Apparatus and methods for electrical signal discrimination

A novel apparatus and method for electronically determining whether audio signals which are present in an electrical audio device (for example, a telephone) are voice signals or, on the other hand, some type of supervisory or system-generated signals. Audio signal lines are connected to an audio detector and a digitizer. The audio detector provides an output which indicates the presence of an audio signal. The digitizer transforms any audio signal into a series of high and low level signals in some manner. The outputs from the audio detector and the digitizer are then provided to a central processing unit which is used to analyze the data. If an audio signal is detected and the digitized audio signal is determined to have a regular periodic pattern, it is presumed that the audio signal is some type of system-generated supervisory tone. If, on the other hand, the digitized audio signal does not have a regular periodic pattern, it is presumed that the audio signal is a voice signal. Accordingly, upon detecting an irregular audio signal, the central processing unit produces an output signal which can be detected by associated circuitry as indicating that voice signals are present.

BACKGROUND 
1. The Field of the Invention 
This invention relates to devices and methods for discriminating between 
different types of electrical signals and, more particularly, to novel 
apparatus and methods for electronically distinguishing between voice 
signals and system-generated signals in an electrical audio device. 
2. The Prior Art 
During recent years, largely as a result of the significant developments in 
the microprocessor arts, a number of devices have been developed which are 
designed to call one or more specified telephone numbers and then transmit 
and/or receive an audio message. For example, security devices have been 
developed which will, upon being actuated, call one or more designated 
telephone numbers and deliver an emergency message and/or a request for 
help. Similar devices are now being used by many doctors, dentists, and 
other professionals to call patients or clients and remind them of 
scheduled appointments. Many department stores are likewise using such 
devices to call customers and inform them that their has order has been 
received. Also, marketing companies and salesmen have begun using 
automatic calling devices to contact potential clients or customers, to 
inform them of a new product or service, and/or to obtain answers to 
particular questions. In fact, there are presently numerous types of such 
microprocessor-based devices which are being used in various ways to 
perform similar automatic telephone calling functions. 
When using any such automatic calling device, it is, of course, intended 
that the audio message will actually be heard by an individual who answers 
the called telephone. Nevertheless, one of the principal challenges 
associated with these devices has been to develop a dependable way of 
determining that a party has, in fact, answered the called telephone. 
The design of early automatic calling devices often simply ignored the 
problem of detecting whether the called telephone had been answered. Thus, 
some of these early devices would simply dial the designated telephone 
number and then deliver the message. It will, of course, be appreciated 
that the called party would often not receive the audio message at all, 
such as, for example, if the party was not home or if the telephone line 
was busy. In other cases, the called party would receive only part of the 
message, since the audio message would have been started before the called 
party ever answered the telephone. 
In an effort to overcome these problems, some of the early automatic 
calling devices were designed so as to repeat the audio message a 
specified number of times after dialing the telephone number and before 
disconnecting. Thus, for example, the device might dial a telephone number 
and then repeat the audio message two times. As a result, a party might 
answer the telephone in the middle of the first time through the message, 
but would still be able to receive the entire message before the device 
was disconnected. Understandably, however, this method of message delivery 
could be rather annoying to call recipients. More significantly, repeating 
the audio message several times still did not ensure that the called party 
ever received the message at all, such as, for example, if the telephone 
line was busy or if the called party did not answer. 
Recognizing the need for some type of telephone answer detection, a number 
of attempts have been made by those skilled in the art to incorporate into 
automatic calling devices some way of detecting that the called telephone 
has been answered. In this way, it was hoped that the device could then be 
designed so as to deliver the audio message only after the called 
telephone was answered, thereby avoiding the problems outlined above. 
However, the devices 22 and methods which have heretofore been used to 
detect whether a called telephone has been answered have proven to be 
largely inadequate and undependable. 
One method which has previously been used to determine whether a called 
telephone has been answered is to require the answering party to transmit 
a recognizable answer signal back to the calling device. For example, the 
calling device could be designed to dial a designated telephone number and 
then wait to receive a specific tone or other answer signal before 
transmitting the audio message. Such an answer signal could, for example, 
be a conventional dual-tone multifrequency signal which is transmitted 
manually from the telephone of the answering party. Alternatively, the 
answer signal could comprise a tone or other signal which is transmitted 
automatically by a second device that is connected in some way to the 
called telephone. 
While this method of telephone answer detection may be quite reliable, it 
has some significant disadvantages. First, since telephone companies do 
not provide any universal telephone answer signal, the calling device 
cannot generally be used to call a particular party until prior 
arrangements have been made with that party. For example, this answer 
detection method typically requires either that the called party first 
know the required answer signal to transmit or that a piece of special 
equipment be previously installed in connection with the party's 
telephone. As a result, this method of detecting that a called telephone 
has been answered has proven to be quite cumbersome and inconvenient and 
may also be relatively expensive. Moreover, due to the required prior 
arrangements, this method of telephone answer detection may even preclude 
the use of the automatic calling device for many of the purposes outlined 
above. 
As alluded to previously, in many areas throughout the United States, the 
telephone company provides an electronic signal indicating that a 
telephone has been answered. This signal generally comprises reversing the 
voltage polarity on the telephone lines (that is, causing the electrical 
current to flow through the telephone lines in an opposite direction after 
the telephone has been answered). A reversal in voltage polarity is 
relatively easy to detect, and such detection can be done with a great 
degree of reliability. Hence, some prior art devices have attempted to 
detect an answered telephone by simply detecting a reversal of voltage 
polarity on the telephone lines. 
The major impediment to this method of detecting an answered telephone is 
that the telephone company in many parts of the United States, as well as 
in foreign countries, does not always provide voltage polarity reversal 
when a telephone is answered. As a result, this method of detecting an 
answered telephone, while simple and quite reliable, cannot be used 
uniformly on all telephone systems. Thus, if a telephone call is placed 
into an area in which voltage polarity reversal is not provided as an 
answer signal, no answer will be detected by the calling device. 
Still other prior art devices have attempted to detect the answering of a 
called telephone by monitoring the telephone system's call progress tones. 
In general, there are only four different types of call progress tones to 
be monitored: (1) a dial tone, (2) a ringing tone, (3) a line-busy signal 
(slow busy signal), and (4) an all trunk busy signal (fast busy signal). 
By monitoring the call progress tones, for example, it can be determined 
whether the called telephone has started ringing. If the telephone 
thereafter stops ringing, it may be presumed that the telephone was 
answered. 
Call progress tones have previously been monitored by distinguishing 
between the timing of the audio signals associated with each tone. For 
example, a ringing tone may comprise a two-second audio tone followed by 
four seconds of silence, with this pattern then repeating. Thus, if this 
audio signal pattern were detected on the telephone lines, the calling 
device would assume that the called telephone is now ringing. 
Again, one of the chief problems associated with this method of detecting 
if a called telephone has been answered is due to the nonuniformity 
between various central offices of the telephone sytem. For example, in 
some areas, a ringing tone may comprise a two-second tone followed by only 
three seconds of silence, while other areas may maintain the pattern 
comprising four seconds of silence, as set forth above. As a result, it 
may not be possible to adequately recognize and/or distinguish between 
various call progress tones; and this nonuniformity in timing thus makes 
this method unreliable for universal use in detecting whether a telephone 
has been answered. 
In addition, the above-described method of detecting an answered telephone 
may not respond quickly enough to the called party. For example, the party 
may answer the telephone in the middle of a ringing tone and say "hello." 
The party may then wait four seconds before saying "hello" again, and the 
calling device would generally perceive this as a continuation of the 
ringing tone. In all likelihood, the called party would soon hang up the 
telephone, and the calling device would not have yet delivered the audio 
message. 
BRIEF SUMMARY AND OBJECTS OF THE INVENTION 
In view of the foregoing, it is a primary object of the present invention 
to provide a dependable apparatus and method for electronically detecting 
that a called telephone has been answered. 
It is also an object of the present invention to provide an apparatus and 
method for detecting that a called telephone has been answered which does 
not require any special action on the part of the called party. 
Additionally, it is an object of the present invention to provide an 
apparatus and method for detecting that a called telephone has been 
answered which does not require the use of any special equipment in 
connection with the called telephone. 
It is an additional object of the present invention to provide an apparatus 
and method for detecting that a called telephone has been answered which 
is not dependent upon any particular signal generated by the telephone 
system. 
Further, it is an object of the present invention to provide an apparatus 
and method for detecting that a called telephone has been answered which 
is capable of reliable, universal application on any telephone system. 
It is a still further object of the present invention to provide an 
apparatus and method for detecting that a called telephone has been 
answered which is not dependent upon any specific timing or pattern of 
audio signals. 
Also, it is an object of the present invention to provide an apparatus and 
method for accurately distinguishing between audio voice signals and any 
supervisory or other system-generated signals which may be present in an 
electrical audio device. 
Consistent with the foregoing objects, the present invention is directed to 
a novel apparatus and method for electronically determining whether audio 
signals which are present in an electrical audio device (such as, for 
example, a telephone), are voice signals or, on the other hand, some type 
of supervisory or system-generated signals. While this invention is 
described herein as it may be applied in detecting that a called telephone 
has been answered, it will be appreciated that numerous other applications 
of this invention can be made. Thus, the following summary and description 
is intended by way of illustration only. 
In accordance with one presently preferred embodiment of the invention, 
telephone lines are connected to an audio detector and a digitizer. The 
audio detector provides an output which indicates either that there is or 
is not an audio signal on the telephone lines. The digitizer transforms 
any audio signal on the telephone lines into a series of high and low 
level signals in some manner. For example, in one presently preferred 
embodiment, the digitizer produces a high level signal whenever the 
instantaneous voltage level of the incoming audio signal is greater than a 
specified reference value, and it produces a low level signal at all other 
times. 
The outputs from the audio detector and digitizer are then provided to a 
central processing unit which is used to analyze the data. If an audio 
signal is detected on the telephone lines and the digitized audio signal 
has a regular periodic pattern, it is presumed that the audio signal is 
some type of supervisory or call progress tone which is generated by the 
telephone system. If, on the other hand, the digitized audio signal does 
not have a regular periodic pattern, it is presumed that the audio signal 
is a voice signal. Accordingly, upon detecting an irregular or random 
audio signal, the central processing unit produces an output signal which 
can be detected by associated circuitry as indicating that the called 
telephone has been answered.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It will be readily appreciated that the components of the present 
invention, as generally described and illustrated in the figures herein, 
could be arranged and designed in a wide variety of different 
configurations. Thus, the following more detailed description of the 
embodiments of the apparatus and methods of the present invention, as 
represented in FIGS. 1 through 9H, is not intended to limit the scope of 
the invention, as claimed, but it is merely representative of the 
presently preferred embodiments of the invention. 
Moreover, as mentioned above, the present invention is described and 
illustrated herein as it may be applied in connection with a telephone 
system to determine whether a called telephone has been answered. Various 
other applications for this invention are, of course, possible, and such 
applications would be readily apparent to those skilled in the art. Thus, 
the following description is not intended to limit this invention to any 
particular application. 
The presently preferred embodiments of the invention will be best 
understood by reference to the drawings, wherein like parts are designated 
with like numerals throughout. 
General Discussion 
If the audio signal on a telephone line was displayed graphically to a 
viewer, one would notice a marked difference between the audio signal 
which is present during any of the system-generated call progress tones 
and the audio signal which is present as a result of a human voice. This 
difference can be appreciated by examining FIGS. 1 and 2. 
In FIG. 1, the audio signal 12 which results from a call progress tone is 
shown graphically. As shown, audio signal 12 has a very regular pattern. 
Moreover, while the specific pattern of audio signal 12 may vary depending 
upon the particular call progress tone which is present on the telephone 
lines, audio signal 12 will have a regular periodic pattern in all cases. 
(As used herein, the term "regular periodic pattern" means a waveform 
which is predictable and which repeats itself over a certain period of 
time.) 
With reference to FIG. 2, the audio signal 14 which results from a human 
voice can be seen to be quite different from audio signal 12 of FIG. 1. 
Unlike audio signal 12, audio signal 14 has a very irregular pattern, and 
it does not repeat itself on any periodic basis. 
The apparatus and methods of the present invention take into account the 
above-noted fundamental differences between system-generated call progress 
tones and voice signals. This is done electronically such that an 
irregular or random audio signal is immediately recognized as a voice 
signal without the need for any human intervention. Significantly, since 
the present invention is based upon the inherent differences between 
system-generated call progress tones and voice signals, it is not 
dependent upon either the use of special equipment in connection with the 
called telephone, an answer signal provided by the telephone company, or 
the specific timing or pattern of any call progress tones. Thus, unlike 
the prior art devices, the present invention is capable of universal 
application for purposes of telephone answer detection on any telephone 
system. 
In order to electronically analyze an audio signal which is present on the 
telephone lines, the audio signal is first "digitized" such that the 
incoming audio signal is transformed into uniform high voltage signals 
(such as, for example, five volts), and uniform low voltage signals (such 
as, for example, zero volts), which can be received and processed by a 
digital computer. This "digitization" can be done in a number of ways. 
For example, the audio signal may be provided directly to a conventional 
analog-to-digital converter which will immediately transform the audio 
signal into a series of digital, numerical values. Such numerical values 
can then be provided to a digital computer for analysis. 
Alternatively, the audio signal may be digitized by transforming it into a 
single series of high voltage signals and low voltage signals. This can be 
done in a number of ways so as to facilitate analyzing the audio signal in 
accordance with the present invention. 
For example, as depicted in FIGS. 1A and 2A, the audio signal may be 
digitized such that a high signal results whenever the instantaneous 
voltage level of the incoming audio signal is greater than a specified 
reference value 16 (see FIGS. 1 and 2), and such that a low signal results 
whenever the instantaneous voltage level of the incoming audio signal is 
below the specified reference value 16. By comparing FIG. 1A to FIG. 1, 
therefore, it will be seen that the digitized audio signal 13 of FIG. 1A 
is high whenever audio signal 12 of FIG. 1 is above reference value 16. At 
the same time, digitized audio signal 13 is low whenever audio signal 12 
is below reference value 16. A comparison between FIGS. 2 and 2A will show 
that audio signal 15 of FIG. 2A similarly corresponds to audio signal 14 
of FIG. 2. Thus, FIGS. 1A and 2A represent one way of digitizing the audio 
signals of FIGS. 1 and 2, respectively. 
Comparing the digitized audio signals 13 and 15, it will be seen that 
digitized audio signal 13, like audio signal 12, still has a regular 
periodic pattern. Likewise, digitized audio signal 15 has an irregular, 
random pattern, as does audio signal 14. Thus, the digitized audio signals 
13 and 15 of FIGS. 1A and 2A, respectively, can be analyzed by a computer, 
and the digitized audio signal 15 of FIG. 2A can be immediately recognized 
as corresponding to an audio voice signal because of its irregularity. 
Another way of digitizing an incoming audio signal 12 or 14 would be to 
provide a high signal whenever the slope of the incoming audio signal is 
positive (that is, when its graphical representation slopes upward and to 
the right), and a low reading whenever the slope of the incoming audio 
signal is negative (that is, when the graphical representation of the 
audio signal slopes downward and to the right). Such a method of 
digitization would also show that any call progress tone results in a 
regular, periodic digitized audio signal, while a voice signal results in 
a very irregular digitized audio signal. 
Once the audio signal on the telephone lines has been digitized, the audio 
signal is then sampled and analyzed during successive time periods. Such 
time periods are represented graphically in FIG. 3. 
FIG. 3 illustrates a time period 18 during which the digitized audio signal 
is sampled and analyzed. Such time period is hereafter referred to as a 
"window." Window 18 is subdivided into a specified number of smaller time 
periods which are hereafter called "frames." Finally, during each frame, 
the digitized audio signal is sampled a specified number of times. 
Due to the high speed of microprocessors and other digital computing 
devices, the required sampling and analyzing steps can be done very 
rapidly. For example, it may be decided to take one sample every 160 
microseconds and to take 132 samples during every frame. A frame would, 
therefore, last 21.12 milliseconds. It might then be decided to have 13 
frames during every window. In such case, a window would last 0.27456 
seconds (274.56 milliseconds), during which period of time 1,716 samples 
of the digitized audio signal would be taken and analyzed. 
Reference is now made to FIG. 4 which illustrates one presently preferred 
embodiment of the present invention. 
As shown, the electrical signal discrimination apparatus and method of the 
present invention begins at step 200 and proceeds immediately to step 201, 
wherein an electrical signal is detected and/or amplified. Thereafter, the 
electrical signal is digitized in some manner, as outlined above, at step 
202. 
At step 203, it is determined whether the energy level of the electrical 
signals is above a specified threshold level. Thus, the analysis of noise 
signals is avoided. In this regard, it is noted that noise signals, like 
voice signals, are random in nature, although the energy level of noise 
signals is significantly less than that of voice signals. Thus, in order 
to avoid the processing of mere electrical noise, the apparatus and method 
of the present invention remains at step 203 until an electrical signal is 
detected which is above the specified threshold energy level. 
Once an electrical signal above the specified threshold level is detected 
at step 203, the system proceeds to step 204 to determine whether it is 
the beginning of such an electrical signal. The purpose of this step is to 
avoid potentially erroneous results due to analyzing only the end of an 
electrical signal. 
After the beginning of an electrical signal which is above the specified 
threshold energy level is detected at step 204, the system proceeds to 
step 205 to begin sampling and analysis of the electrical signals. 
Advantageously, step 205 may not actually be commenced simultaneously with 
the beginning of the electrical signal in order to avoid detecting 
irregularities which may be present only at the very beginning of the 
signal. Thus, for example, one or more frames of data (see FIG. 3) may be 
discarded prior to the commencement of a window 18 in step 205. 
At step 205, the designated number of samples are taken and analyzed during 
the first time period. Then, at step 206, it is initially determined that 
a frame standard has not yet been set. Accordingly, a frame standard is 
established at step 207, and the system returns to step 205 to continue 
sampling. As set forth in more detail below, the frame standard serves as 
a comparison reference for comparing the signal samples taken during the 
various subsequent frames in a window. 
Samples are then taken at step 205 during subsequent frames of window 18. 
After the completion of each frame, the system passes through step 206 to 
step 208 to determine whether the signal during the frame just completed 
was sufficiently close to the frame standard. If it is determined that the 
signal was not sufficiently close to the frame standard, a counter is 
incremented at step 209 to indicate the possibility that random voice 
signals may be present, since the digitized electrical signal does not 
appear to have any regular periodic pattern. The system then moves through 
step 210 and back to step 205 to complete the remaining frames in the 
window. 
At the completion of the window, the system moves from step 210 to step 211 
and determines whether the counter has been incremented more than a 
specified number (x) of times, thereby indicating a very irregular or 
random signal pattern. If the counter has been incremented more than a s 
specified number of times, the system moves to step 215 and generates an 
output signal indicating the presence of random electrical signals. In the 
case of telephone audio signals, this indicates the presence of voice 
signals and that the telephone has been answered. 
In addition, after the completion of a window in which the counter has not 
been incremented more than the specified number of times, the system moves 
to step 212 to determine whether a window standard has been set. If not, 
the system proceeds to step 213 to establish such a window standard. The 
window standard may, for example, comprise the frame standard which was 
set in the first window during which the electrical signal in question was 
analyzed. 
Once a window standard has been set, the system thereafter, upon reaching 
step 212, moves to step 214 to determine whether the signals analyzed 
during the window just completed are sufficiently similar to the window 
standard. If the signals are sufficiently close, the system returns to 
step 203 and commences a new window. If, on the other hand, the signals 
are not sufficiently close to the window standard, the system moves to 
step 215 and generates an output signal indicating an irregular random 
signal pattern. 
It will be appreciated by those skilled in the art that the digitized audio 
signal may be sampled and analyzed in accordance with the present 
invention in any of a number of ways. In one presently preferred 
embodiment, for example, the system is designed to count the number of low 
to high transitions which occur in the digitized audio signal during each 
frame. Thus, the digitized audio signal is sampled a specified number of 
times during each frame and a counter is incremented each time it is 
detected that the digitized audio signal has jumped from a low level 
reading to a high level reading. The numerical value stored in the counter 
at the end of each frame thus indicates the number of low to high signal 
transitions which occurred during that frame. Finally, the number of low 
to high transitions during the various frames and windows is then 
compared, as outlined above, and an irregularity in the number of low to 
high transitions is used as an indication that voice signals are present 
on the telephone lines. 
Alternatively, or in combination with the foregoing method, the "duty 
cycle" of the digitized audio signal could be sampled and analyzed. The 
duty cycle is defined as the ratio of the time during which the digitized 
audio signal is high to the total time during which samples are taken. 
Thus, a timing device could be used to keep track of the time during which 
the digitized audio signal is high during each frame. This measured time 
value could then be compared for subsequent frames and windows, as 
outlined above, with any irregularity in the duty cycle indicating the 
presence of voice signals on the telephone lines. Alternatively, signals 
could be sampled and analyzed during successive equal time periods and the 
number of times during each such time period that the digitized audio 
signal is a high voltage signal could be counted. That number would then 
be proportional to the "duty cycle" of the digitized audio signal. 
2. The System Structure 
One presently preferred embodiment of a system, generally designated at 10, 
which may be used to carry out the present invention is illustrated in the 
functional block diagram of FIG. 5. It will, however, be appreciated that 
a number of different system configurations may be used in accordance with 
the present invention. For example, those skilled in the art will readily 
recognize a number of obvious modifications to the functional block 
diagram of FIG. 5 which are, nevertheless, within the scope of the present 
invention. 
As depicted in FIG. 5, in one presently preferred embodiment of detection 
system 10, the audio signal from the telephone lines is provided to 
detection system 10 on line 40 as an input to an audio detector device 20. 
Such audio signal is then provided through audio detector 20 on line 22 to 
a digitizer 24. Alternatively, the audio signal may be provided 
simultaneously to both audio detector 20 and digitizer 24 without any 
interconnection between audio detector 20 and digitizer 24. 
The purpose of audio detector 20 is to indicate the presence of any audio 
signal on the telephone lines, which is above a certain threshold level, 
such as, for example, -40 dBm. Upon detection of such an audio signal, 
therefore, audio detector 20 transmits a signal on line 42 to input port 
26. Audio detector 20 makes no distinction between the various types of 
audio signals which may be present on the telephone lines. Thus, audio 
detector 20 detects any clicks, hissing noises, voice signals, and call 
progress tones which are above the specified threshold level and, in each 
case, indicates by a signal on line 42 that some type of audio signal is 
present. 
Digitizer 24 digitizes the incoming audio signal in some acceptable manner, 
as outlined above. The digitized audio signal is then transmitted on line 
44 to input port 26. 
The data provided by audio detector 20 and digitizer 24 to input port 26 is 
sampled and analyzed by a central processing unit (CPU) 30. The operating 
instructions for CPU 30 are stored in a suitable memory device 32, and 
such instructions are communicated in a conventional fashion to CPU 30 on 
data bus 28 in response to signals generated by CPU 30 on address bus 33. 
The timing of the sample taking and the analysis of data by CPU 30 is 
controlled by a timer 34 which is connected to CPU 30 as shown. 
In accordance with the foregoing, in response to both the instructions 
stored in memory device 32 and the timing signals generated by timer 34, 
CPU 30 periodically samples and analyzes the data provided at input port 
26. After the completion of a window, CPU 30 then determines whether the 
sampled data has an irregular, random pattern and whether voice signals 
are, therefore, present on the telephone lines. If voice signals are 
determined to be present, CPU 30 stops sampling the data and generates an 
answer signal on line 50. 
Reference is next made to FIGS. 6, 6A and 7, which illustrate in more 
detail preferred embodiments of a schematic diagram derived from the block 
diagram of FIG. 5. Those of ordinary skill in the art will, of course, 
appreciate that various modifications to the detailed schematic diagrams 
of FIGS. 6, 6A and 7 may be easily made without departing from the 
essential characteristics of the invention, as described above. Thus, the 
following description of the detailed schematic diagrams of FIGS. 6, 6A 
and 7 is intended only as an example, and simply illustrates one presently 
preferred embodiment of a schematic diagram that is consistent with the 
foregoing description of FIG. 5 and the invention as claimed herein. The 
various circuit stages corresponding to each of the functional blocks of 
FIG. 5 are identified in FIGS. 6, 6A and 7 with like numerals. 
FIG. 6 illustrates the analog portion of the functional block diagram of 
FIG. 5. That is, FIG. 6 is a representation of one presently preferred 
embodiment of audio detector 20 and digitizer 24. 
As shown, audio detector 20 may comprise two operational amplifiers (op 
amps) U1A and U1B. Amplifier U1A receives the audio signal from contact 
point 40 through capacitor C1 and resistor R1 and provides a large 
increase in signal strength (gain). For example, resistors R1 and R2 may 
be selected so as to provide op amp U1A with a signal gain of 
approximately 200. 
The output of op amp U1A is used to charge capacitor C2 through diode D1, 
with capacitor C2 discharging through resistor R3. Op amp U1B is then used 
to compare the voltage across capacitor C2 with a reference voltage which 
is determined by the voltage divider comprising resistors R5 and R6. Op 
amp U1C acts as a voltage follower to further stabilize the reference 
voltage, and capacitor C4 reduces noise. Op amp U1B thus acts as a 
comparator and provides a high signal at contact point 42 whenever the 
level of the audio input is greater than a specified threshold level. For 
example, the various components of audio detector 20 may be selected such 
that a high signal is provided at contact point 42 whenever the level of 
the audio signal at contact point 40 is greater than approximately -40 
dBm. 
As also shown in FIG. 5, digitizer 24 may comprise an op amp U1D. As shown, 
the amplified audio signal from op amp U1A is provided on line 22 to op 
amp U1D, which functions as a voltage comparator. Thus, whenever the 
instantaneous voltage level of the amplified audio signal is above the 
specified reference level, a high voltage signal will be provided at 
contact point 44. (See FIGS. 1 and 1A and FIGS. 2 and 2A.) 
It will be appreciated, therefore, that the embodiment of audio detector 20 
and digitizer 24 which is illustrated in FIG. 5 provides a high reading at 
contact point 42 whenever audio is present and provides a digitized audio 
signal at contact point 44. Hence, the analog audio signal which was 
provided at contact point 40 has now been digitized into two separate 
digital data signals which are provided at contact points 42 and 44. 
FIG. 6A illustrates a second preferred embodiment of digitizer 24. As 
shown, digitizer 24 may comprise two op amps U2A and U2B. Op amp U2A acts 
as a voltage follower and is connected to line 22, as shown. The output of 
op amp U2A is then provided to two switches S1 and S2. Switches S1 and S2 
may, for example, comprise analog gates which are connected as shown to a 
positive voltage source and to a frequency source 25. Frequency source 25 
may, for example, oscillate at a frequency of 23 kHz. In this way, switch 
S2 is continuously opened and closed while switch S1 is always open. 
As illustrated in FIG. 6A, a capacitor C5 is connected between switch S2 
and op amp U2B. As a result, capacitor C5 acts as a sample and hold device 
and is charged to the voltage level on line 22 whenever switch S2 is open. 
Finally, op amp U2B acts as a voltage comparator to compare the voltage 
capacitor C5 with the instantaneous voltage on line 22, as transmitted 
through op amp U2A and switch S1. It will, therefore, be appreciated that 
op amp U2B produces a high voltage signal at contact point 44 whenever the 
instantaneous slope of the incoming audio signal is positive and produces 
a low voltage signal at contact point 44 at all other times. 
One presently preferred embodiment of the digital portion of the block 
diagram of FIG. 5 is illustrated in FIG. 7. The various circuit components 
depicted in FIG. 7 are connected together as indicated in conventional 
fashion by the pin numbers adjacent integrated circuits U5 through U9. 
Thus, for example, line 27, which corresponds to bit A12 of address bus 
33, is connected to pin number 19 of integrated circuit U5. 
As shown in FIG. 7, input port 26 may comprise an integrated circuit U5. 
Contact points 42 and 44 are connected to integrated circuit U5, as shown, 
and U5 thus receives the above-mentioned digitized data from the circuitry 
of FIG. 5. 
Integrated circuit U5 is also connected, as shown, to data bus 28. Thus, 
when an enable signal is received by s integrated circuit U5 on line 27, 
the digitized data at contact points 42 and 44 is transmitted through 
integrated circuit U5 onto data bus 28. As previously indicated, line 
As also shown in FIG. 7, CPU 30 may comprise an integrated circuit U6. A 
clock pulse may be provided to U6 at contact point 46. For example, 
contact point 46 may be connected to a clock pulse having a frequency of 2 
megahertz. 
A means is also provided for enabling integrated circuit U6. This enabling 
means may, for example, comprise a logic gate U7, the associated resistors 
R8 and R9 and a transistor Q1. In this way, whenever a low voltage signal 
is received simultaneously at both contact points 48 and 49, integrated 
circuit U6 will be enabled and will commence operation. If, on the other 
hand, either of contact points 48 or 49 has a high voltage signal, 
integrated circuit U6 will be disabled. 
It will be readily appreciated that a number of different methods could be 
used to enable integrated circuit U6. The circuitry illustrated in FIG. 7 
has been configured in order to facilitate using this invention in 
conjunction with an automatic calling device having a tone generator of 
some sort. Contact points 48 and 49 may thus be connected to such tone 
generator such that integrated circuit U6 is enabled whenever no tone is 
being produced by the tone generator. 
The operating instructions for integrated circuit U6 are stored in memory 
device 32 which comprises an integrated circuit U8, as shown. Integrated 
circuit U8 may advantageously comprise a read-only memory device, and 
integrated circuit U8 is connected to integrated circuit U6, by means of 
both data bus 28 and address bus 33, in a conventional fashion such that 
appropriate instructions which are stored in integrated circuit U8 may be 
transmitted on data bus 28 for execution by integrated circuit U6. 
Timer 34 of answer detection device 10 may comprise an integrated circuit 
U9, which is configured, as shown, so as to comprise a monostable device. 
Thus, whenever a trigger pulse is received on line 36 (which may, as 
shown, correspond to bit A13 of address bus 33), integrated circuit U9 
would generate a high voltage signal on output line 37 for a period of 
time which is determined by the values of capacitor C3 and resistor R10. 
At the end of that time period, integrated circuit U9 would produce a low 
signal on line 37 which would then interrupt integrated circuit U6. As set 
forth in more detail below, this configuration may be used to assure that 
data samples are taken by integrated circuit U6 at regular intervals. 
In operation, integrated circuit U6 will not function until it is enabled 
by low voltage signals at contact points 48 and 49. Once enabled, 
integrated circuit U6 will receive and execute instructions from 
integrated circuit U8. At the appropriate times, integrated circuit U6 
will provide a signal on bit A12 of address bus 33, which bit corresponds 
to line 27 connected to integrated circuit U5. The signal on line 27 will 
enable integrated circuit U5 and data will then be transmitted through 
integrated circuit U5 and onto data bus 28, such that the data can be 
sampled and analyzed by integrated circuit U6. 
Prior to taking each sample, integrated circuit U6 will also send a signal 
on bit A13 of address bus 33, which bit corresponds to line 36 connected 
to integrated circuit U9. As outlined above, this will result in a high 
voltage signal on line 37 for the duration of the predesignated time 
period. When the signal on line 37 again is dropped to a low voltage 
level, integrated circuit U6 will be interrupted, thereby indicating that 
the next data sample should now be taken. 
Upon sampling and analyzing the data, integrated circuit U6 may determine 
that the digitized audio signal does not have any regular pattern. In such 
case, all data processing will be halted, and integrated circuit U6 will 
produce a low voltage signal at contact point 50. Such low voltage signal 
may then be detected by associated circuitry as indicating that the called 
telephone has been answered. 
Table I below indicates various values and types for the specific circuit 
components illustrated in FIGS. 6, 6A and 7. Those skilled in the art will 
readily appreciate that variations may be made to the specific values and 
component types without departing from the spirit or scope of the present 
invention. Thus, the specific components identified in Table I below are 
intended to be an illustration only, and the present invention is not 
intended to be limited to any specific circuit components. 
The components set forth in Table I below are identified by the same 
reference numerals which are used in FIGS. 6, 6A and 7. Resistance is 
stated in ohms; and capacitance is stated in microfarads. 
TABLE I 
______________________________________ 
LISTING OF CIRCUIT COMPONENTS 
______________________________________ 
Capacitors 
No. Capacitance 
______________________________________ 
C1 0.1 
C2 10 
C3 0.01 
C4 10 
C5 0.05 
______________________________________ 
Resistors 
No. Resistance 
______________________________________ 
R1 2.7K 
R2 470K 
R3 18K 
R4 10K 
R5 1K 
R6 470 
R7 10K 
R8 2.7K 
R9 10K 
R10 16K 
______________________________________ 
Diodes 
No. Type 
______________________________________ 
D1 1N914A 
Transistors 
No. Type 
______________________________________ 
Q1 2N4124 
______________________________________ 
Analog Gates 
S1 4066 
S2 4066 
______________________________________ 
Integrated Circuits 
No. Type 
______________________________________ 
U1A-U1D LM324 
U2A-U2B LM324 
U5 74LS241 
U6 Z80 
U7 74LS32 
U8 2716 
U9 74LS123 
______________________________________ 
3. The System Operation 
One presently preferred embodiment of the method of operation of the 
present invention is illustrated by the flow chart of FIGS. 8 through 8H. 
Those skilled in the art will readily appreciate that other specific 
methods may be used in accordance with the present invention and that the 
flow chart of FIGS. 8 through 8H may be modified without departing from 
the scope of the present invention. 
Moreover, the flow chart of FIGS. 8 through 8H illustrates one way of 
implementing the present invention using a Z80 microprocessor and taking 
advantage of the particular characteristics of that type of 
microprocessor. Such characteristics include, for example, the 
availability of internal registers for storing the results of the data 
analysis which is performed during the course of the system's operation. 
Thus, the following description of the flow chart of FIGS. 8 through 8H is 
intended by way of illustration only, and not limitation. 
Furthermore, the flow chart of FIGS. 8 through 8H is described as it would 
be implemented in the form of computer software. Those skilled in the art 
will readily appreciate that the method could also be implemented in the 
form of system hardware, and such hardware implementation is also within 
the scope of this invention. 
With reference to FIG. 8, the system will begin operating at step 100 as 
soon as CPU 30 is enabled (see FIGS. 5 and 7). The system then moves 
immediately to step 101 and initializes all registers which will be used 
to store data during sampling and analysis. 
Next, the system moves to step 102 and determines whether audio is 
initially present on the incoming audio line. The system thus samples line 
42 of FIGS. 5 through 7 to determine whether audio detector 20 indicates 
that an audio signal is present. If audio is initally present, the system 
remains at step 102. If audio is not initially present, or as soon as 
audio detector 20 indicates that no audio is now present, the system moves 
to step 103. 
At step 103, the system again determines whether audio is present by 
reading the output of audio detector 20. As long as no audio is present, 
the system stays at step 103. Once audio is detected, the system moves 
immediately to step 104. 
Steps 102 and 103 are designed to ensure that the audio data is sampled 
from the beginning of an audio signal, rather than at the end or in the 
middle of a signal. In this way, erroneous readings which might otherwise 
result from sampling only the tail end of an audio signal can be 
eliminated. 
At step 104, a storage register within the microprocessor which is 
hereinafter referred to as the window counter is loaded with a numerical 
value equal to N+2. As illustrated in FIG. 3, N is the number of frames 
which will be included within a window. Thus, N may be any suitable 
number, such as, for example, 13. In such case, the window counter would 
initially be set to a value of 15 at step 104. 
The system next moves to step 105, and a storage register which is 
hereinafter called the odd counter is set to a value of 0. As described 
more fully below, the odd counter will be used to indicate the number of 
frames within a given window wherein the data sampled is significantly 
different from the average data characteristic for that window. Thus, the 
odd counter will be incremented whenever the system determines that it is 
possible that voice signals are present. 
After initializing the odd counter, the system moves to step 106 and 
initializes a storage register which is hereinafter called the transition 
counter. Throughout this flow chart, it will be assumed that the system is 
analyzing the data by determining the number of low to high transitions in 
the digitized audio signal during each frame. During the analysis, 
therefore, the transition counter will keep track of the number of such 
transitions; and the transition counter is thus initially set to a value 
of 0 at step 106. 
The system next moves to step 107 and sets a storage register which is 
hereinafter called the frame counter to a value of S. The frame counter is 
used to keep track of the number of samples which have been taken during a 
given frame. Thus, the number S indicates the number of samples to be 
taken during each frame and may be set to any suitable number in step 107, 
such as, for example, 132. 
The system then moves to step 108 and resets to zero one bit of a storage 
register which is hereinafter called the transition flag. As will become 
more readily apparent from the discussion which follows, the transition 
flag indicates whether the digitized audio signal was high or low the last 
time it was sampled. Thus, the transition flag will be used to determine 
whether a low to high transition has occurred. 
Next, the system moves to step 109 and enables the interrupt function of 
CPU 30 (see FIGS. 5 and 7). At this point, the system will begin to 
recognize an interrupt signal which is present on line 37 of FIG. 7. As 
set forth more fully below, such an interrupt signal indicates that a data 
sample should now be taken by the system. 
After enabling the interrupt, the system moves to step 110 and initializes 
the "stack." Actually, the system described and illustrated herein does 
not use any memory space which could be characterized as a "stack." 
However, a stack is initialized at step 110 in order to provide an unused 
memory address to which the system can write when it is interrupted. 
Normally, when a microprocessor system is interrupted, it automatically 
records an address indicating the memory location to which the processor 
should return after it has completed the interrupt routine. However, as 
will be seen from the discussion which follows, such a return address is 
not required by the present system, but a "phantom" stack is provided so 
as to avoid potential conflict on the data bus should CPU 30, when 
interrupted, attempt to write to memory device 32. 
After completing step 110, the system repeats steps 109 and 110 until an 
interrupt signal is received. Upon receiving an interrupt signal, the 
system immediately moves to step 111 of FIG. 8A. 
It should be noted that, when the system is first enabled, an interrupt 
signal will initially be present, since a low voltage signal will be 
present on line 37 of FIG. 7. Therefore, the first time that the system 
reaches step 109 it will immediately recognize the interrupt signal and 
jump to step 111. 
At step 111, the system restarts the interrupt timer 34 (see FIGS. 5 and 
7). Referring to FIG. 7, at step 111, CPU 30 sends a signal on line 36 to 
timer 34 such that interrupt line 37 is then maintained at a high voltage 
level for a time period determined by capacitor C3 and resistor R10. This 
procedure will enable the system to take data samples at regular 
intervals, as will be explained more fully below. 
Following step 111, the system moves to step 112 and decrements the frame 
counter. This indicates that the system is about to take a sample, and the 
value in the frame counter now indicates the number of additional samples 
which will need to be taken during the current frame. From step 112, the 
system moves to step 113 and determines whether the frame counter is equal 
to 0. In other words, the system determines at step 113 whether all of the 
samples in the current frame have been taken. If all of the samples have 
not been taken, the system moves to step 122 of FIG. 8B, as indicated by 
the flags labeled "B." 
At step 122, the system determines whether the digitized audio data 
generated by digitizer 24 is high at that moment in time. In other words, 
the system samples the signal from line 44 of FIGS. 5 through 7 and 
determines whether the digitized audio signal at that point in time is a 
high voltage signal or a low voltage signal. 
If the audio signal is not a high signal, the system then moves directly to 
step 123 and resets the transition flag. As indicated by the flags labeled 
"A," the system then returns to step 109 of FIG. 8 to await the next 
interrupt. 
If the instantaneous signal from digitizer 24 is high at step 122, the 
system moves to step 124. At step 124, the system determines whether a low 
to high transition has taken place. In other words, the system determines 
whether the digitized audio signal was at a low voltage level the last 
time it was sampled. This is done by examining the transition flag. If the 
transition flag is 0, this is an indication that the last audio signal 
sampled was a low value and that a transition has, therefore, taken place. 
If the transition flag is set to 1, on the other hand, this is an 
indication that the last audio signal sampled was also at a high value and 
that there has been no transition. 
If it is determined at step 124 that a transition from a low voltage level 
to a high voltage level has occurred, the system moves to step 125 and 
sets the transition flag to a value of 1. As set forth above, this 
indicates that the audio signal just sampled was at a high voltage level. 
The system then moves to step 126 and increments the transition counter to 
indicate that a low to high transition has taken place. Thereafter, the 
system returns to step 109 of FIG. 8 to await the next interrupt, as 
indicated by the flags labeled "A." 
If it is determined at step 124 that no transition has taken place, the 
system moves to step 127. This would be the case when the audio signal 
just sampled has a high value and the last audio signal sampled was also a 
high value. At step 127, the transition flag is set to normal, that is, it 
is reset to the same state it was in before step 124. The system then 
returns to step 109 of FIG. 8 to await the next interrupt, as indicated by 
the flags labeled "A." 
As indicated above, prior to taking each sample, the interrupt timer is 
restarted at step 111, which means that a pulse is sent on line 36 of FIG. 
7 to timer 34 so as to bring line 37 of FIG. 7 high for a time period 
determined by capacitor C3 and resistor R10. When a sample is taken, 
therefore, the steps illustrated in FIG. 8B will be executed, and the 
system will then return to step 109 of FIG. 8. When the system returns to 
step 109, line 37 of FIG. 7 will still be high. The system will then 
continuously execute steps 109 and 110 of FIG. 8 until line 37 of FIG. 7 
goes low, at which time the system will again jump to step 111 of FIG. 8A, 
as explained above. 
It will be appreciated from the foregoing, therefore, that the time 
constant determined by capacitor C3 and resistor R10 of FIG. 7 determines 
the time between successive samplings of the output from digitizer 24 (see 
FIGS. 5, 6 and 6A). For example, C3 and R10 may be selected such that a 
sample is taken every 160 microseconds. Of course, other suitable time 
durations could also be chosen, as desired. 
The system will continue to take samples as set forth above and keep track 
of the number of low to high transitions until all of the samples in a 
frame have been taken. That will be determined by the system at step 113 
of FIG. 8A. 
If the system determines at step 113 that all of the samples in a frame 
have been taken, the system will move to step 114. At step 114, the system 
determines whether the window counter is less than or equal to the number 
of frames which are to be included within the window. 
It will be recalled that at step 104 the window counter was set to a value 
of two greater than the number of frames in the window. In this way, two 
frames could be ignored prior to the beginning of the window. Of course, 
the window counter could be set at step 104 to any value greater than or 
equal to the number of frames to be included in the window such that more 
or less than two frames are ignored prior to the commencement of the 
window, as desired. 
If it is determined at step 114 that the window counter is not less than or 
equal to the number of frames in the window, this is an indication that 
the frame which has just been completed is to be ignored. Accordingly, the 
system then moves to step 115 and decrements the window counter and then 
returns to step 106 of FIG. 8, as indicated by the flags labeled "C." 
Beginning at step 106, the transition counter and frame counter and 
transition flag are all reset such that a new frame can be started. The 
system then moves to steps 109 and 110 to await an interrupt signal 
indicating the beginning of the next frame and that the first sample of 
that frame should be taken. 
After the first two frames have been ignored, the system will determine at 
step 114 that the window counter is equal to the number of frames to be 
included in the window. Accordingly, the system will then go to step 116. 
At step 116, the system determines whether the window counter is exactly 
equal to the number of frames to be included in the window. If so, this 
will indicate that the frame just completed is the first frame in this 
window for which data is to be analyzed. If it is determined at step 116 
that this is the first frame of data to be analyzed, the system moves to 
step 117. 
At step 117, it is determined whether the value in the transition counter 
is greater than four. That is, whether there were more than four low to 
high transitions during that frame. If it is determined that there were 
not more than four transitions, this frame will be discarded and an 
entirely new window will be commenced, in that the system will then move 
immediately to step 104 of FIG. 8, as indicated by the flags labeled "F." 
The purpose of the determination in step 117 is to eliminate the data due 
to "clicks" or other voltage spikes on the telephone lines. A click may 
sometimes be heard on the telephone lines when a call is being routed 
through several central offices of the telephone system, and such clicks 
will be perceived as an audio signal. Accordingly, the system will begin 
sampling. However, there will be relatively few low to high transitions 
during that frame, since the "click" or spike will not result in any 
sustained audio signal on the telephone lines. Hence, step 117 is intended 
to eliminate potentially erroneous results which could otherwise occur due 
to such clicks or other voltage spikes. 
Accordingly, as outlined above, if it is determined at step 117 that fewer 
than four transitions were detected during a given frame, the window 
counter and odd counter will be initialized at steps 104 and 105, 
respectively, thereby indicating the beginning of a new window. The system 
will then move through steps 106 through 108 and down to steps 109 and 
110, where the system will again wait for an interrupt signal to begin 
sampling. Of course, any suitable number either greater or less than four 
could be used as the test criterion at step 117. The appropriateness of 
using any particular number will depend upon the number of samples being 
taken during each frame. 
If it is determined at step 117 of FIG. 8A that more than four low to high 
transitions occurred during the frame, the system will move to step 118 
and save the transition counter value in a separate storage register. The 
transition counter value is saved so that it can be averaged with the 
value obtained during the next frame and so that the average value can 
then be compared with the transition counter value resulting from 
subsequent frames. 
Following step 118, the system initializes the transition counter and odd 
counter in step 119 and resets the frame counter to the appropriate value 
in step 120. Finally, the window counter is decremented at step 121, 
indicating that the first frame from which data is to be analyzed has now 
been completed. Thereafter, the system returns to steps 109 and 110 of 
FIG. 8, as indicated by the flags labeled "A." 
Once the first frame of data which is to be analyzed has been saved in step 
118, as set forth above, the system is ready to begin taking samples which 
will comprise the second frame of data to be analyzed. This data will be 
taken in exactly the same manner as set forth above, as the system 
executes the appropriate steps illustrated in FIG. 8B. Then, when the 
system determines in step 113 that all of the samples in this second frame 
have now been taken, the system will again move to step 114, and from step 
114 to 116. 
At step 116, the system will now determine that the window counter is not 
exactly equal to the total number of frames to be included in the window. 
This is due to the fact that the window counter was previously decremented 
below the value of the total number of frames in the window as a result of 
step 121. Therefore, the system will now move to step 128 of FIG. 8C, as 
indicated by the flags labeled "D." 
At step 128, the system will determine whether the window counter is equal 
to one less than the number of frames in the window. In other words, step 
128 determines whether this is the second frame in the window for which 
data is to be analyzed. If it is determined at step 128 that this is the 
second frame of data in the window which is to be analyzed, the system 
moves to step 129 of FIG. 8D, as shown by the flags labeled "G." 
At step 129, it is again determined whether the value in the transition 
counter is greater than four. Step 129 serves the same purpose as step 117 
in eliminating potentially erroneous results due to clicks and other 
voltage spikes on the telephone lines. Thus, if the transition counter 
does not have a value greater than four, the system would again return to 
step 104 and begin a new window, as outlined above. 
If the value in the transition counter in the second frame is, however, 
greater than four, the system would move to step 130 and calculate the 
average value of the transition counter for the first and second frames. 
In step 131, the calculated average value would then be stored in a 
register hereinafter called CNT 1. In step 132, a value of two greater 
than the average value would be stored in a register hereinafter called 
CNT 2; and a value of three less than the average value would be stored in 
a register hereinafter called CNT 3 in step 133. 
In step 134, the transition counter and odd counter are reset to zero. The 
frame counter would be reset to indicate the appropriate number of samples 
in step 135. Finally, the window counter would be decremented in step 136 
to indicate the completion of another frame. The system would then return 
to steps 109 and 110 of FIG. 8, as indicated by the flags labeled "A." 
Upon receiving the next interrupt, the system would again take samples and 
count the low to high transitions until the completion of the next frame. 
The system would then move through steps 114 and 116 of FIG. 8A to step 
128 of FIG. 8C, as explained previously. 
At step 128, the system would again determine whether the window counter is 
equal to one less than the total number of frames in the window. However, 
since this is now the third frame in the window, the value of the window 
counter is now two less than the total number of frames in the window. 
Accordingly, the system would move to step 137 of FIG. 8E, as indicated by 
the flags labeled "H." 
At step 137, it is determined whether the transition counter for the 
present frame is less than the value stored in the register CNT 2. It will 
be remembered that the value stored in CNT 2 is two greater than the 
average number of transitions in the first two frames. Thus, step 137 is 
determining whether the number of transitions in the present frame are 
less than the upper limit which is stored in CNT 2. 
If the transition counter is not less than the upper limit stored in CNT 2, 
the system moves directly to step 147 of FIG. 8F, as indicated by the 
flags labeled "I." Step 147 will be explained in more detail below. 
If, on the other hand, the transition counter is found in step 137 to be 
less than the upper limit stored in register CNT 2, the system moves to 
step 138. At step 138, the system determines whether the value of the 
transition counter is greater than the lower limit which is stored in 
register CNT 3. 
It will be remembered that the lower limit which was stored in register CNT 
3 in step 133 is equal to three less than the average number of 
transitions for the first two frames in the window. Thus, at step 138 it 
is determined whether the number of low to high transitions in the present 
frame is greater than that lower limit. If it is determined that the value 
in the transition counter is not greater than the designated lower limit, 
the system then moves immediately to step 147 of FIG. 8F, as indicated by 
the flags labeled "I." 
Steps 137 and 138 of FIG. 8E are, therefore, steps which determine whether 
the number of low to high transitions in the present frame are within 
acceptable limits of the average number of transitions in the first two 
frames of this window. If the number of transitions is not within 
acceptable limits, the system moves to step 147 of FIG. 8F. If, on the 
other hand, the number of transitions is within acceptable limits, the 
system moves to step 139 where the registers are returned to normal and 
the system may continue its analysis. 
It will be appreciated that any of a number of upper and lower limits could 
be established in steps 132 and 133 of FIG. 8D. However, the upper and 
lower limits specified in steps 132 and 133 of FIG. 8D represent the 
presently preferred values. 
As set forth above, if it is determined at either step 137 or step 138 that 
the number of transitions in the present frame is not within the 
designated limits, the system moves to step 147 of FIG. 8F. At step 147, 
the registers are again returned to their normal values, and the system 
moves to step 148. 
At step 148, the odd counter is incremented. The odd counter, as explained 
above, is used to keep track of the number of frames in which the number 
of low to high transitions varied significantly from the average number of 
low to high transitions which was calculated from the first two frames in 
the window. Thus, at step 148, the odd counter is incremented to indicate 
that the number of transitions in the present frame was not within the 
designated, acceptable limits, and the system then returns to step 140 of 
FIG. 8E, as indicated by the flags labeled "M." 
At step 140, the window counter is decremented to indicate the completion 
of another frame in the window. The system then moves to step 141. 
At step 141, it is determined whether the window counter is equal to zero. 
In other words, at step 141 a determination is made as to whether all of 
the frames in the window have been completed. If all of the frames have 
not been completed, the system returns to step 106 of FIG. 8, as indicated 
by the flags labeled "C," such that a new frame can be started, as 
outlined above. 
The system thus continues to take samples and complete frames and to 
compare the number of low to high transitions in each frame with the 
average value calculated from the first two frames until all of the frames 
in the window have been completed. With each comparison, the odd counter 
is incremented at step 148 of FIG. 8F if it is determined that the number 
of low to high transitions in a given frame is significantly different 
from the average number of transitions calculated from the first two 
frames in the window. 
At the completion of all of the frames in the window, the system moves from 
step 141 of FIG. 8E to step 142. At step 142, it is determined whether the 
odd counter has a value greater than five. That is, whether more than five 
frames within the window had low to high transitions which were 
significantly different from the average value for that window. If such is 
the case, it is assumed that the telephone has been answered and that the 
audio signal detected is a voice signal. Accordingly, the system moves to 
step 152 of FIG. 8H, as shown by the flags designated "J." Step 152 will 
be described in more detail below. 
If it is determined at step 142 that the odd counter does not have a value 
greater than five, the system moves to step 143. At step 143, it is 
determined whether a standard has yet been set. A standard is set during 
the first window after the system is first enabled. Thus, after the 
completion of the first window, a standard would not have been set and the 
system would, therefore, move to step 144. 
At step 144, the system stores the average number of low to high 
transitions which was calculated for that window, which was previously 
stored in register CNT 1, in another register hereinafter designated AVG 
1. The system then moves to step 145 and stores the value of the odd 
counter in a register hereinafter designated ODD 1. Finally, the system 
moves to step 146 and sets one bit of a register which is hereinafter 
called the standard flag to indicate that a standard has now been set. The 
system then returns to step 102 of FIG. 8 to begin a new window, as 
indicated by the flags labeled "L." 
At the completion of each window, the odd counter is again tested at step 
142 of FIG. 8E. If the value of the odd counter is greater than five, the 
system again moves to step 152 of FIG. 8H, as indicated previously. 
However, if the value of the odd counter is not greater than five, the 
system again moves to step 143. 
At this point, the system will determine at step 143 that a standard has 
now been set. Thus, the system will move to step 149 of FIG. 8G, as 
indicated by the flags labeled "K." 
At step 149, it is determined whether the odd counter is greater than two. 
In other words, it is determined whether three or more frames in the 
window had a number of low to high transitions which was significantly 
different from the average value determined during the first two frames of 
that window. If such is the case, the telephone is presumed to have been 
answered and the system moves to step 152 of FIG. 8H. 
If the value of the odd counter is not greater than two, the system moves 
to step 150 and determines whether the average number of low to high 
transitions which was calculated for the present window is greater than 
two more than the average number of transitions which was calculated for 
the first window. In other words, step 150 is determining whether the 
window which has just been completed had significantly more low to high 
transitions than the first which was analyzed. If so, the system again 
moves to step 152, as indicated by the flags labeled "J." 
If the average number of low to high transitions for the present window is 
not significantly greater than the average number of low to high 
transitions in the first window, the system then passes to step 151. Step 
151 is designed to determine whether the number of low to high transitions 
is significantly less for the present window than for the first window. 
Thus, if the average number of transitions for the present window, which 
is stored in register CNT 1, is not greater than three less than the 
average value determined for the first window, the system again goes to 
step 152 of FIG. 8H. If, however, it is determined at step 151 that the 
average value for the present window is not significantly less than the 
average number of transitions for the first window, the system returns to 
step 102 of FIG. 8 to begin a new window, as indicated by the flags 
labeled "L." 
As set forth above, the data collected during each window is analyzed at 
several points to determine whether there are significant variations in 
the number of low to high transitions, either within the window itself or 
between the present window and the first window, to justify the conclusion 
that the audio signal on the telephone lines cannot be a supervisory or 
call progress tone but must be a voice signal. If at any of these 
determining points, it is determined that the difference is significant 
enough to warrant that determination, the system jumps to step 152 of FIG. 
8H. 
It will be readily appreciated, of course, that a number of different 
standards could be set to determine when one is justified in concluding 
that voice signals are present on the telephone lines. Thus, the standards 
illustrated in the figures herein are not intended by way of limitation 
but only by way of example. Those criteria do, however, represent the 
presently preferred system operation. 
Thus, as indicated at step 142, it will not be concluded that voice signals 
are present on the telephone lines during the first window of sampling 
unless more than five frames of that window have a number of low to high 
transitions which is significantly different from the average number 
calculated from the first two frames in that window. On subsequent 
windows, however, voice signals will be presumed to be present if there 
are only three or more frames in the window which are significantly 
different from the average. Thereafter, subsequent windows will also be 
compared with the first window of data analyzed, and it will be concluded 
that voice signals are present if the average number of low to high 
transitions calculated for the present window is either significantly 
greater than or significantly less than the average number of low to high 
transitions which was calculated for the first window. Once any of these 
criteria are met, the system moves to step 152 of FIG. 8H, as explained 
previously. At step 152 the interrupt function is disabled, thereby 
indicating to the microprocessor that all further interrupts are to be 
ignored and that no further samples are to be taken. The system then moves 
to step 153. 
At step 153, the system executes a halt instruction. With the hardware 
illustrated in FIG. 6 herein, a halt instruction will cause contact point 
50 to go low. This low value can, therefore, be detected by associated 
circuitry and will indicate that the answer detection device and method of 
the present invention has detected that voice signals are present on the 
telephone lines and that the telephone has, therefore, been answered. The 
associated circuitry can then be activated to transmit an appropriate 
audio message to the called party. 
One presently preferred embodiment of a set of operating instructions which 
is designed to accomplish the foregoing steps and objectives is set forth 
in the Appendix hereto. The instructions listed therein were designed to 
be executed by a Z80 microprocessor. It will be readily appreciated that 
other specific instructions could achieve the same result. Thus, the 
specific listing set forth in the Appendix is intended by way of example 
only. 
A second presently preferred embodiment of the method of operation of the 
system of the present invention is illustrated in FIGS. 9 and 9A. FIGS. 9 
and 9A illustrate one way in which the present invention can be carried 
out using a conventional analog-to-digital converter wherein the incoming 
electrical signals are converted directly to a digital, numerical value 
which is proportional to the instantaneous voltage level of the incoming 
electrical signal. The numerical values are then analyzed by CPU 30 to 
determine whether the electrical signal has a regular periodic pattern or 
is a random electrical signal. 
Referring to FIG. 9, the system begins at step 160 and starts an interrupt 
timer at step 161. The system then moves to step 162. 
As shown in FIG. 9A, step 162 commences a sample loop during which samples 
of the digitized data are taken. Thus, the system initially resets all 
counters at step 173 and then determines whether audio signals above a 
given energy threshold level are present at step 174. If such audio 
signals are not present, the system remains at step 174. 
As soon as audio is detected, the system moves to step 175 and samples the 
digitized data. At step 176, the system then stores the sample value just 
taken and takes an additional sample at step 177. 
At step 178, the system determines whether the sample value just taken is 
greater than the first sample value taken. In other words, the system 
determines whether the instantaneous voltage level of the incoming audio 
signal is increasing. If so, the system returns to step 176 and repeats 
steps 176 through 177 until it is determined at step 178 that the sample 
value is not increasing. 
At that point, the system moves to step 179 and increments a transition 
counter. The system then moves to step 180 and stores the sample value 
just taken and takes an additional sample in step 181. 
At step 182, the system now determines whether the sample values are 
decreasing. In other words, the system determines whether the 
instantaneous voltage level of the incoming electrical signal is 
decreasing over time. If so, the system returns to step 180 and step 181 
until it is determined at step 182 that the sample value is not 
decreasing. 
Upon such a determination at step 182, the system moves to step 183 and 
again increments a transition counter. The system then returns to step 176 
and proceeds as outlined above. 
It will, therefore, be appreciated that the steps illustrated in FIG. 9A 
are designed to count the number of times the slope of the incoming 
electrical signal changes from positive to negative and from negative to 
positive. The system is thus counting the number of relative maxima and 
minima of the voltage curve of the incoming electrical signals. 
The steps illustrated in FIG. 9A are continued indefinitely until the timer 
which was set at step 161 in FIG. 9 indicates that sampling is to cease. 
At that point, the system moves to step 163 of FIG. 9 and saves the value 
in the transition counter. 
Thereafter, at step 164, the system again starts a timer and then again 
proceeds to step 162 of FIG. 9A and takes samples in the manner previously 
outlined. 
As soon as the timer again indicates that sampling is to cease, the system 
now returns to step 165 of FIG. 9 and determines the average value of the 
transition counter over the two periods of sampling. That average value is 
then saved at step 166. 
At step 167, the timer is again started and the system then proceeds again 
to step 162 of FIG. 9A and takes samples. Upon completing the sample 
taking as indicated by the timer, the system now returns to step 168 of 
FIG. 9 and determines whether the value in the transition counter is 
sufficiently close to the average value which was calculated at step 165. 
If so, the system moves to step 171 which will be described below. 
If it is determined at step 168 that the value in the transition counter is 
not sufficiently close to the average value previously calculated, the 
system moves to step 169 and increments an odd counter. Then, the system 
proceeds to step 170 to determine whether the odd counter value is greater 
than some predetermined reference value (x). 
If the odd counter is not greater than the predetermined reference value, 
the system moves to step 171 and determines whether the window has yet 
been completed. If not, the system returns to step 167 and proceeds as 
outlined above. If the window is completed, the system moves to step 161 
to begin a new window. 
As soon as the system determines at step 170 that the value in the odd 
counter is greater than the specified reference value such as, for 
example, five, the system moves to step 172. At step 172, the system 
generates some output signal indicating the presence of random, irregular 
electrical signals. 
4. Summary 
From the above discussion, it will be appreciated that the present 
invention provides a dependable voice detection apparatus and method for 
detecting that a called telephone has been answered. Since the answer 
detection device and method of the present invention uses only those 
signals which are normally present on the telephone lines, the present 
invention provides an apparatus and method for detecting that a called 
telephone has been answered which does not require any special action on 
the part of the called party. Moreover, the answer detection device and 
method of the present invention does not require the installation or use 
of any special equipment in connection with the called telephone. 
Importantly, the answer detection device and method of the present 
invention can be used on any telephone system, because it both establishes 
a different reference standard each time it is used and it is designed to 
consider only the regularity of the incoming audio tones and not the 
specific timing or other characteristics of any such tones. 
The invention may be embodied in other specific forms without departing 
from its spirit or essential characteristics. The described embodiments 
are to be considered in all respects only as illustrative and not 
restrictive. The scope of the invention is, therefore, indicated by the 
appended claims, rather than by the foregoing description. All changes 
which come within the meaning and range of equivalency of the claims are 
to be embraced within their scope. 
APPENDIX 
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