Note: Descriptions are shown in the official language in which they were submitted.
74~
DETECTING THE PRESENCE OF A FIRST VARYING SIGNAL IN THE
PRESENCE OF A SECOND VARYING SIGNAL
Background of the Invention
This invention relates to a method, apparatus and
system for detecting the presence of a first varying
signal component in a signal having a second varying
component where the degree of amplitude variation of
the first signal is greater than that of the second signal.
The invention is particularly useful for determining
the presence of a voice signal in a signal that contains
both noise signals and voice signals and has particular
utility in a hands-free communication system. While
the invention will be described hereinafter in such
an environment and in connection with voice and noise
signals, it is to be understood that its utility and
applicability is not restricted thereto.
Summary of the Invention
Apparatus for processing a signal containing
~irst and second components the amplitudes of which
both vary, the first components from time-to-time
having greater amplitudes than that of the second
components, to detect the presence of the first components
comprising means for peak detecting said signal, said
peak detecting means having an output terminal and
providing at said output terminal a peak detected
output signal having a level proportional to the
maximum level of said signal; means connected to said
output terminal for valley detecting the valleys of
said peak detected output signal to produce a valley
detected output signal having a level proportional
to the lowest level of said peak detected output signal;
means for relatively decreasing the level of said
signal containing said first and second components
relative to the level of said valley detected output
signal; and means for comparing the levels of the two
last-mentioned signals to produce an output signal when
the level of the first of the two last-mentioned
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signals exceeds the level of the second of the two
last-mentioned signals.
Brief Description of the Drawin~
This invention will become more apparent from the
following detailed description, taken in conjunction
with the appended drawings, in which:
Figure 1 is a circuit diagram of one form of the
present invention; and
Figures 2a-2d are waveforms useful in
understanding the operation of the embodiment of the
invention shown in Figure 1.
Detailed Descri~tion of the Invention Includin~
_ _
the Preferred Embodiment
Referring to Figure 1, lla designates a microphone
but may be any source of signals that are derived, in
the illustrated embodiment, from acoustic noise and
voice sources. The output signal of microphone lla is
supplied to one input terminal (pin 3) of an
operational amplifier top amp) 16a whose output
terminal (pin 1) is connected to one of the input
terminals (pin 5) of another op amp 20a that provides
additional amplification.
The output terminal (pin 7) of op amp 20a is
connected to one input terminal (pin 3) of an op amp
30a, and the input signal on pin 3 of op amp 30a is
that which is used by the automatic background level
sensing circuitry of the instant invention.
Op amp 30a, diode CR2, resistor R13 and capacitor
C6 constitute a peak detector, the latter two
components being the low pass and storage device of the
peak detector. The output of op amp 30a and diode CR2
is, within very small tolerances, the output voltage
expected from an ideal diode placed at pin 3 of op amp
30a, but the output signal of op amp 30a and diode CR2
is buffered and is of low impedance to drive resistor
R13 and capacitor C6.
The output of capacitor C6 (at terminal A) with
respect to the + 4 volt reference voltage thus is the
approximate peak voltage of the voice frequency
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characteristic signals (when present), and when the
voice inpu-t is between syllables and words, the peak
voltage decays down to the peak value of the character-
istic background noise.
Referring to Figure 2a, the noise and voice signals
which are applied to pin 3 of op amp 30a are shown by
reference numeral 2, the largest amplitude portion 3 of
these signals being voice signals and the other portion
4 of the signals being noise signals. The peak detected
voltage is shown at 5 in Figure 2b and appears at
terminal A in Figure 1.
An op amp 31a and diode CR3 functioning as an
ideal diode together with a resistor R18 and a capacitor
C7 constitute what could be referred to as a valley
detector. Capacitor C6 (terminal A) is connected to
one input terminal (pin 5) of op amp 31a, and op amp
31a with diode CR3 thus tracks the lowest voltage that
appears on capacitor C6 which, of course, is the back-
ground noise voltage. The buffered output of op amp
31a through diode CR3 rapidly discharges capacitor C7
to the minimum voltage on capacitor C6, i.e., the
background noise voltage. The voltage on capacitor C7
at terminal B (Figure 1) is shown at 6 in Figure 2B.
Op amps 30a and 31a and their associated components
constitute a background level sensing network 21a.
Capacitor C7 can be charged in several ways, e.g.,
by a constant current; by a resistor to a constant
voltage; by a resistor to a voltage proportional to the
voltage (at terminal A) on capacitor C6; or by a current
proportional to the voltage at terminal A. In the
embodiment illustrated, which is the preferred embodiment,
the technique of charging capacitor C7 by a resistor
(R14) to a voltage proportional (1:1) to tha-t at terminal
A has been chosen.
The voltage decay of capacitor C6 to the minimum
L6
4a
levels allowed by the noise peaks is via resistors R12
and R14, semiconductor leakage and bias currents. The
voltage charging of capacitor C7 is via resistor R14,
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semiconductor leakage and bias currents. In order to
prevent very small voice and noise peaks, as well as
bias voltages, from activating the circuit, there may
be provided a bias circuit (resistors R9 and Rll) that
provides a small bias which prevents a comparator 22a,
to be discussed hereinafter, from detecting below a
minimum threshold.
The result of the foregoing is that through the
manipulation of the time constants in the circuit,
different voice frequencies and noise frequencies can
be detected, with the valleys of the peak detected
noise voltage, i.e., signal 6 in Figure 2b, being
placed on one input terminal (pin 2) of a comparator
22a.
The other input terminal (pin 3~ of comparator 22a
is connected via an attenuator and, optionally, a
filter, constituted by resistors R8, Rl5 and R16 and
capacitors C3, C4 and C5 to the output terminal (pin 7)
of op amp 20a and thus receives attenuated and,
optionally, filtered voice and noise signals.
The voice and noise signals applied to pin 3 of
comparator 22a are shown at 7 in Figure 2c, while the
background noise level (shown at 6 in Figure 2b) also
is shown at 6 in Figure 2c. These two signals 6 and 7
are referenced to one voltage, in the present circuit +
4 volts, making ratiometric comparison possible, this
being achieved by comparator 22a. As a result,
whenever signal 7 exceeds signal 6, as shown at 8 and 9
in Figure 2c, there is an output signal, which is shown
at 8a and 9a in Figure 2d, at the output terminal (pin
l) of comparator 22a that signals this condition and
thus the presence of, in this case, voice signals.
In the illustrated embodiment of this invention
the voice and noise signals (signal 2) at the output
terminal of op amp 20a are attenuated by a ratio of 1:3
before being applied to pin 3 of comparator 22a, as a
result of which signal 7 is one third the amplitude of
signal 2. If this were not done, the relative levels
of signals 6 and 7 would be such that comparator 22a
7~6
could not differentiate between noise signals, on the
one hand, and voice signals alone or with noise
signals, on the other hand, because signal 6 always
would be below signal 7 in level. It should be
appreciated, however, that the same desired result may
be achieved by amplifying signal 6 to, say, enhance its
amplitude by 100~ as shown at 6a in Figure 2b, while
not attenuating signal 7, or attenuating it to a lesser
degree than otherwise. The sensitivity of the circuit
is directly related to the degree of amplification
and/or attenuation. Care must be taken, of course, not
to attenuate signal 7 or amplify signal 6 to the point
where the voice signals do not exceed the level of
signal 6 and hence never are detected.
While preferred embodiments of the invention have
been described herein, those skilled in the art will
appreciate that changes and modifications may be made
therein without departing from the spirit and scope of
the invention as defined in the appended claims.
