Input level supervisory system for level regulator

Disclosed is a system for supervising the level of input applied to a level regulator using a pilot signal. According to the invention, an input level supervisory circuit is provided which utilizes a level adjusting variable element of the level regulator, or employs a particular variable element arranged to be driven by means of the control voltage for said level adjusting variable element simultaneously with the latter element and having characteristics equal thereto, and which has a transmission ratio in a reciprocal or inverse relationship to the alternating current transmission ratio of a main signal alternating current transmission line of the level regulator. The input level supervisory circuit is supplied with an input voltage or current from a direct current constant-voltage power source or a direct current constant-current power source to generate an output voltage by means of which supervision of the level of the pilot signal input to the level regulator is performed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an input level supervisory system in an 
automatic gain controller, and more particularly to a system for 
supervising the level of input applied to a level regulator of carrier 
frequency terminal equipment, which has highly accurate performance and 
can be manufactured at low costs. 
2. Description of the Prior Art 
The level regulator used in carrier frequency terminal equipment of a 
frequency division multiplex communication system employs an electric 
current of a frequency outside the carrier frequency band as a supervisory 
current (pilot signal). Said electric current is transmitted with a 
constant amplitude and variations in the level of said electric current 
are detected to obtain equalization of characteristics deviation, change 
in the temperature, etc., occurring in the transmission line, terminal 
stations, etc. The level regulator changes the value of a variable element 
thereof, such as a thermistor or a field effect transistor (FET), in 
accordance with the level of the electric current thus detected, so as to 
compress the variations in the level of the electric current into smaller 
values, thus regulating the level of the input. 
In addition to said function of automatic level control, the level 
regulator is further provided with a function of supervising the level of 
the pilot signal. The pilot signal level being supervised is utilized for 
exhibiting an alarm function of transmitting a signal for lighting an 
alarm lamp and ringing an alarm bell when the pilot signal level exceeds a 
range that can be regarded as normal, and; for a pilot signal level 
indicating function of constantly indicating and recording the pilot 
signal level through a meter or a recorder for improved maintenance. 
Generally, supervision of the pilot signal level includes supervision of 
the level of input to the level regulator and supervision of the level of 
output therefrom. Of these types of supervision, the output level 
supervision has conventionally been employed. The two types of supervision 
are different in the supervising accuracy of the input level. It goes 
without saying that it is more desirable for higher accuracy to supervise 
the input level than to supervise the output level which has the same 
relation to the input level in a compressed state. Further, the 
International Telegraph and Telephone Consultative Committee recommends 
the input level supervision as a preferable one. In a conventional method 
of this art, the input level supervision uses an electric voltage which 
varies approximately in accordance with a decibel change in the level of 
input to the level regulator, which voltage is available from the control 
voltage for driving the variable element of the level regulator or a like 
voltage, thus supervising a pseudo input level. To be concrete, if there 
is a change in the input level, the change is detected so that the gain is 
controlled by the thermistor or the FET of the level regulator. Since the 
output voltage from the control circuit for driving the thermistor or the 
FET is somewhat mutually related to the input level, said output voltage, 
i.e., control voltage, is directly used to supervise a pseudo input level. 
Further, an improved method of the above-mentioned conventional supervising 
method has been proposed by German Patent P 22 35 230.3, owned by Siemens 
Aktiengesellischaft. According to this method, said control voltage is 
input to a non-linear element of a correcting circuit to produce an 
electric voltage more proportional to the input level, to be used as a 
supervisory output for a pseudo input level. However, this method is not 
sufficiently high in accuracy. This is because, the resulting amount of 
change may be too large (an elongation of the input level) or too small (a 
contraction of the input level) with respect to the change in the input 
level to perform accurate supervision of the input level. 
Further, there is another conventional method, according to which the input 
to the level regulator is branched. A particular narrow-band pilot signal 
filter is provided for extracting the pilot signal, and the pilot signal 
thus extracted is amplified and detected for performing supervision of the 
input level. To be concrete, for the level regulating operation, the 
output is fed back to produce a predetermined electric voltage through a 
control circuit, and said voltage is used to drive the variable element of 
the level regulator to control the gain. While, for the level supervising 
operation, a supervisory circuit is provided separately from said gain 
control loop, in which an input consisting of a voice signal and a pilot 
signal is applied to a pilot signal filter to extract only the pilot 
signal, which is then amplified and rectified to be obtained as a level 
supervisory output. However, since this method requires a particular 
narrow-band crystal filter with sharp attenuation characteristics and a 
particular rectifier circuit, it is disadvantageous with respect to 
manufacturing costs and simplicity in construction. 
SUMMARY OF THE INVENTION 
The present invention has been devised to obviate the aforementioned 
conventional problems, and it is an object of the invention to provide an 
input level supervisory system which is capable of supervising the input 
level with very high accuracy and which is negligibly influenced by 
external disturbances such as variations in temperature and in the 
electric voltage of power supply, and can be constructed of a small number 
of parts and simple circuits, thereby permitting reduction in production 
costs and compacting the construction thereof. 
According to the invention, there is provided a system for supervising the 
level of the input signal applied to a level regulator including a level 
adjusting circuit of a first transfer function having at least one first 
variable element and a terminal for inputting said input signal containing 
a pilot signal, and a control circuit connected to an output terminal of 
said level adjusting circuit for extracting said pilot signal to control 
said first variable element so that the transfer amount of said input 
signal is varied, characterized in that said system comprises an input 
level supervisory circuit of a second transfer function, said supervisory 
circuit comprising an input terminal which is supplied with a constant 
input and at least one second variable element having characteristics 
equal to those of said first variable element and being varied by said 
extracted pilot signal from said control circuit so that the transfer 
amount of said constant input is varied. 
Further, according to the present invention, there is also provided a 
system for supervising the level of an input signal applied to a level 
regulator including a level adjusting circuit for a first transfer 
function having a terminal for inputting said input signal containing a 
pilot signal and a control circuit connected to an output terminal of said 
level adjusting circuit for extracting said pilot signal to control the 
transfer amount of said input signal, characterized in that said system 
has an input level supervisory circuit of a second transfer function, said 
input level supervisory circuit comprising an input terminal which is 
supplied with a constant input, said level adjusting circuit of said first 
transfer function and said input level supervisory circuit of said second 
transfer function provide commonly a variable element, and said variable 
element is varied by said pilot signal extracted from said control circuit 
so that the transfer amount of said constant input is varied. 
Further features and advantages of the present invention will be apparent 
from the ensuing description with reference to the accompanying drawings 
to which, however, the scope of the invention is in no way limited.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates an example of the conventionally proposed systems, which 
appears in German Patent P 22 35 230.3, owned by Siemens 
Aktiengesellschaft. The system of FIG. 1 used an electric voltage varying 
approximately in accordance with the decibel change in the input level 
applied to the level adjusting circuit 1, which voltage is available from 
the control voltage for driving the variable element used in the lever 
adjusting circuit, or a like voltage, thus performing supervision of a 
pseudo input level. More specifically, in FIG. 1, if there is a change in 
the input level e.sub.in, said change is detected so that gain control is 
effected by the thermistor or the FET of the level adjusting circuit 1. 
Since the output voltage from the control circuit 2, for driving the 
thermistor or the FET is somewhat mutually related to the input level, 
this control voltage is directly used as an output V.sub.01 for 
supervision of a pseudo input level. Alternatively, said control voltage 
is applied to a non-linear element of the input level supervisory circuit, 
i.e., the base of transistor TR.sub.1, to produce an electric voltage more 
proportional to the input level e.sub.in as a supervisory output V.sub.02 
for a pseudo input level. That is, although the output from the control 
circuit 2 is input to the base of transistor TR.sub.1, a change in this 
input voltage is not equal to the decibel change in the input level. 
Therefore, this unequalness is compensated for by means of the non-linear 
impedance characteristics of a diode D.sub.1 which is biased by a voltage 
divider consisting of resistors R.sub.3 and R.sub.4 located on the emitter 
side of transistor TR.sub.1. Thus, the input level supervising voltage is 
available by picking out an electric current from the emitter of 
transistor TR.sub.1, which has been made to approximate the decibel change 
in the input level as an electric voltage by means of collector resistor 
R.sub.1. 
FIG. 2(a) shows the characteristic curve of the output voltage V.sub.01 
obtained by the circuit arrangement shown in FIG. 1, with reference to an 
ideal curve. In FIG. 2(a), Curve I represents an ideal curve which is 
theoretically obtained with respect to the input level, and Curve II a 
change in the output voltage level V.sub.01 with respect to the input 
level. The deviation of Curve II from Curve I is calculated to be +1.8 dB 
and -1.6 dB respectively at the input levels of -6 dB and +6 dB of output 
voltage V.sub.01. To reduce the deviation, Siemens Aktiengesellschaft 
employs the circuit 3 in which a diode is used to improve the accuracy in 
the input supervisory voltage with respect to the input level. However, 
such circuit has been designed by first determining Curve II and then 
approximating it to the ideal Curve I, and accordingly has the following 
drawbacks. 
(a) Since the circuit in FIG. 1 is an approximating circuit making use of 
the non-linear impedance of a diode, transistor or FET, there is a limit 
in the accuracy of approximation. That is, there still occurs a deviation 
of about 1 dB in an input level range of from -6 dB to +6 dB. In this 
regard, particularly in carrier frequency terminal equipment, the 
deviation should desirably be kept within .+-.0.3 dB in the input level 
range of from -6 dB to +6 dB, as shown in FIG. 2(b). 
(b) A diode, transistor and FET suffer large fluctuations in performance 
due to temperature fluctuations in the region in which they have 
non-linear impedances, so that there occur fluctuations in the input level 
supervisory voltage due to temperature fluctuations. 
(c) The operating voltage for non-linear elements to achieve good 
approximating characteristics thereof vary with each piece, and 
accordingly, inspection or adjustment of such elements is required piece 
by piece before use. 
Thus, the system in FIG. 1 is not sufficiently high in accuracy. The system 
suffers an excessive amount of change (elongation) or a small amount of 
change (contraction) with respect to a change in the input level, thus 
being unable to perform accurate supervision of the input level. 
Next, the circuit shown in FIG. 3 will be discussed. The circuit in FIG. 3 
includes a narrow-band pilot signal filter 4 for branching an input 
e.sub.in to a level adjusting circuit to extract a pilot signal which is 
then amplified at 5 and detected at 6, thus performing supervision of the 
input level. More specifically, for the level regulating operation, the 
output from the level adjusting circuit 1 is fed back through the control 
circuit 2 to produce a predetermined voltage for driving the variable 
element of the level adjusting circuit 1 to control the gain; whereas for 
the level supervising operation, independently of said gain control loop, 
the input e.sub.in consisting of a voice signal and a pilot signal is 
applied to a pilot signal filter 4 for extracting only the pilot signal. 
The pilot signal thus extracted is then amplified and rectified to be a 
level supervisory output V.sub.03. However, the system of FIG. 3 is 
disadvantageous with respect to manufacturing costs and compactness in 
construction, because it requires use of a particular narrow-band crystal 
filter having sharp attenuation characteristics and a particular rectifier 
circuit. 
Details of the present invention will be described hereinbelow with 
reference to the drawings. 
FIG. 4 is a graph showing the compression characteristics of the level 
regulator, FIG. 5 a block diagram showing the input level supervisory 
circuit, FIGS. 6A, 6B, 6C, 6D and 6E are connection diagrams showing 
concrete examples of the level adjusting circuit and input level 
supervisory circuit of FIG. 5, and FIG. 7 a comparison table showing 
various types of circuits embodying the circuits of FIG. 5. 
In FIG. 5, numeral 11 denotes a level adjusting circuit including a level 
adjusting variable element Rv, 12, a control circuit comprising a pilot 
filter, a rectifier, an amplifier and a discriminator, 13 a supervisory 
circuit which consists of the same level adjusting variable element of the 
level adjusting circuit 11 or another variable element which is arranged 
to be driven simultaneously with said level adjusting variable element and 
has characteristics equal to those of the same element, 14 an amplifier 
which is dispensable and which is adapted to amplify the output from the 
supervisory circuit 13 to a required level, V.sub.s a reference voltage, 
and V.sub.out an input level supervisory output which is to be connected 
to an alarm circuit, a meter or recorder. 
To explain the principle of operation of the invention embodied in FIG. 5, 
the principle of how to regulate the input level is hereby formulized. 
Assuming that when there is a change .DELTA.eN (dB) in the input level, 
the resistance value of the adjusting variable element is Rv at a normal 
input level and the amount of adjustment by the regulating operation is 
.DELTA.Rv, the following relationship is established: 
##EQU1## 
wherein .DELTA..delta.(dB) is the balance of adjustment, symbol .+-. of 
.+-..DELTA.Rv the polarity of the system, and f(Rv) the transmission ratio 
of the level adjusting circuit. 
The above formula (1) represents that the change .DELTA.eN (dB) in the 
input signal level is compensated by the change 20 
log{[f(Rv.+-..DELTA.Rv)]}-20 log f(Rv) in the transmission ratio of the 
level adjusting circuit 11, so that the output consists merely of a change 
in the balance of adjustment .DELTA..delta.(dB). 
Now, assume that a function F(Rv) has the following relation to the 
transmission ratio of f(Rv) of the level adjusting circuit 11 in said 
formula (1): 
##EQU2## 
If said function F(Rv) is substituted into said formula (1), the following 
equation is obtained: 
##EQU3## 
If the function F(Rv) is thus substituted, the right and left members of 
the equation have the same symbol .+-., and also, the change in the 
transmission ratio becomes nearly equal to the change .DELTA.eN in the 
input level. That is, it will be noted that, if a level supervisory 
circuit 13 is provided which has a reciprocal transmission ratio 
F(Rv)=1/f(Rv) as against the transmission ratio f(Rv) of the level 
adjusting circuit 11, the decibel change .DELTA.eN in the input level 
applied to the level adjusting circuit 11 and the change in the 
transmission ratio of the output from the level supervisory circuit 13 are 
equal to each other both in amount of change (dB) and polarity (.+-.), 
except for the balance of the adjustment of the input level 
.DELTA..delta.(dB). 
Although the level supervisory circuit 13 having a transmission ratio as 
expressed by Formula (2) contains as an error a term .DELTA..delta.(dB), 
representing the balance of adjustment as evident from Formula (3), the 
value of said term can be sufficiently reduced to a negligible value by 
suitably setting the compression ratio and range of adjustment of the 
level adjusting circuit 11. Further, said error can be minimized by 
correcting the gain of the amplifier 14 of FIG. 5 according to necessity. 
That is, the compression characteristics of the level adjusting circuit 11 
present a curve as shown in FIG. 4 for instance, according to which, when 
there is a change of +5 dB in the input with respect to the normal level N 
of input e.sub.in, there is only a change of +0.5 dB in the output 
e.sub.out from the level adjusting circuit 1. In this instance, the 
compression ratio of the level adjusting circuits 11, 12 is 10:1, and said 
value of +0.5 dB is the balance of adjustment .DELTA..delta.(dB). Since 
usually the compression ratio is set at a large ratio of from 10:1 to 
20:1, the error can thus be small. If the plus side and the minus side of 
the input level are symmetrical with respect to the normal input level N, 
that is, the balances .DELTA..delta. on the plus and minus side are equal 
to each other, the error can be reduced to zero: that is 
{(.vertline.+.DELTA..delta..vertline.-.vertline.- 
.DELTA..delta..vertline.)/2} by applying a gain correction of 
(.vertline.+.DELTA..delta..vertline.+.vertline.-.DELTA..delta..vertline.)/ 
2 to the amplifier 14 of FIG. 5. However, since in general the range of 
adjustment is applied to changes of .+-.5 dB or +4 dB in the input level, 
the level regulator is operated within a small range of balance of 
adjustment .DELTA..delta. so that practically high accuracy is available. 
Referring to FIGS. 6A, 6B and 6C the description is now directed to 
concrete examples of the level adjusting circuit 11 and level supervisory 
circuit 13 of FIG. 5, in which the feedback amplifier is embodied by an 
operational amplifier. 
FIG. 6A shows an arrangement in which the level adjusting circuit 11 
consists of a non-inverted amplifier, the transmission ratio of which is 
expressed as: 
EQU f(Rv)=1+(Ro/Rv)=(Rv+Ro)/Rv (4) 
where Rv is a resistance value of variable element and Ro a value of fixed 
resistance. 
Incidentally, FIG. 6D shows another example of the circuit having the same 
type of transmission ratio as that of FIG. 6A, but composed of discrete 
transistors, and FIG. 6E a further example of the circuit embodying a 
hybrid feedback amplifier. 
FIG. 6B shows an arrangement embodying the level supervisory circuit 13 
which has a transmission ratio in a reciprocal or inverse relationship to 
the transmission ratio of the level adjusting circuit 11 of FIG. 6A (f(Rv) 
of said Formula (4)). In FIG. 6B, the transmission ratio F(Rv) to the 
output of the input supplied from a constant-voltage power source is 
expressed as: 
EQU F(Rv)=-Rv/(Rv+Ro) (5) 
In this instance, Rv, the resistance value of the variable element of said 
Formula (5), represents either the resistance value of the level adjusting 
variable element itself or the resistance value of another variable 
element, specially provided for level supervision, which is driven 
simultaneously with said level adjusting variable element and has 
characteristics equal to the latter element. Therefore, the level 
supervisory circuit 13 of FIG. 6B has an output voltage expressed as: 
##EQU4## 
Said output voltage is a supervisory voltage which is free of both 
contraction and elongation of the input level with respect to the decibel 
change in the input level, owing to the relationship of said Formula (3). 
As described above, according to the present invention, either a level 
adjusting variable element itself or another particular variable element 
having characteristics equivalent thereto is used to provide a circuit 
which has a transmission ratio in a reciprocal relationship to the 
transmission ratio of the level adjusting circuit with respect to a 
constant direct current power source, thus permitting supervision of the 
input level free of contracting and elongation of said level. 
In addition, the variable element Rv in the level adjusting circuit shown 
in FIG. 6A and the variable element Rv in the level supervisory circuit 
can be commonly used as one variable element. Since the level adjusting 
circuit shown in FIG. 6A produces an alternating current and the level 
supervisory circuit produces a direct control current, the alternating 
current and the direct current can be separated by using a coil. For the 
purpose of using the variable element Rv commonly in the level adjusting 
circuit and the level supervisory circuit, the earth terminals of the 
variable element Rv shown in FIG. 6A and of the variable element Rv shown 
in FIG. 6B are connected as one common terminal, and the other terminals 
of the variable elements are connected via a coil L so that the 
characteristics of the level adjusting circuit and of the level 
supervisory circuit are not affected by the use of the common variable 
element. 
Further, according to the invention, not only the supervisory voltage is 
indicated in decibels, but also the output supplied to the recorder and 
the meter can be indicated in linear representation corresponding to the 
decibel change in the pilot input level. Meters in general have linear 
scales, while recorders should also desirably have linear scales because 
the scales in decibel representation have too small of a space between 
scale lines towards the lower-level ends thereof. Further, linear 
representation is preferable for the operation to obtain a higher degree 
of accuracy of reading. 
The arrangement of FIG. 6C is based upon the same principle of that of FIG. 
6B, in which an admittance transmission ratio containing the transmission 
ratio f(Rv) of the level adjusting circuit 11 is available in a manner 
that an electric current I.sub.D corresponding to the decibel change in 
the input level applied to the level adjusting circuit 11, expressed as: 
##EQU5## 
is obtained and supplied to a logarithmic-to-linear-conversion element (a 
diode in FIG. 6C). The relationship of the current I.sub.D to the voltage 
between the opposed ends of the diode as characteristics of the diode can 
be expressed in the equation: 
##EQU6## 
where I.sub.S is a saturation voltage inherent in the diode. The two 
members of said equation (8) can be expressed in the form of natural 
logarithm as: 
##EQU7## 
where c is a constant. 
As will be evident from the equation (9), if the current flowing through 
the diode varies with coefficient ln, the voltage Vo between the opposed 
ends of the diode varies in a linear manner. That is, an equation 
(q/kT)=(constant) is set up, if the change in temperature is not 
contemplated, and accordingly, Vo.alpha. log I.sub.D is established, 
except the constant term. Since the natural logarithm ln and the common 
logarithm log are in proportion to each other, the decibel change in the 
input level can be supervised in linear representation if the linear 
output voltage Vo is picked up as input level supervisory output 
V.sub.out. 
Since the input level supervisory voltage Vo subjected to 
logarithmic-to-linear conversion is thus in linear form, and its manner of 
increasing is identical to its manner of descreasing in amount and curve 
with respect to the normal or central value thereof, the admittance 
transmission ratio may be composed of either a transmission ratio the same 
as the transmission ratio of the level adjusting circuit 11 or a function 
containing a transmission ratio in a reciprocal relationship to said 
transmission ratio of the circuit 11. By thus utilizing the voltage Vo 
subjected to logarithmic-to-linear conversion, supervision of the input 
level is available through a meter or a recorder which can easily be 
monitored by the operator. 
FIGS. 7A, 7B and 7C show six types of circuits embodying the level 
adjusting circuit 11 and the input supervisory circuit 13 coupled thereto 
shown in FIG. 5. Although the level adjusting circuit itself is usually 
composed of discrete transistors to meet strict requirements of non-linear 
distortion, etc., the circuits of FIG. 7 according to the present 
invention are classified by operational amplifiers. The input level 
supervisory circuit 13 is largely classified into those which produce 
logarithmic outputs and those which produce linear outputs. The 
logarithmic output type is further classified into constant-voltage type 
and constant-current type, as well as into convertion type and 
non-conversion type. It should be noted that a transmission ratio is 
indicated under each circuit diagram. In the type producing logarithmic 
output with respect to the transmission ratio f(Rv) of the level adjusting 
circuit, the transmission ratio of the input level supervisory circuit is 
expressed as a reciprocal transmission ratio 1/f(Rv), while in the type 
producing linear output which is provided with a logarithmic-to-linear 
conversion circuit, the transmission ratio is expressed as an admittance 
transmission ratio f(Rv)/Ro (or 1/(f(Rv).Ro), Ro/f(Rv). 
As described above, according to the system of the present invention, a 
change in the transmission ratio, inclusive of that of a variable element 
controlled by a closed loop, is subjected to voltage conversion or current 
conversion, followed by logarithmic-to-linear conversion to obtain a 
linear voltage as an input supervisory voltage. Thus, the system is 
negligibly influenced by external disturbances such as temperature 
variation and fluctuations in the power supply voltage. Further, by 
employing an operational amplifier, it has become possible to easily 
obtain a reciprocal transmission ratio and an admittance transmission 
ratio from the alternating current transmission ratio of the level 
adjusting circuit, thereby permitting supervision of the input level with 
very high accuracy. Furthermore, the system according to the present 
invention can be constructed of a small number of parts and of simple 
circuits, thus leading to compactness in construction and reduction in 
production costs.