Speech detector capable of avoiding an interruption by monitoring a variation of a spectrum of an input signal

In a speech presence detector, the input signal (speech plus noise) is detected for power and spectral-variation per unit time. Speech presence is decided if high-power or a sudden large variation in spectral-distribution (for example, unvoiced to voiced sound) is detected.

BACKGROUND OF THE INVENTION 
This invention relates to a speech detector responsive to an input signal 
including a speech or voice signal as a desired signal for detecting 
presence and absence of the speech signal. 
It has already been pointed out that a normal telephone conversation 
effectively utilizes only about 40% of time on unidirectionally 
transmitting a speech signal along a transmission line and uselessly 
wastes the remaining time. Thus, a utilization rate during which the 
transmission line is effectively utilized is very low in the normal 
telephone conversation. In order to raise the utilization rate, a speech 
transmission system has been proposed which can realize effective 
transmission of the speech signal by transmitting the speech signal only 
during presence thereof and, otherwise, any other data signals. A speech 
detector of the type described is used in such a speech transmission 
system to detect presence and absence of the speech signal. 
A conventional speech detector monitors electric power of an input signal 
to determine presence of the speech signal when the monitored electric 
power becomes higher than a predetermined or fixed threshold level. Let an 
ambient noise or background noise be included, as an undesired signal, in 
the input signal in addition to the speech or desired signal. When the 
electric power of the input signal is monitored to be compared with the 
predetermined threshold level, it may always exceed the predetermined 
threshold level. As a result, the speech detector wrongly detects presence 
of the speech signal and brings about deterioration of the utilization 
rate. On the other hand, a higher threshold level gives rise to an 
interruption at the beginning of each talk or speech. In view of the 
circumstances, it is possible to adaptively vary a threshold level in 
response to a level of the undesired signal. However, the interruption at 
the beginning of each speech inevitably takes place when the level of the 
undesired signal is equal to or higher than a level of the speech signal. 
In IEEE Transactions on Communications, vol. COM-26, No. 1, pp. 140-145 
(January, 1978), P. G. Drago et al have proposed a digital dynamic speech 
detector which detects a speech signal by deriving an envelope of the 
speech signal to successively monitor relative variations of the envelope 
between two adjacent time instants. With this speech detector, it is 
difficult to correctly detect presence of the speech signal when each 
relative variation is narrow, such as vowels. 
In U.S. Pat. No. 4,401,849 issued to Akira Ichikawa et al, a speech 
detecting method is disclosed which monitors partial auto-correlation 
coefficients determined in relation to a frequency spectrum of the input 
signal. The speech detecting method is disadvantageous in that the 
undesired signal will be erroneously detected as a desired signal when the 
undesired signal exhibits the partial auto-correlation coefficients which 
are similar to those of the desired signal. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a speech detector which is 
capable of reducing wrong detection of a speech signal. 
It is another object of this invention to provide a speech detector of the 
type described, which is capable of avoiding an interruption at the 
beginning of a speech or talk. 
It is a further object of this invention to provide a speech detector of 
the type described, which is capable of detecting presence of the speech 
signal even when a level of a background noise is higher than a level of 
the speech signal. 
A speech detector to which this invention is applicable is responsive to an 
input signal comprising a desired signal and an undesired signal for 
detecting presence of the desired signal. The desired and the undesired 
signals are representative of a speech and otherwise, respectively. The 
input signal has a spectrum variable with time in dependence on the 
desired and the undesired signals. According to this invention, the 
detector comprises first means responsive to the input signal for 
detecting electric power of the input signal to produce a first signal 
representative of the electric power, second means responsive to the input 
signal for detecting a variation of the spectrum to produce a second 
signal representative of the variation, and third means responsive to the 
first and the second signals for producing a third signal representative 
of presence of said desired signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, principles of this invention will be described to 
facilitate an understanding of a speech detector according to this 
invention. It is assumed that the speech detector is supplied with an 
input signal IN which has a wave form specified by an input voltage V and 
includes a speech signal beginning at a start time instant t.sub.s, as 
illustrated in FIG. 1(A). A background or an ambient noise is stationarily 
included in the illustrated input signal IN, as depicted on the lefthand 
side of the start time instant t.sub.s. 
Let electric power P.sub.0 be calculated about the input signal IN in a 
known manner. In this event, the electric power P.sub.0 exhibits a power 
wave form illustrated in FIG. 1(B). The electric power P.sub.0 scarcely 
varies at the start time instant t.sub.s. It is therefore difficult to 
detect the start time instant t.sub.s only by monitoring the electric 
power P.sub.0. This gives rise to an interruption at the beginning of each 
speech. 
Herein, consideration will be directed to that spectrum dispersed within a 
frequency band and which is specified by spectra of the ambient noise and 
the speech signal. As is known in the art, the spectrum of the ambient 
noise would be stationary or invariable with time, if such an ambient 
noise results from a stationary noise source, such as a motor, or from an 
electric power source generating a hum. However, it is difficult to 
preliminarily estimate the spectrum of the ambient noise. Therefore, the 
speech signal can not be distinguished from the ambient noise even when a 
plurality of threshold levels are prepared in relation to various 
different frequencies to monitor each component at the respective 
frequencies. On the other hand, the spectrum of the speech signal is 
nonstationary at the beginning of each speech and, therefore, exhibits a 
transient spectrum thereat. Such a transient spectrum is conspicuous 
particularly in fricative consonants. The transient spectrum does not 
appear during continuation of single sounds, such as vowels. In this case, 
it is possible to distinguish between the ambient noise and the beginning 
of each speech by monitoring the transient spectrum. Under the 
circumstances, a variation of the spectrum of the input signal IN is 
successively detected in the form of a variation of electric power 
relating to the spectrum. The variation of electric power may be a 
difference between electric power derived at two adjacent time instants. 
The difference of electric power varies as illustrated in FIG. 1(C) and 
exhibits a steep variation at the start time instant t.sub.s. Thus, the 
steep variation results from the transient spectrum. 
The spectrum of the input signal IN, namely, the electric power relating to 
the spectrum can be specified at each time instant by each partial 
autocorrelation coefficient calculated at each time instant, in the manner 
known in the art. Taking the above into account, operation is carried out 
in the speech detector to successively calculate the partial 
autocorrelation coefficients at the respective time instants and to obtain 
differences between the partial autocorrelation coefficients calculated at 
two adjacent ones of the time instants. 
Let only the differences between the partial autocorrelation coefficients 
be monitored and detected to produce an output signal representative of 
presence of the speech signal. In this event, those of the vowels which 
include continuation of single sounds may objectionably be lost from the 
output signal. 
The speech detector according to this invention detects not only the 
differences between the partial autocorrelation coefficients but also the 
electric power illustrated in FIG. 1(B). Therefore, both of the beginning 
of each speech and the vowels can correctly be detected by the speech 
detector. Any other coefficients or factors may be monitored instead of 
the partial autocorrelation coefficients in order to successively detect 
the spectrum at two adjacent ones of the time instants. 
Referring to FIG. 2, a speech detector according to a preferred embodiment 
of this invention is operable in response to an analog input signal AIN to 
deliver first, second, and third output signals OUT1, OUT2, and OUT3 (as 
will become clear later) to a speech synthesis unit (not shown). The 
analog input signal AIN is supplied through a low pass filter (LPF) 11 to 
an analog-to-digital (A/D) converter 12 to be converted into a succession 
of digital signals. 
The digital signal succession is processed at each frame having a frame 
period shorter than 30 milliseconds. The frame period is, for example, 20 
milliseconds. The digital signal succession is sent to a buffer memory 13 
having a first and a second memory section (not shown). The digital signal 
succession is alternatingly distributed to the first and the second memory 
sections at each frame period under control of the control circuit 14. The 
stored digital signal succession is selectively read out of the first and 
the second memory sections by the control circuit 14 to be delivered to a 
power detector 16 and an autocorrelator 17 in parallel. The power detector 
16 and the autocorrelator 17 are synchronously put into operation by the 
control circuit 14 so as to process the read out digital signal 
succession. The read out digital signal succession is processed in a 
manner similar to the input signal IN described in conjunction with FIG. 
1. The read out digital signal succession may be regarded as the input 
signal IN described in FIG. 1. 
The power detector 16 may be a multiplier for successively calculating a 
square of each digital signal. The square of each digital signal specifies 
electric power of each digital signal. The power detector 16 therefore 
produces a first power signal representing the square of each digital 
signal to specify the electric power. The first power signal is sent to a 
first comparator 21 and to the speech synthesis unit as the first output 
signal OUT1. A first threshold circuit 22 produces a first threshold 
signal TH1 representative of a first threshold level predetermined in 
relation to the electric power of each digital signal. The first 
comparator 21 compares the first power signal with the first threshold 
signal TH1 to produce a first signal representative of a result of 
comparison. A combination of the power detector 16, the first comparator 
21, and the first threshold circuit 22 serves as a first detection circuit 
for detecting the electric power of each digital signal and, therefore, 
the first signal may be called a first detection signal DET1 
representative of a result of the above-mentioned detection. 
It should be noted here that the first comparator 21 itself need not avoid 
an interruption occurring at the beginning of each speech. The first 
threshold level is therefore selected at a comparatively high level in 
which the interruption may occur at the beginning of each speech. 
Responsive to the digital signal succession read out of the buffer memory 
13, the autocorrelator 17 calculates a partial autocorrelation coefficient 
dependent on the spectrum. The partial autocorrelation coefficient may be 
either a first-order partial autocorrelation coefficient or a second-order 
partial autocorrelation coefficient. Such calculation of a partial 
autocorrelation coefficient is readily possible in a well-known circuit. 
Therefore, the autocorrelator 17 will not be described in detail herein. 
Anyway, the autocorrelator 17 produces a succession of coefficient signals 
each of which is representative of the partial autocorrelation 
coefficient. 
The coefficient signal succession is delivered to a delay circuit 25 and a 
subtractor 26. The coefficient signal succession is furthermore delivered 
to the speech synthesis unit as the second output signal OUT2. The second 
output signal OUT2 is processed by the speech synthesis unit in a known 
manner. The delay circuit 25 provides a predetermined delay to the 
coefficient signal succession to produce a succession of delayed 
coefficient signals. The predetermined delay is equal to the frame period. 
The subtractor 26 successively subtracts the delayed coefficient signal 
succession from the coefficient signal succession to calculate a 
difference between each delayed signal and each coefficient signal to 
produce a difference signal representative of the difference. Inasmuch as 
each delayed signal is delayed by the frame period, the difference 
specifies a variation between two adjacent ones of the frames. The 
difference signal is sent to a power calculator 28 which may be a 
multiplier and which is similar to the power detector 16. The power 
calculator 28 calculates a square of the difference to produce a square 
signal representative of the square. The square signal specifies 
additional electric power determined by the variation of the spectrum, 
namely, by the difference of two adjacent ones of the partial 
autocorrelation coefficients. Thus, the square signal has a variable level 
in accordance with the difference. 
A second threshold circuit 32 produces a second threshold signal TH2 
representative of a second threshold level predetermined in relation to 
the additional electric power. The second threshold level is selected such 
that the beginning of each speech can be detected when the square signal 
succession is monitored. 
A second comparator 34 compares the square signal succession with the 
second threshold signal TH2 to produce a second signal indicative of 
comparison. A combination of the autocorrelator 17, the delay circuit 25, 
the subtractor 26, the power detector 28, the second threshold circuit 32, 
and the second comparator 34 serves as a second detection circuit for 
detecting the variation of the spectrum. In this connection, the second 
signal may be called a second detection signal DET2 representative of the 
variation of the spectrum. In the second detection circuit, the power 
calculator 28, the second threshold circuit 32, and the second comparator 
34 are operable to derive the additional electric power, specifying the 
variation, from the difference signal succession. 
The first and the second detection signals DET1 and DET2 are sent through 
an OR gate 36 to a hangover circuit 38. The hangover circuit 38 provides a 
delay to a signal passing through the OR gate 36 in a known manner to 
produce a third signal representative of presence of the speech signal. 
The hangover circuit 38 serves to avoid objectionable abrupt interruptions 
or pauses. Such a hangover circuit 38 may be structured by a counter or 
the like. The delayed signal is supplied from the hangover circuit 38 to 
the speech synthesis unit as the third output signal OUT3. 
While this invention has thus far been described in conjunction with a 
preferred embodiment of this invention, it will readily be possible for 
those skilled in the art to put this invention into practice in various 
manners. For example, any other factors which specify the spectrum may be 
used instead of the partial autocorrelation coefficients. The spectrum may 
be divided into a plurality of partial spectra so as to detect the 
difference of the spectrum by monitoring the partial spectra as the 
factors. The first and the second threshold levels may adaptively be 
varied in response to the input signal.