a hearing aid. The hearing aid includes a microphone to
receive an input signal, a speaker to reproduce the input signal,
and a processor. The processor processes the input signal using a
gain. The processor includes an inhibitor, which inhibits
distortions, and an adjuster, which adjusts the gain. The inhibitor
acts to smooth an envelope of the input signal to inhibit undesired
modulation. The adjuster adjusts the gain if the envelope is either
above or below a threshold.
Inventors: |
Kindred; Jon Schmidt;
(Minneapolis, MN) ; Sorensen; Bryant; (Colorado
Spring, CO) ; Wahl; Jerry; (Woodland Park,
CO) |
Correspondence
Address: |
Schwegman, Lundberg, Woessner & Kluth, P.A.
P.O. Box 2938
Minneapolis
MN
55402
US
|
Assignee: |
AMI Semiconductor, Inc.
|
Family ID: |
24934362 |
Appl. No.: |
12/356256 |
Filed: |
January 20, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09730200 |
Dec 5, 2000 |
7489790 |
|
|
12356256 |
|
|
|
|
Current U.S.
Class: |
381/104 ;
330/278 |
Current CPC
Class: |
H03G 1/04 20130101; H04R
25/505 20130101; H03G 3/3005 20130101; H04R 25/356 20130101; H03G
7/08 20130101; H03G 3/301 20130101 |
Class at
Publication: |
381/104 ;
330/278 |
International
Class: |
H03G 3/00 20060101
H03G003/00 |
Claims
1. A digital analog gain control, comprising: a detector to detect
an envelope of an input signal using Hilbert filters; an adder to
provide a difference between the envelope and a threshold; and an
adjuster that adjust a gain if the difference is one of two
conditions, wherein the two conditions includes being greater than
zero and being less than zero.
2. The digital analog gain control of claim 1, further comprising a
filter that removes low frequencies, wherein the filter receives
the input signal, removes frequencies less than about 100 Hertz
from the input signal, and presents the input signal to the
detector.
3. The digital analog gain control of claim 2, further comprising a
digital delay element that receives the input signal and presents a
delayed input signal.
4. The digital analog gain control of claim 3, further comprising a
first Hilbert filter and a second Hilbert filter, wherein the first
Hilbert filter receives the delayed input signal and filters the
delayed input signal to form the first filtered input signal, and
wherein the second Hilbert filter receives the input signal and
filters the input signal to form the second filtered input
signal.
5. The digital analog gain control of claim 4, further comprising:
a first multiplier and a second multiplier, wherein the first
multiplier receives the first filtered input signal and squares the
first filtered input signal to form a first squared signal, and
wherein the second multiplier receives the second filtered input
signal and squares the second filtered input signal to form a
second squared signal; another adder to add the first squared
signal and the second squared signal to form a sum-of-square
signal; and a limiter that receives the sum-of-square signal,
limits the sum-of-square signal to a desired range, and presents a
limited signal to the adder that provides the difference between
the envelope and the threshold.
6-7. (canceled)
8. The digital analog gain control of claim 1, wherein: the
adjuster receives the difference, a release time constant, and an
attack time constant, wherein the adjuster adjust a gain if the
difference is one of two conditions, and wherein the adjuster
decreases the gain if the difference is positive.
9. The digital analog gain control of claim 8, wherein: the
adjuster receives a previous gain, wherein if the difference is
negative, the adjuster increases the gain by shifting the bits of
the previous gain to the right by the release time constant to form
a new gain and taking the negative of the new gain, and wherein if
the difference is positive, the adjuster decreases the gain by
shifting the bits of the difference to the right by the attack time
constant to form the new gain.
10. (canceled)
11. The digital analog gain control of claim 9, further comprising:
a width adjuster that adjusts the word with of the previous gain
and presents an adjusted previous gain; another adder that adds the
new gain and the adjusted previous gain to form the gain; a limiter
to the limit the range of the gain so that the gain is positive; a
buffer that stores the gain and presents the stored gain, wherein
the stored gain is defined as the previous gain, which is presented
to the adjuster and the width adjuster; and a rounding circuit that
rounds the stored gain to a smaller precision value so as to be
compatible with the input width of subsequent circuitry that
includes a digital-to-analog converter.
12-15. (canceled)
16. The digital analog gain control of claim 1, further comprising:
a filter to block low frequencies from the input signal; and
wherein the adjuster receives the difference, a release time
constant, and an attack time constant, wherein the adjuster
increases the gain if the difference is negative, and wherein the
adjuster decreases the gain if the difference is positive.
17. The digital analog gain control of claim 16, wherein: the
filter includes a first digital delay that receives the input
signal and presents a delayed input signal; the filter includes a
first adder that determines a difference between the input signal
and the delayed input signal; the filter includes a first
multiplier that multiplies the difference and a scale to form a
scaled signal, wherein the scaled signal inhibits the filter from
overflow.
18-19. (canceled)
20. The digital analog gain control of claim 17, wherein: the
filter includes a second adder that adds the scaled signal and a
blocked signal to form a filtered signal; the filter includes a
second digital delay that receives the filtered signal and presents
a filtered signal that is delayed; and the filter includes a second
multiplier that multiplies the filtered signal that is delayed and
an alpha signal to form a blocked signal, wherein the alpha signal
determines a range of frequencies that will be blocked by the
filter.
21-22. (canceled)
23. A digital analog gain control, comprising: a detector to detect
an envelope of the input signal using infinite-impulse-response
(IIR) filters; an adder to provide a difference between the
envelope and a threshold; and an adjuster that receives the
difference, a release time constant, and an attack time constant,
wherein the adjuster adjust a gain if the difference is one of two
conditions, wherein the two conditions includes being a negative
number and being a positive number, wherein the adjuster increases
the gain if the difference is negative, and wherein the adjuster
decreases the gain if the difference is positive.
24. (canceled)
25. The digital analog gain control of claim 23, wherein each
infinite-impulse-response filter includes a first delay, a second
delay, and a scale element, wherein the input signal is delayed by
the first delay, delayed by the second delay, and scaled by the
scale element to form a scaled signal.
26. The digital analog gain control of claim 25, wherein: each
infinite-impulse response filter includes a first adder that
determines a difference between the input signal and a feedback
signal; each infinite-impulse-response filter includes a multiplier
that multiplies the difference and a beta signal to form a modified
signal, wherein the beta signal modifies the phase of the
difference; each infinite-impulse-response filter includes a third
delay that delays the modified signal to form a filtered signal;
and each infinite-impulse-response filter includes a fourth delay
that delays the filtered signal to form the feedback signal.
27-29. (canceled)
30. A method for controlling a gain of an amplifier, comprising:
forming an envelope of an input signal that lacks modulation using
Hilbert filters; and subtracting the envelope from a threshold to
form a difference, wherein the difference is used to control the
gain.
31. The method of claim 40, wherein blocking includes blocking low
frequencies that are less than about 100 Hertz.
32. The method of claim 30, further comprising: determining if the
difference is greater than zero; shifting the bits of the
difference to the right by an attack constant to form a decreased
gain; and shifting the bits of a negated signal to the right by a
release constant to form an increased gain.
33-34. (canceled)
35. The method of claim 32, further comprising switching for
presenting the decreased gain as the gain if the difference is
greater than zero, or else the act of switching presents the
increased gain as the gain if the difference is less than zero.
36. The method of claim 35, further comprising: summing the gain
and the feedback signal that is delayed to form a modified gain
signal; presenting a final gain to an analog-to-digital converter,
wherein the final gain is zero if the modified gain signal is less
than or equal to zero, and wherein the final gain is one if the
modified gain signal is greater than one; delaying the final rain
to produce the feedback signal that is delayed; and negating the
feedback signal that is delayed to form the negated signal.
37-39. (canceled)
40. The method of claim 30, further comprising blocking low
frequencies from the input signal.
Description
RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 09/730,200 filed Dec. 5, 2000, which is incorporated herein by
reference and made a part hereof.
TECHNICAL FIELD
[0002] The present invention relates generally to audio signal
processing. More particularly, it pertains to inhibiting
distortions that arise from adjusting gains of preamplifiers.
BACKGROUND
[0003] Sound systems can be broken down into three general
components: an input device, such as a microphone; a processing
system; and an output device, such as a speaker. Sounds are picked
up by the microphone, transmitted to the processing system where
they are processed, and then projected by the speaker so that the
sounds can be heard at an appropriate distance. Both the microphone
and the speaker are generally considered to be transducers.
[0004] A transducer is a device that transforms one form of energy
into another form of energy. In the case of a microphone, sound
energy, which can be detected by the human ear in the range of 20
Hertz to 20,000 Hertz, is transformed into electrical energy in the
form of an electrical signal. The electrical signal can then be
processed by a processing system. After the signal is processed,
the speaker transforms the electrical energy in the electrical
signal to sound energy again.
[0005] Before reaching the processing system, the electrical signal
is amplified by a preamplifier using a certain gain. However, if
the electrical signal already represents a powerful sound energy,
the amplified electrical signal may be at a level beyond the linear
operating range of the signal processing circuitry following the
preamplifier. To limit the electrical signal to the operating range
of the signal processing circuitry, an automatic gain control is
used.
[0006] The automatic gain control detects the level of the waveform
of the electrical signal, compares the level to a threshold, and
adjusts the gain of the preamplifier to decrease the level of the
electrical signal if the envelope is higher than the threshold.
When the level is below the threshold, the automatic gain control
increases the gain to its uncompressed level.
[0007] However, the automatic gain control, which is supposed to
help, also hinders by adding undesired distortions to the
electrical signal. These undesired distortions are frustrating to
users of sound systems in general, but are particularly
debilitating for users of hearing aids since these users depend
upon such aids to maintain their ability to communicate. Without an
acceptable solution to the undesired distortions, the optimum level
of performance desired by the end user will not be achieved.
[0008] Thus, what are needed are systems, devices, and methods to
inhibit AGC-induced distortions in sound systems, such as hearing
aids.
SUMMARY
[0009] The above-mentioned problems with distortions in audio
signal processing as well as other problems are addressed by the
present invention and will be understood by reading and studying
the following specification. Systems, devices, and methods are
described which inhibit AGC-induced distortions.
[0010] One illustrative embodiment includes a method for providing
automatic gain control. The method includes smoothing an envelope
of an input signal having a gain and adjusting the gain that is
applied to the input signal. The act of adjusting is dependent on
the level of the envelope relative to a threshold. The act of
smoothing inhibits distortions arising from apparent modulation of
the input signal.
[0011] Another illustrative embodiment includes a hearing aid. The
hearing aid includes an adjuster to adjust the gain so as to
amplify an input signal, and a detector to form a smooth envelope
that is a rectified version from the input signal. The detector
presents the smooth envelope to the adjuster. The adjuster adjusts
the gain that is applied to the input signal. The adjuster adjusts
the gain based on the level of the envelope relative to a
threshold.
[0012] The digital system as will be described has a number of
benefits not seen before. One benefit is an enhanced manufacturing
process that reduces a need for external components, such as
capacitors, and the need to couple the external components to a
circuit through I/O pins. Another benefit includes a reduction in
the die area required to implement the digital automatic gain
control loop. Other benefits include an enhanced control of the
tolerance of the bandwidth of the automatic gain control, and the
tolerance of the loop time constants of the automatic gain control.
The system also benefits from an enhanced power efficiency and low
operating voltage performance. Additionally, the system allows a
non-linear signal processing by selectively controlling the gain of
the preamplifier or providing information to a Nyquist-rate digital
signal processor to compensate for adaptive gain changes in the
preamplifier.
[0013] These and other embodiments, aspects, advantages, and
features of the present invention will be set forth in part in the
description which follows, and in part will become apparent to
those skilled in the art by reference to the following description
of the invention and drawings or by practice of the invention. The
aspects, advantages, and features of the invention are realized and
attained by means of the instrumentalities, procedures, and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a block diagram of a system according to one
embodiment of the invention.
[0015] FIG. 2 is a graph of a signal according to one embodiment of
the invention.
[0016] FIG. 3 is a graph of a signal according to one embodiment of
the invention.
[0017] FIG. 4 is a graph of a signal according to one embodiment of
the invention.
[0018] FIG. 5 is a graph of a signal according to one embodiment of
the invention.
[0019] FIG. 6 is a block diagram of a system according to one
embodiment of the invention.
[0020] FIG. 7 is a block diagram of a filter according to one
embodiment of the invention.
[0021] FIG. 8 is a block diagram of a filter according to one
embodiment of the invention.
[0022] FIG. 9 is a process diagram of a method according to one
embodiment of the invention.
DETAILED DESCRIPTION
[0023] In the following detailed description of the invention,
reference is made to the accompanying drawings that form a part
hereof, and in which are shown, by way of illustration, specific
embodiments in which the invention may be practiced. In the
drawings, like numerals describe substantially similar components
throughout the several views. These embodiments are described in
sufficient detail to enable those skilled in the art to practice
the invention. Other embodiments may be utilized and structural,
logical, and electrical changes may be made without departing from
the scope of the present invention.
[0024] The embodiments of the invention focus on inhibiting
distortions that arise from automatic adjustments of the gain of
preamplifiers in sound systems. An ear-worn hearing aid is an
example of such a sound system. As discussed hereinbefore, the
automatic gain control, which helps in adjusting the gain of the
preamplifier, also hinders by adding undesired distortions to the
electrical signal.
[0025] The automatic gain control detects the envelope of the
waveform of the electrical signal, compares the envelope to a
threshold, and adjusts the gain of the preamplifier. The act of
detecting the envelope includes sampling the waveform of the
electrical signal to form samples of the envelope that are
representative of the magnitude of the waveform. Each sample of the
envelope is then compared to the threshold by the act of comparing.
If any of the samples is greater than or less than the threshold,
the gain of the preamplifier is adjusted by the act of adjusting.
After the gain is adjusted, the preamplifier amplifies the
electrical signal so as to form an amplified electrical signal.
[0026] A curious phenomenon may occur during the acts of detecting,
comparing, and adjusting: The magnitude of the waveform of the
amplified electrical signal, for certain frequencies, appears as if
it has been modulated so as to form an amplitude modulation. This
amplitude modulation is unwanted because it will give rise to the
undesired distortions. This phenomenon occurs when the frequency of
the electrical signal is slightly removed from a rational factor of
the sampling frequency.
[0027] Each sample of the envelope that includes the apparent
modulation is then compared to the threshold by the act of
comparing. If any of the samples is greater than or less than the
threshold, the gain of the preamplifier is adjusted by the act of
adjusting. However, because of the apparent modulation in the
envelope, the gain no longer tracks the true envelope of the signal
but varies periodically. This gain, which varies periodically, is
applied to the electrical signal by the preamplifier. The
preamplifier produces an amplitude modulation as a result of the
application of the gain, which varies, to the electrical
signal.
[0028] This amplitude modulation adds undesired frequency
components to the electrical signal. These undesired frequency
components are distortions which are inhibited by the embodiments
of the invention. The embodiments of the invention solve this and
other problems as discussed hereinbelow.
[0029] FIG. 1 is a block diagram of a system in accordance with one
embodiment. A system 100 includes a microphone 102. The microphone
102 transduces sound energy into an electrical signal. The
microphone 102 is powered by a voltage supply 104. The microphone
102 also couples to ground 106.
[0030] The electrical signal is presented to a capacitor 108. The
capacitor 108 removes the direct-current (DC) component of the
electrical signal and presents the electrical signal to a
preamplifier 110 without the direct-current component. The
preamplifier 110 amplifies the electrical signal using a gain. As
discussed hereinbefore, the electrical signal may be at a level
that is too weak for subsequent circuitry to process. The
preamplifier 110 adjusts the level of the electrical signal so that
the electrical signal is within a range that is appropriate for
further processing.
[0031] The electrical signal, which has been amplified, is
presented to an analog-to-digital converter 112. The
analog-to-digital converter 112 converts the electrical signal from
an analog form to a digital form. The digital form includes a
desired number of bits (N) at a predetermined sampling rate
(F.sub.s) The electrical signal, which is in the digital form, is
presented to a filter 114. The filter 114 blocks the DC component
of the electrical signal. The filter 114 removes low frequencies
from the electrical signal. In one embodiment, the low frequencies
include frequencies less than about 100 Hertz. The electrical
signal with the low frequencies removed is presented as a signal
116. The signal 116 is presented to the rest of the system 100 for
processing.
[0032] The signal 116 also forms a feedback signal 118. The
feedback signal 118 is presented to a detector 120. In one
embodiment, the detector 120 inhibits apparent modulation in the
feedback signal 118 so as to inhibit distortions in the signal 116.
In another embodiment, the detector 120 forms a smooth envelope of
the feedback signal 118. The smooth envelope is a filtered estimate
of the feedback signal 118. The smooth envelope lacks the apparent
modulation. Because of the absence of the apparent modulation in
the smooth envelope, distortion of the signal 116 is inhibited.
[0033] The detector 120 presents the smooth envelope to an adjuster
122. The adjuster 122 adjusts the gain of the preamplifier 110 if
the smooth envelope is above or below a threshold. The adjuster 122
adjusts the gain of the preamplifier 110 by producing an adjustment
signal. In one embodiment, the adjustment signal is in a digital
form. The digital form includes a desirable number of bits (M) at a
predetermined sampling rate (F.sub.s).
[0034] The adjuster presents the adjustment signal to a
digital-to-analog converter 124. The digital-to-analog converter
converts the adjustment signal from the digital form to an analog
form. In analog form, the adjustment signal is an analog adjustment
that is used by the preamplifier 110. The adjustment signal lacks
the apparent modulation. The preamplifier 110 amplifies the
electrical signal using the adjustment signal so as to form an
amplified electrical signal. The amplified electrical signal
excludes the amplitude modulation that would have formed if the
adjustment signal were to include the apparent modulation. Thus,
the amplified electrical signal contains desired frequency contents
and lacks the amplitude modulation that gives rise to
distortions.
[0035] In one embodiment, the detector 120 includes a Hilbert
filter. The Hilbert filter receives the feedback signal 118 and
produces two signals that are 90 degrees out of phase with each
other. The detector 120 squares each signal of the two signals. The
detector 120 then sums the two squared signals to form the smooth
envelope. In another embodiment, the detector 120 takes the square
root of the sum of the two squared signals to form the smooth
envelope.
[0036] FIG. 2 is a graph of an input signal according to one
embodiment of the invention. The following discussion of FIG. 2 is
for the purpose of illustration only. The graph 200 graphs a signal
that is present in a digital automatic gain control loop. This
signal exists after the act of detecting the envelope but before
the act of adjusting the gain. The abscissa of the graph 200
represents time in seconds. The ordinate of the graph 200
represents amplitude of the signal.
[0037] The signal is a 5.01 kHz sine wave that has been sampled at
20 kHz. 5.01 kHz does not divide 20 kHz by exactly an integer
fraction. Thus, according to the discussion hereinbefore, the
signal appears as if it includes an apparent modulation. The graph
200 confirms that the amplitude of the signal appears modulated.
The apparent modulation occurs as if the waveform of the electrical
signal is modulated with another signal. Mathematically, this other
signal appears to be a rectified sine wave with a frequency value
of n[F.sub.sm/n-F.sub.input]. n includes a set of whole numbers
that is greater than 1. F.sub.s is the sampling frequency. m
includes a set of whole numbers excluding 0. F.sub.input is the
frequency of the electrical signal being input into the automatic
gain control.
[0038] This apparent modulation is the genesis that causes
distortions when the apparent modulation is transferred to the gain
during the act of adjusting the gain and eventually to the signal
during the act of amplifying the signal by the preamplifier. It is
this apparent modulation that is inhibited by the embodiments of
the invention.
[0039] The graph 200 shows that the apparent modulation includes a
depth of modulation. This depth of modulation can be used in this
circumstance to understand how much distortion is present in the
signal: the deeper the depth of modulation, the greater the
distortion. The depth of the modulation depends on whether the
frequency of the signal is evenly divisible by the sampling
frequency. If it is evenly divisible, or a rational factor, the
depth of modulation depends on the difference of the frequency of
the signal and the nearest rational factor of the sampling
frequency, the actual frequency of the signal, and the bandwidth of
the control loop. The smaller the difference and the higher the
signal frequency, the greater the depth of modulation, for signals
within the control bandwidth.
[0040] What is shown in the graph 200 is the apparent modulation
that may give rise to the amplitude modulation and hence the
distortions when the signal is amplified by the preamplifier. The
amplitude modulation will also include a depth of modulation. This
depth of modulation tends to be greater as the level of the signal
rises above the threshold of the adjuster of the digital automatic
gain control.
[0041] FIGS. 3-4 are graphs of a signal according to one embodiment
of the invention. These graphs are for the purpose of illustration
only. FIG. 3 shows a graph 300A of an input signal into a sound
system having a digital automatic gain control. The graph 300A
graphs an input signal that is presented to a digital automatic
gain control. The abscissa of the graph 300A represents time in
seconds. The ordinate of the graph 300A represents amplitude of the
signal.
[0042] The graph 300A graphs a portion 302A of the signal that has
an amplitude above the threshold of the digital automatic gain
control. As discussed hereinbefore, the digital automatic gain
control will reduce the amplitude of the input signal in the
portion 302A by adjusting the gain of the preamplifier. A portion
304A of the graph 300A has an amplitude below the threshold of the
digital automatic gain control. As discussed hereinbefore, the
digital automatic gain control will increase the amplitude of the
input signal in the portion 304A by adjusting the gain of the
preamplifier.
[0043] FIG. 4 shows a graph 300B of an output signal in a sound
system having a digital automatic gain control. The graph 300B
graphs an output signal that is produced by a digital automatic
gain control. This output signal is processed from the input signal
as shown in the graph 300A of FIG. 3. The abscissa of the graph
300B represents time in seconds. The ordinate of the graph 300B
represents amplitude of the signal.
[0044] A portion 302B of the graph 300B reflects the effort of the
digital automatic gain control to reduce the amplitude of the input
signal. The peaks of the signal in portion 302B tend to be
discontinuous. These discontinuous peaks of the portion 302B are
indicative of distortion in the signal. This distortion arises from
the amplitude modulation of the signal that is inhibited by the
embodiments of the invention. A portion 304B of the graph 300B
reflects the effort of the digital automatic gain control to
increase the amplitude of the input signal. The portion 304B shows
a gradual increase in the amplitude over time.
[0045] FIG. 5 shows a graph 400 of an output signal in a sound
system having a digital automatic gain control. The graph 400
graphs an output signal that is produced by a digital automatic
gain control. This output signal is processed from the input signal
as shown in the graph 300A of FIG. 3. The abscissa of the graph 400
represents time in seconds. The ordinate of the graph 400
represents amplitude of the signal.
[0046] A portion 402 of the graph 400 indicates that the amplitude
of the input signal is successfully reduced. Note that the peaks of
the output signal are parabolic and not discontinuous. This
indicates that the signal lacks the distortion that is caused by
the amplitude modulation as discussed hereinbefore. A portion 404
of the graph 400 shows that the amplitude of the input signal is
successfully increased.
[0047] FIG. 6 is a block diagram of a system according to one
embodiment of the invention. A system 500 receives a signal, which
represents sound energy, from a microphone 502. The signal enters a
preamplifier 504. The preamplifier 504 amplifies the signal so that
the signal has strength for subsequent processing by the system
500. The signal, which is amplified, enters an analog-to-digital
converter 506. The analog-to-digital converter 506 converts the
signal to a digital signal. The digital signal is in a form that
can be easily processed by a digital integrated circuit. The
digital signal enters a decimator 508. The decimator 508 reduces
the number of samples while increasing the word length in the
digital signal for subsequent processing of the digital signal. The
digital signal, which has been decimated, enters an interpolator
512. After interpolation by the interpolator 512, the digital
signal enters a digital-to-analog converter 514. The
digital-to-analog converter 514 converts the digital signal to an
analog signal. The analog signal enters a speaker 516. The speaker
516 reproduces sounds from the analog signal.
[0048] The digital signal, which has been decimated by the
decimator 508, is also processed by a digital automatic gain
control 517. Recall that the digital automatic gain control 517
helps to change the gain of the preamplifier 504. Specifically, the
digital signal enters a filter 518. The filter 518 filters out low
frequencies in the digital signal. In one embodiment, the low
frequencies include frequencies below 100 Hertz.
[0049] The digital signal, which has been filtered, enters a
detector 519. The detector 519 uses Hilbert filters to detect the
envelope of the digital signal. Specifically, the digital signal
enters a digital delay element 520. The digital delay element 520
delays the digital signal and produces a delayed signal. The
delayed signal enters a first Hilbert filter 524. The first Hilbert
filter comprises an infinite impulse response filter. The first
Hilbert filter 524 filters the delayed signal to form a first
filtered signal. Besides presenting itself to the digital delay
element 520, the digital signal also enters a second Hilbert filter
522. The second Hilbert filter comprises another infinite impulse
response filter. The second Hilbert filter 522 filters the digital
signal to form a second filtered signal.
[0050] The first filtered signal enters a first multiplier 528. The
first multiplier 528 squares the first filtered signal to form a
first squared signal. The second filtered signal enters a second
multiplier 526. The second multiplier 526 squares the second
filtered signal to form a second squared signal. Both the first
squared signal and the second squared signal enter an adder 530.
The adder 530 adds the first squared signal and the second squared
signal together to form a sum-of-square signal.
[0051] The sum-of-square signal enters a limiter 532. The limiter
532 limits the digital range of the sum-of-square signal to a
desired operating range. The sum-of-square signal then enters an
adder 536. The adder 536 determines the difference between the
sum-of-square signal and a threshold 534. The sum-of-square signal
is an envelope of the digital signal that is produced by the
detector 519. Thus, in another view, the adder 536 determines the
difference between the envelope of the digital signal and a
threshold 534. As will be discussed, this difference is used to
adjust the gain of the preamplifier 504.
[0052] The difference determined by the adder 536 enters an
adjuster 538. The adjuster 538 also receives the previous gain, an
attack time constant, and a release time constant. The previous
gain is the gain previously adjusted by the adjuster 538. The
attack time constant is used to decrease the gain, and the release
time constant is used to increase the gain.
[0053] If the difference is negative, the adjuster 538 increases
the gain of the preamplifier 504. The gain is increased by shifting
the bits of the previous gain to the right by the release time
constant, and taking the negative of the result of the shifting. In
other words, when the envelope of the digital signal is below the
threshold 534, the gain of the preamplifier 504 should be
increased. Such increase depends on the previous gain. The new gain
is obtained by multiplying the previous gain by the inverse of a
power of two. The modifier in this instance has a direct
relationship to the release time constant. The discussed
implementation uses shifts, which is equivalent to multiplications
by inverse powers of two, to implement the time constants, but it
should be understood that these time constants can be implemented
by other techniques, such as by regular multiplies.
[0054] If the difference is positive, the adjuster 538 decreases
the gain of the preamplifier 504. The gain is decreased by shifting
the bits of the difference to the right by the attack time
constant. In other words, when the envelope of the digital signal
is above the threshold 534, the gain of the preamplifier 504 should
be decreased. Such decrease depends on the difference between the
envelope of the digital signal and the threshold. The new gain is
obtained by multiplying the difference by the inverse of a power of
two. The modifier in this instance has a direct relationship to the
attack time constant.
[0055] The new gain enters an adder 540. The adder 540 adds the new
gain to an adjusted previous gain to form the gain. The adjusted
previous gain is formed from a width adjuster 542 that adjusts the
width of the word of the previous gain. The gain enters a limiter
544. The limiter 544 limits the range of the gain. The gain then
enters a buffer 546. The buffer 546 stores the gain and presents
the gain to a rounding circuit 548. The buffer 546 also feeds back
the gain to the width adjuster 542 and the adjuster 538. The
rounding circuit 548 rounds the gain to a smaller precision value
so as to be compatible with the input width of subsequent
circuitry.
[0056] The gain, which is rounded, enters a digital-to-analog
converter 550. The digital-to-analog converter 550 converts the
gain from digital to analog and presents the gain, which is now
analog, to the preamplifier 504. The preamplifier 504 uses the gain
to amplify the signal, which represents sound energy, from the
microphone 502.
[0057] FIG. 7 is a block diagram of a filter according to one
embodiment of the invention. The filter 600 acts to filter out low
frequencies from a digital signal. The digital signal enters both a
first adder 604 and a first digital delay element 602. The first
digital delay element 602 delays the digital signal to produce a
delayed digital signal. The adder 604 determines the difference
between the digital signal and the delayed digital signal. This
difference enters a multiplier 606. The multiplier 606 multiplies
the difference by a scale 608 to produce a scaled signal. The scale
608 is used to inhibit the filter 600 from overflow. The scaled
signal enters a second adder 610. The second adder 610 adds the
scaled signal with a block signal to produce a filtered signal. The
block signal will be discussed hereinafter. The filtered signal
enters a second digital delay element 616. The second digital delay
element 616 delays the filtered signal. The filtered signal then
exits the filter 600. A portion of the filtered signal feeds back
into a second multiplier 614. The second multiplier 614 multiplies
the filtered signal, which is delayed, by an alpha signal to form
the blocked signal. The alpha signal determines a range of
frequencies that will be blocked by the filter 600.
[0058] FIG. 8 is a block diagram of a filter according to one
embodiment of the invention. A filter 700 is an infinite-impulse
response filter. The filter 700 is configured as a two-zeros
two-poles filter. The filter 700 can be used as a Hilbert filter in
a detector as part of a digital automatic gain control circuit. The
digital signal enters a first digital delay element 702, a second
digital delay element 704, and a scale element 712. Thus, the
digital signal is delayed by the first digital delay element 702,
delayed by the second digital delay element 704, and scaled by the
scale element 712 to produce a scaled signal.
[0059] The digital signal also enters a first adder 706. The first
adder 706 determines the difference between the digital signal and
the feedback signal. The difference enters a multiplier 708. The
multiplier 708 multiplies the difference and a beta signal 710 to
form a modified signal. The beta signal 710 acts to control the
phase of the difference. The beta signal contains a number of bits
that is used to represent a desired number to be input into the
multiplier 708.
[0060] The modified signal enters a third digital delay element
716. The third digital delay element 716 delays the modified signal
to form a filtered signal. The filtered signal exits the filter 700
to be used by other circuitry. A portion of the filtered signal
enters a fourth digital delay element 718. The fourth digital delay
element 718 delays the filtered signal to form the feedback
signal.
[0061] FIG. 9 is a process diagram of a method according to one
embodiment of the invention. The process 800 discusses the feedback
loop that analyzes the digital signal and determines whether the
level of the digital should be adjusted. The process 800 begins at
an act 802. The act 802 converts an analog signal to a digital
signal. The digital signal is presented to an act 804. The act 804
blocks low frequencies from the digital signal to produce a
filtered signal. In one embodiment, the low frequencies, which are
blocked, are less than about 100 Hertz.
[0062] The filtered signal is presented to an act 806. The act 806
forms an envelope that lacks the apparent modulation. One suitable
technique of forming an envelope that lacks the apparent modulation
includes using Hilbert filters. The envelope is presented to an act
810. The act 810 subtracts the envelope from a threshold to form a
difference. The difference is presented to acts 812 and 814.
[0063] The act 812 determines if the difference is greater than
zero. If the difference is greater than zero, the gain should be
decreased. In other words, the envelope of the digital signal is
greater than the threshold. The digital signal is at a level beyond
the operating range of a processing system and such level should be
decreased. If the difference is less than zero, than the gain
should be increased. When the envelope of the digital signal is
less than the threshold, the digital signal should be strengthened
by increasing the gain for subsequent processing.
[0064] The result of the act 812 is presented to an act 818. The
act 818 uses the result of the act 812 to select the result of
either act 814 or act 816 to form a gain. Thus, the act 818
switches between the result of the act 814 or the act 816 depending
on the result of the act 812. If the gain needs to be decreased,
the act 818 selects the result of the act 814. The act 814
decreases the gain by shifting the bits of the difference to the
right by an attack constant. If the gain needs to be increased, the
act 818 selects the result of the act 816. The act 816 increases
the gain by shifting the bits of the feedback signal, which is
delayed and negated, to the right by a release constant.
[0065] The gain, which is formed by the act 818, is presented to an
act 824. The act 824 sums the gain and the feedback signal, which
is delayed. The feedback signal, which is delayed, is formed by an
act 822. The act 820 negates the feedback signal, which is delayed,
and presents the result to the act 816 as discussed
hereinbefore.
[0066] The act 826 equates the gain to 0 if the gain is less than
or equal to zero. Otherwise, the act 828 equates the gain to 1 if
the gain is greater than 1. The result of the act 826 and the act
828 is presented to an act 830. The act 830 converts the digital
form of the gain to an analog form, which is suitable for an analog
preamplifier.
CONCLUSION
[0067] Thus, systems, devices, and methods have been discussed for
inhibiting undesired amplitude modulation which causes distortions
in the amplified signal in a sound system. The embodiments of the
invention inhibit such undesired amplitude modulation by reducing
apparent sampling rate distortion.
[0068] The digital system as described has a number of benefits not
seen before. One benefit is an enhanced manufacturing process that
reduces a need for external components, such as capacitors, and the
need to couple the external components to a circuit through I/O
pins. Another benefit includes a reduction in the die area required
to implement the digital automatic gain control loop. Other
benefits include an enhanced control of the tolerance of the
bandwidth of the automatic gain control, and the tolerance of the
loop time constants of the automatic gain control. The system also
benefits from an enhanced power efficiency and low operating
voltage performance. Additionally, the system allows a non-linear
signal processing by selectively controlling the gain of the
preamplifier or providing information to a Nyquist-rate digital
signal processor to compensate for adaptive gain changes in the
preamplifier.
[0069] Although the specific embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that any arrangement which is calculated to achieve the
same purpose may be substituted for the specific embodiment shown.
This application is intended to cover any adaptations or variations
of the present invention. It is to be understood that the above
description is intended to be illustrative and not restrictive.
Combinations of the above embodiments and other embodiments will be
apparent to those of skill in the art upon reviewing the above
description. The scope of the invention includes any other
applications in which the above structures and fabrication methods
are used. Accordingly, the scope of the invention should only be
determined with reference to the appended claims, along with the
full scope of equivalents to which such claims are entitled.
* * * * *