DE2117291B2 - Telephone for four-wire connection - Google Patents

Description

Connections A ₂ , B ₂ , C ₂ , D ₂ , E ₂ and F ₂ . Each bridge consists of six branches that are joined together to form a circle. Opposite connections are referred to as diagonal points. The opposite bridge branches are the same forms excluded ^r>, thereby achieving the ground symmetry. The values of the bridge branches are indicated with a, b and c , followed by a 1 in the case of the receiving bridge and a 2 in the case of the sending bridge. Additional trailing digits can be used to avoid confusion.

A receive line with the impedance U is connected to the diagonal points A \ D ₁ and the receiving ι ^r> bridge two isolation capacitors Cl. Similarly, a transmission line with the impedance L? connected to the diagonal points A ₂ and D _{2 of} the transmission bridge, but in this case a battery E _e 2 »is in the telephone exchange via the windings of a differential relay Äy and via a network comprising two resistors Rd and a capacitor Cd for direct current supply of the telephone set to the transmission line connected. In the receiving bridge, a receiver R is connected between the 2> diagonal points C \ and Fi. In the transmitter bridge, a microphone, a microphone amplifier and a simple voltage divider are shown within a dashed box M , the output of the microphone amplifier being connected to the diagonal points C ₂ and jo F ₂ . These components present in box M are simply referred to as a microphone, unless it is necessary to discuss the individual components. The values of the components a, b and c in each bridge together with the impedance R of the listener or the impedance M of the microphone are such that the remaining third pairs of diagonals B, E are conjugated with the third pair of diagonals A, D. These two third pairs of diagonal points are connected to one another either directly -to or, as shown, via two identical apparent resistances q . In Fig. 1, the connection point B \ is connected to the connection point E ₂ , while the connection point B _{2 is connected} to the connection point E \ . Of course, the connection point B \ can also be connected to the connection point B ₂ and the connection point E ₁ to the connection point E ₂ . Since both six-pole bridges are balanced, the receiving line and the sending line are very effectively decoupled from one another. Since the pairs of diagonal points C, Fund B, and E are not assigned to each other, a sidetone current from the microphone via the transmitter bridge and the receiver bridge to the listener R, but because of the assignment of the diagonal point pairs ßi, Ei to the diagonal point pair A \, A, cannot be transmitted to the line L \ flow.

If in each bridge the required value of the input impedance Zoi. and the impedance of the listener or the microphone is given and if there are three values of the apparent resistances a, b and c, then the first value can be chosen so that the bridge is balanced, and a second value can be chosen so that the required input impedance is achieved, and the third value can be selected so that the back listening is determined. The back-listening can also be controlled by the apparent resistances q if the insertion of these apparent resistances is desired.

Before naming typical values for the components in the exemplary embodiment according to the invention, an analysis of the values shown in FIG. The circuit arrangement shown can be made. For this purpose Maxwell's circulating currents are shown in the drawing, for example a current f / i) i flows from the line L \ to the receiving bridge and a current (/ 2) 2 from the line L ₂ to the transmitting bridge. The current through the receiver R or the microphone M is denoted by i ₂ , it being possible for this denomination to be followed by a suitable further number (omitted if not necessary in order to avoid confusion). K is the current flowing between the bridges and / 3 is the circulating current in each bridge. The impedance seen by the connections BE \ n the other bridge is denoted by Q and a number after it, while the impedance seen by these connections BE in its own bridge is denoted by Zqq with another suitable number after it, if necessary explain which bridge is meant.

For the time being, it will suffice to limit the consideration to one of the bridges. For this purpose, the impedance arranged between the connections C and F is denoted by S, where 5 also Äbzw. M can be, depending on whether the receiving bridge or the sending bridge is being considered.

It is assumed that an electromotive force causes ei on the line with the line impedance L. There are four mesh equations (in which, as mentioned, a second digit 1 is placed after the / values):

I ₁₁ (L + α + b + c) - I ₂₁ c —1 ₃₁ (α + b + c) + I ₄₁ α = e _x -'ii ^c + '21CS + 0 + b + c) + I ₃₁ ( a + b + c) -i ^ b = -'11 + hi + 2I ₃₁ -I ₄ I = O

- '11 ^α - hi b - '3i (° + b + c) + in [Q + α + b + c) = It is common to use I to denote the determinant:

I =

L + a + b + c -c

- (a + b + c)

- c

_ J

S + a + b + c a + b + c -b

2 -1

-b -ia + b + c) O + a + b + c

Krs denotes the sub-determinant that is formed by deleting the r-th column and the s-th row. It surrenders

J ₄₁ \ / e, = K ₄₁ = S ', a - (b + c) \ - 2bc. (6)

The prerequisite for a bridge adjustment is that Z _n = 0, ie that

a = b + 2bc / S

(7)

In the following analysis, one has to reckon not only with an electromotive force ei in series with the impedance Z, but also with an electromotive force s in other meshes. The basic mesh equations remain the same as in lines (1) to (4), with the exception of the equation that contains the expression for the electromotive force. If the electromotive force applied is used, for example, in the nth mesh, then the result for the current in the rth mesh, which is _{denoted by / r} ":

= K "eJ I

(8th)

Sib

2bc \\ Q {p

K ₁₁ = 2

K ₁₂ = K ₂₁ = 2 - \ QiS +

/ C ₂₄ = K ₄₂ = 2 - ^ \ L (S + b)

Sib +

2hc \ (9)

(10)

2 /> ci, (11)

K ₁₄ = 2

S + c

\ L (S

S (b

2hc \. (13)

7 _ SIb-

2bc

S {b + c) + 2bc

Ύ + c

(14)

obtained, and c to be dimensioned so that the bridge circuit is matched to the line. The return loss is determined, among other things, by the relative values of Zoo and Q for both bridges. As will be shown later, the insertion loss and the return loss can be matched, with or without the aid of the apparent resistances q inserted in the connections between the two bridges.

The values of the branch resistances b and c can be expressed in terms of the input resistances of the bridges as follows:

20th

If a is eliminated by equation (7), the following expressions result for Δ and for the sub-determinants of Δ:

in

4 «

Input resistance

From equations (8), (9), (10) and (13) it can be seen that

is. It should be noted that for the case c = ύ the relation Zo /. = Zoq - 26 applies.

A normal requirement is that Z ₍ ». = L , neglecting the reactive components of L and the capacitances Ci . It will be shown in the following that the transmission or reception power losses can be calculated very simply by expressing L, S and b Although it is possible to allow small deviations from the value of 5, the impedances of the listener and microphone are usually fixed, so it is desirable to choose b to give an acceptable loss of power η -

- Z _O

I _- Z Og_ + S (Zg ₁ - Zqq)

2Z ₁

(15)

OQ

The other impedance of interest is Q, and it is shown that:

Q ₁ =

; Q ₂ =

'OQl

(16)

Normally L and Srein are ohmic resistances in order to meet the requirements of bridge balancing and impedance matching, in this case the six branch impedances of each bridge are also purely ohmic. In the circuit in FIG. 1, both bridges are provided with capacities in some bridge branches. In the case of the transmission bridge, this is intended to compensate for the drop in the transmission characteristic at high frequencies, as will be explained in more detail below. In the case of the receiving bridge, the capacitances in the branches A \ B \ and Ei Di serve to compensate for the inductance of the receiver R, as far as the bridge balancing is concerned, although the insertion of these capacitances is not always necessary in practice. This fact will now be explained.

Inductance of the listener

If the value 5 in equation (7) is equated with the value s + jwi , then the following applies for the bridge adjustment:

a = b + c + -

2bc

s + j κι I

2bc / s

= b

c + _T -

assuming that

R _x = 2bc / s and C, -

(17)

(18)

Therefore, the effect of the inductance of the listener by connecting a capacitance in parallel with one of the

with resistances of the branches A \ B, and D \ E \ are compensated, as shown in FIG. This inductance of the listener also affects the input impedance of the bridge if b and c are different and, as previously assumed, purely ohmic, but if,

πι as has been explained in connection with equation (14), c - b is made, the input impedance is independent of S. Therefore, the inductance does not affect the value Z,> i. or Z ₁ ^ a. It

it can quickly be shown that for the case that b and c are purely ohmic, the expression b = c is the necessary condition for an input impedance which remains independent of the frequency. However, if the phase angle of the input impedance has the expression (b-c) , in many practical cases the effect of the listener inductance and input impedance will be small. This effect can be neglected, although it may be necessary to take the inductance into account for the bridge balancing.

Performance losses

The maximum power Wor that can be fed in from the line L] is eiVLi, where the

Power source is then adapted to its load resistance. The instantaneous power Wr, which is fed to the listener, is k \ ² R. The insertion loss P / f of the receiving bridge is thus given by

W _n ,

4K ₂₁ ¹ L ₁ R ^(19>

The maximum power Was that is fed into the transmitter bridge by the microphone is similarly C ₂ ² I (AM), the microphone being adapted to its load resistance. The instantaneous power Ws fed to the line L ₂ is / 12 ² L ₂ , and the insertion loss P.y of the transmitter bridge is given by

W _x

4K ₁₂ ² LS

I.
4LS

(21)

This value P becomes a minimum when Z ₍ > i. = L , and then becomes

_ fS + M ² L = \ h] S-

(22)

In order to send a power μ Wo to the line, an electromotive force e2o in M is required, such that

III / • Where = ('

^ J ² V

c ₂₁₁ ² L ₂ .

(24)

The electromotive force fto lets a current (U
flow into the receiving bridge:

('42) 2 =

I /

(25)

υ As far as the receiving bridge is concerned, a current (Ua) \ occurs according to an electromotive force in series with Q \ :

20th

('44). = ('42) 2 · (26)

The resulting current in the handset is
K ₁

^ 24

44

and the listening performance is W _s = (Z ₂₄ )! ² /
Then

K ₁ ^v

< ²⁷ >

Since K \ 2 = K21 , the insertion loss for r, both bridges can be expressed as follows:

or

/i.v

K ₄₂ ) ² RW "

(28)

W _K is given in expressions W " by equation (23). So is

Back listening

In order to be able to specify the desired back loss, let us first consider a nearby and a distant station, each of which alternately transmits signals that reach the distant station with an arbitrary power level Wo. It is assumed that the power Wo fed to the receiving bridge by the line Li results in a power level WR at the receiver and that a power μ Wo is _{supplied to L 2} during transmission, so that a desired power pcgcl Wo arrives at the remote station. If transmission is carried out at this level, for example a back-hearing performance Wv is supplied to the listener. This means that Ws = Wh / s, that is to say that the sidetracking pcgcl is 10 log s db below the level at the listener. thats why

W »W _H

123)

From equations (10) to (13) and (15), the in equation (29) can be reduced to the following form:

I. . "1" T

or

OQl

2 c /

■ OQ \

OQl

HL ₁ L ₂ ). (31)

As has been mentioned before, the branch impedance values b are practically determined by the desired transmission or reception insertion loss in the transmission or reception bridge. In other words, b is given by the values of P. Sund L

is determined in equation (22). Resistance c is then given a value which is dependent on b and 5, and therefore P and L , to provide the required value of Zol , which is normally L. The value of zoo ^'st therefore also a function of insertion loss and of L and S. It follows that, apart from exempted impedances q. the sidetracking performance is determined at the same time by the transmit and receive insertion losses of the two bridges. If π is set for the amount ] / (PS / L) for the sake of convenience, b can be expressed in terms of the insertion loss from equation (22) in the following form:

b / L =

S / L n- 1 '

Then from equations (14) we get

clL =

η-Sj L

and

n + 1 + \) S / L (H Dd + S / L) (H- + \) S / L (n

(n- I) (H- S / L)

(32)

(33)

(34)

(35)

If relations (34) and (35) are introduced into equation (31), where S = R for the case of the receiving bridge, S = Mouth η, = i / (PrR / L \) and for the case of the sending bridge n ₂ = 1 / (PsMZL ₂ ) is set, then the sidetracking factor ßs results directly in terms of the transmit and receive insertion loss and the impedance ratios of the two lines, the microphone, the listener and the exposed impedance q. The resulting, somewhat complicated expression is not given, but a practical example is given below in which the impedances of the two lines of the microphone and the listener are the same.

L ^ = L ₂ = M = R

In the event that L \ = L ₂ =
the equations (32) to (35)

M = R results for

HI

7Γ + Ί

(36)

π + 1

~ ° ⁰ '2 (HI) ■

Equation (31) is reduced to

(37)

whereby

(38)

(39)

Equation (39) gives the value of the hearing factor (Rus). where q is - 0. In Fig. 2 is the set 20 log y in

Expressions of the insertion loss Pr (in decibels) of the receiving bridge for different works of the insertion loss Ps of the sending bridge, although due to the symmetry of equation (39)

^r > does not matter if the transmit and receive insertion losses are interchanged. The ordinate therefore indicates the level of sidetracking below that of the listener when the apparent resistances q are omitted. Here are the signal levels at the input connections

κι of the transmitting and receiving stations the same when receiving (i.e. the transmission level is higher than the Reception level due to the line attenuation). Negative values for the sidetone level mean that the Listening to the level of the signals received at the

r> listener exceeds if there is no additional hearing loss caused by the apparent resistances q. The use of these curves and equations (38) and (39) will be explained by numerical examples.

Examples

It is assumed that the required sidetone level is 15 db below the level of the level that is arriving at the listener Signals and that when sending a line

r> attenuation of 2 db must be taken into account. Then lOlogjtis = 17. It is also assumed that the Reception insertion loss is kept as small as possible, but that a maximum transmission insertion loss of 10 db can be permitted. It

κι can be recognized immediately from the curve Ps = 10 db that the reception losses cannot be reduced by less than about 3.3 db without exceeding the permitted transmission losses. Hence n \ = \, 46 ₂ and n ₂ = 3.162 and q = 0. With these values n \ and n ₂

Ji results from equations (36)

bJL = 2.17; cJL = 0.187;
bi / L = 0.462; C ₂ / L = 0.562.

In order to round off these values to manageable values, because the reduction of b \ and thus the increase of Pr as well as the increase of 62 and thus the reduction of Ps is possible, b \ = 2 and bi = 0.5

^v> can be selected. Hence, n \ = ^ and n ₂ = 3. This

results in a value y = 7 (16.9 db) instead of the required value 7.08 (17 db). It is unlikely that a correction to these Vi values will be necessary in practical cases. If not, the value of q can be found from equation (38) to take account of the additional back loss of 0.1 db, so that q / L = 0.02.
In the example above, the expression is

= - has been made, and then the expression

C ₂ IL same value. It should be noted that if o and c are made the same in both bridges, where η = 3 for both bridges, the value of y = I, which means that in the absence of additional attenuations caused by further apparent resistances q , the sidetone level is the same as large as the received signal level would be. If, on the other hand, the two bridges are similar, but b and c are not the same, then the transmission attenuation cn and the insertion attenuation are the same, and the sidetracking loss falls very quickly as the value of the insertion loss decreases, as indicated by the curve Pi - Pv in F i G. 2

is shown. With an insertion loss of 6 db in both bridges, the available sidetone level is 15.6db.

The following values are given as a further example, with the line resistances and the impedance of the microphone and the receiver are each 300 ohms:

öi -250Ohm; η = 75 ohms and a, = 4500hm;
Iy ₂ = C ₂ = 150Ohm; a ₂ = 4500hm.

These values result in η = 5/3 (4.44 db) and n ₂ = 3 (9.5 db). From Fig. 2 it can be seen that the sidetone level is about 13.2 db below the level of the received signal. A request was made to reduce this level to a value of 15 db. This means that in equation (39) the value y = 4.5 (13.06 db) instead of the desired value of 5.62 (15 db). In order to set the sidetone level, it was possible either to reduce the insertion loss of the receiving bridge by 4 db (and thereby make b \ IL = 1510 ohms without having to change the corresponding other branch resistances) or to insert the apparent resistances q. If the latter is done then the required value of q / L is given by

4q / L = j ■ 2 (5.62-4.5) = y

So <7 = 112 ohms.

1.12.

Compensation for low transmission frequencies

In a particular embodiment of the invention, it was required to extend the transmission range in the transmission direction to lower frequencies than is usual in telephone systems. On the other hand, the level of sidetracking at these low frequencies was not important. It was therefore decided to increase the level of the low -to frequencies transmitted on line L ₂ at the expense of sidetracking. Because of the assignment of the bridge diagonal points AD and BE , this can only be achieved by changing the bridge branch impedances, because the level of the signals sent to line L ₂ is independent of ■)> the impedance between points B ₂ E ₂ . The compensation of the low transmission frequencies is explained in the following by means of an analysis and a numerical example.

The ratio of the voltage applied to line L ₂ to the electromotive force in the microphone is

i'2

hL ■ h) (L + Z ₀₁ )

(40)

Wl

The following number 2 has been omitted because only the transmission bridge is affected and no mix-up of the impedances of the receiving bridge can occur.

The value of Zoi. is given by equation (14), where S = M and c is chosen so that Zoi. = L can be made if all values are purely ohmic. It is assumed that the links L, M, and C over the frequency range considered purely resistive ti ^r> remain, however, that an additional capacitance Cb is connected in series with b, but without changing the value of c. If the value (b + I / jmCh) is divided by b in

equations (40) and (14) is set and if c is eliminated, then equation (40)

1 (3M + 4b) + M ^r + 2 / (M + b) (M + 2b) b <; C _h '

At very low frequencies you get

) ₁ , = A \ speak, =

L (M + 2b)

(42)

L (IM + 4b) + M
and for the upper end of the frequency range

(VIe) ₁₁ = ß ", speak, =

2 (M + b)

τ · < ⁴³ >

If | V / e \ ^{2 is} related to the logarithm of the _Ίΐ frequency, so that log ω = χ , then I V / e \ ² can be expressed in the following form with the aid of equations (41) and (42):

e
whereby

and

= —T- ^ß - + - ~ tanMX, - *), (44)

.v, = log <

LQ M + 4B) + M ² ~ 2 ~ LTm + b) (M + 2b)

(45)

is. The expression on the right-hand side of equation (44) is point-symmetric about the point Ar = x \ and has upper and lower asymptotic values A and B. As previously shown in connection with the back listening, the value of £> in expressions of M / L and the transmission insertion loss Ps . The upper and asymptotic values A and S are thus determined by these two terms M / L and Ps ; they can therefore be chosen within wide limits. The frequency of the deflection point of the curve can also be selected in terms of the capacitance Ct and the quantities M / L and Ps.

When M = L , b can be eliminated in the above expressions by the substitution of Ub = η - 1, where n ² = Ps . Where Pv is the transmission insertion loss of the network when all components are purely ohmic, as in the previous discussion of sidetracking. The following convenient formulas result;

Hj-I

4 π

and

Λ B.

ii + l

+ 1) '
4th

(46)

(47)

The quantity A / B indicates the amount of boost available for lower frequencies.

In the particular embodiment mentioned at the beginning of this section, the measurement of the Switching system including frequency response,

that an increase in low frequencies by 6 db was necessary if the frequency drop point of the tanh compensation curve Dei should be around 160 Hz. The size A / B is then 4, where π = 3 and α> ι LCb = 1. If a> \ = 2π χ 160 and L = 300 Ohm is set, the required value of Cb = 3.33 μΚ The calculated values for the compensation arrangement were found to create the necessary correction of the entire frequency response and so the Justify assumptions made in the above analysis.

In Fig. 1, the bridge branch resistors B ₂ C ₂ and E ₂ F _{2 have} the capacitance C _b in series with the resistor b . In order to obtain bridge equilibrium, it is necessary to insert a capacitance Ca in series with a resistance a in each of the branches A ₂ B ₂ and D ₂ E _2. The required value Ca according to equation (7) is C _b (1 + 7c / L), and using equation (33) results

(48)

In the above-mentioned special example with η = 3, C "= 2Cb results. From Fig. 1 it can be seen that a direct current path for the power supply of the microphone amplifier via the bridge branches A ₂ F ₂ and C ₂ D _{2 is} maintained.

The previous discussion has dealt with low frequency compensation. Although inductivities in the bridge branches have been avoided in the special design, compensation of higher frequencies can generally be achieved if the procedure is analogous to that described for the series connection of the inductance with b and to maintain the bridge equilibrium with a .

It has been shown that the invention meets the strict requirements for the decoupling of the transmission and Receiving line, to the impedance matching and to the bridge earth symmetry met, while at the same time the DC power issues and the compensation of the high and low frequencies in solved an extended transmission frequency range will.

For this purpose 2 sheets of drawings

Claims (8)

Claims: Pole bridges are connected to each other in pairs via two equal resistors (q, gj. 1. Telephone set for 4-wire connection with> a microphone, a handset and a back hearing attenuation circuit, characterized in that the back hearing attenuation circuit has a six-pole end bridge associated with the microphone and a six-pole receiver associated with the listener, with the same apparent resistances arranged in opposite branches of these six-pole bridges are that the transmission bridge with two first diagonal points (Q, F ₂ ) to the microphone (M) and with two second diagonal points (A ₂ , Di) to the transmission line (L ₂ ) and the receiving bridge with two corresponding first diagonal points (Q, Fi) connected to the listener (R) and with two corresponding second diagonal points (A], A) to the receiving line (L ₁ ) as well as the corresponding third diagonal points (Bi, Ei, Bu E \) of these bridges in pairs directly or indirectly with one another are connected and that the values of the apparent resistances in the six branches of each bridge and the apparent >> resistances of the microphone and the listener are chosen such that they are adapted to the predetermined values of the apparent resistances of the transmission line and the reception line. 2. Telephone set according to claim 1, characterized in that circuit elements are arranged in the bridge branches which do not have any Hearing frequencies have effective inductance. 3. Telephone according to claim 2, characterized in that in order to avoid the ^{bridge detuning occurring due to the J r} > inductance of the listener, each branch lying between a second and third diagonal point of diagonally opposite receiving bridge branches from a series connection of two resistors (b \ + C \\ R ₃ ) and a capacitor (Ct) connected in parallel to one (R ₃ ) of these resistors. 4. Telephone according to claim 3, characterized in that the other branches of the Receiving bridge consist of the same, purely ohmic resistors. 5. Telephone according to one of claims 1 to 4, characterized in that each branch lying between a first and second diagonal point of diagonally opposite transmitting bridge branches consists of a resistor (Ci) through which the direct current supply for the microphone flows. 6. Telephone according to claim 5, characterized in that to compensate for the drop v> the transmission characteristic curve at low or high frequencies due to the backing up, the other branches of the receiving bridge each have a series connection of a resistor (a, bi) and a capacitor (C ₃ , Cb ) show. w> 7th Telephone apparatus according to one of Claims 1 to 6, characterized in that the microphone is connected to the input of an amplifier, the output of which is connected to the first diagonal points (Ci, Fi) of the transmitter bridge. <> 5 8. Telephone according to one of claims 1 to 7, characterized in that the each other corresponding third diagonal points of the six The invention relates to a telephone set for Four-wire connection with a microphone, a handset and an anti-hearing circuit. With such telephone sets it is difficult to achieve the required level of back loss, i. H., to ensure a high degree of decoupling between the transmission line and the reception line for voltages that appear on both lines. Further The requirements are that the lines are terminated in a suitable manner on the telephone set must, usually with an ohmic resistance, the value of which is similar to that of the characteristic Impedance of the line leans. It is common to use fork transmitters or other inductive networks Coupling of the lines with the microphone and the receiver and for decoupling the microphone and the To use the earpiece. Circumstances arise where the inductive components of the network are undesirable, if only because of the bulky shape and weight of the transducers. In the present invention, the coupling network or the back hearing attenuation circuit without inductive components are built up, although the use of inductors should not be excluded in general. The invention used for the Back hearing attenuation circuit a bridge circuit that can only be formed with ohmic resistors can. The invention is characterized in that the back hearing attenuation circuit has a six-pole transmitter bridge assigned to the microphone and one to the listener has associated six-pole receiving bridge, with the same apparent resistances being arranged in opposite branches of these six-pole bridges, that the transmission bridge with two first diagonal points to the microphone and with two second diagonal points to the transmission line and the receiving bridge with two corresponding second diagonal points to the Receiving line connected as well as the corresponding third diagonal points of these bridges are directly or indirectly connected in pairs and that the values of the apparent resistances in the six branches of each bridge and the Impedance resistances of the microphone and the receiver are chosen such that they match the specified values the apparent resistances of the transmission line and the receiving line are matched. The invention is explained in more detail using an exemplary embodiment. It shows F i g. 1 shows a circuit arrangement for a telephone set for four-wire connection according to the invention, the two six-pole bridges with their Connections to the microphone, the handset and the lines are shown and 2 shows a family of curves for the sidetone level as a function of the insertion loss. The receiving bridge in FIG. 1 has six connections Ai, Bt, Ci, Di, Ei and F \, while the transmitter bridge has six