FR2517148A1 - Solid state network for subscriber stations - uses active element to couple transmitter and receiver to telephone lines and non-inductive component for compensation - Google Patents

Abstract

A preferred embodiment comprises a solid state subscriber station network which provides high quality transmission and reception of communication signals on standard telephone lines. A two wire telephone line is coupled through a diode bridge circuit to the receiver and transmitter of the subset. A first active element, such as a transistor is associated with both the transmitter and receiver. A second active element is associated with the receiver. More particularly, the transmitter is coupled across the telephone lines in series with the collector and emitter of an amplifying transmitting transistor. The receiver is coupled across the line through that same transistor in series with a receiving transistor. The base of the receiving transistor is connected to the A.C. component of the line through a side tone controlling network that extends across the line. The receiving transistor obtains signals that appear at both the line and at the emitter of the transmitting transistor. The ratio of these signals controls the gain of the receiving transistor and similarly, the ratio of the transmitted signals controls the side tone.

Description

 Subscriber extensions generally include components for signaling an incoming call, for indicating the busy or idle state of the extension, for directing outgoing calls, and for communicating with other subscriber extensions. Among the components used to facilitate communication between connected subscriber stations, there are inductive elements, such as transformers. These inductive elements, in addition to being expensive, are bulky, especially in comparison with modern solid state and integrated circuits. As the inductive components are not susceptible to hybridization, their presence in a circuit limits the advantages which one could otherwise obtain from miniutarization in the solid state and from hybridization.

 The inductive elements are used to adapt the load of the subscriber station circuits. The charge includes the telephone lines connecting the subscriber stations to the telephone exchange. The characteristics of the lines can vary with variables such as the ambient climatic conditions. Circuits using inductive components are used to adapt the substation to an average line length and to an average ambient state. The inductive components cannot simply be varied for reasons of cost and physical constraints. As a result, the average station is not, in practice, properly adapted to its telephone line.

 The case of the substation which is not correctly adapted and which causes the most difficulties is that where the transmission and reception of communication signals are very attenuated. With the advent of solid state electronics, it has become easy to equip the station with amplifiers to help overcome impedance matching problems. This is obviously an expensive way to fix things, so that the cost-effectiveness of the system quickly deteriorates.

 Inductors are also traditionally used to obtain the four-wire-two-wire conversion necessary to couple the microphone and the earpiece of the set to two-way two-wire lines connecting the set to the telephone exchange.

These elements are necessary to ensure isolation between the signals transmitted and received simultaneously.

 these coupling nouctazces have been eliminated, ccplces gyrtcur or bridge circuits using in practice several active devices have been solid. Such a circuit based on this design is described in US Pat. No. 3,823,272 entitled "Electronic telephone transmission circuit". Another circuit of this kind is described in US Pat. No. 4,071,713 entitled "Telephone speaking circuit". These circuits of the prior art cannot be implemented in series and parallel series configurations where no more is used. of two transistors.

 According to the invention, a microphone is mounted between the wires of a normal telephone line in a serial circuit comprising the microphone, an active transmission device, such as a transistor, and passive components. More particularly, the microphone is mounted between the line wires in series with the collector-emitter circuit of a simple transmission amplifier transistor and of the passive components, thus creating a simple serial circuit spanning the telephone line.

 In a first embodiment, a listener is mounted between the line wires in a circuit comprising the listener, an active reception device, the active transmission device mentioned above and passive coupling components. More particularly, the earpiece is mounted between the line wires in series, at the same time, with the collector-emitter circuit of a simple reception transistor and the collector-emitter circuit of the simple emission transistor, and of passive components of coupling. Simultaneous use of the transmission transistor for the reception function is achieved by connecting the serial transmission circuit mentioned above to the transmitter of the reception transistor, so that the reception circuit is in parallel with the microphone of the circuit transmission series.

 The base of the receiving transistor is alternately coupled to the signal from the telephone line by a passive network which is mounted between the line wires. This basic signal is in phase with the signal from the emitter of the emitting transistor applied to the emitter of the receiving transistor. The ratio of these signals in phase controls the gain of the reception transistor and, similarly, the ratio of the transmitted signals controls the amplitude of the ambient noises.

 In another exemplary embodiment, the earpiece is connected to one of the line wires by a first active device and is connected to the other line wire via the microphone. More particularly, the first active device is a reception transistor mounted as an emitter follower.

 A second active device is mounted between the first line wire and the point common to the microphone and the earpiece. More particularly, the point common to the earpiece and the microphone is connected to the line wires by a transmission transistor creating a series circuit comprising the collector-emitter circuit of the transmission transistor, passive components and the microphone. This serial circuit is connected by the earpiece and passive components to a reception circuit, so that this reception circuit is in parallel on only part of the serial transmission circuit, this part comprising the emission transistor.

 The earpiece is mounted between the emitter of the receive transistor and the emitter of the send transistor. Thus, phase transmission signals are applied to the two terminals of the earpiece, with their relative levels controlled by a voltage divider circuit controlling ambient noise and formed of passive components coupling the microphone to the emission transistor.

The characteristics of the invention mentioned above, as well as others, will appear better on reading the following description of exemplary embodiments, the description being made in relation to the accompanying drawings, among which:
Fig. 1 is a diagram of a subscriber station circuit in solid state connected to a dial numbering system,
Fig. 2 is a diagram of a subscriber station circuit in solid state connected to a multi-frequency signaling system,
fig. 3 is a diagram of a solid state subscriber station circuit coupled with a commercially available integrated electronic numbering system,
Fig. 4 show diagrams illustrating certain output signals from the electronic numbering system, and
Fig. 5 is a diagram of another exemplary embodiment of a solid state subscriber station circuit connected to the dial numbering system of FIG. 1.

 The Fi. 1 is divided by a dashed line. To the left of this line. is the part of a standard subscriber telephone set connected to the telephone system by wires T and R. To the right of this line is the solid-state circuit 12 of the invention.

The wire T is connected to the rest of the telephone circuit by the hook contacts HS-1. As is known, these contacts are open in the normal position until the handset is lifted or any other online switching means "is activated. The pulse contacts of the DP-1 dial are in series with the HS contacts -1 On the DP-1 contacts, a filter is mounted in parallel comprising a CF capacitor in series with the RF resistance.

 The wire R is shown connected to ground by the well-known ringing circuit 13. The wire R is also shown connected to the circuit 12 by the hook contacts HS-2.

In the solid state telephone circuit 12, means are provided to protect the circuit against overcurrents and reverse polarity. More particularly, the wires T and R are in parallel with a device for suppressing overvoltages D1. A rectifier bridge comprising the D-2-D5 diodes is placed immediately after the suppression device. The loop current normally flows from terminal T through the normal components of a subscriber unit, shown at 11, to section 14 of wire T of the two-wire telephone line, then through the rectifier bridge, the- section 16 of line T1, an NPN transistor Q2, a resistor R4, the microphone 16, a section 18 of wire R of the line, then returns via the diode bridge on wire R, section 19 and the hook contacts
HS-2, towards terminal R. Thus, the emission transistor Q2 is in series with the microphone in a series circuit connecting T and R and comprising the collector-emitter circuit of Q2, the connection resistor R4 and the microphone.

 The user of the telephone, speaking into the microphone 17, modulates the loop current, thus applying an alternating current signal to the emitter of the transistor Q2 on the conductor 22. The transistor Q2 operates as a common base amplifier on this current module. The signal amplified at the collector of transistor Q2 is applied to the line wire T by conductors 23 and 16, the diode bridge, conductor 14, the contacts DP-1 and HS-1.

Means are provided for controlling the transmission gain of the transistor 92. More particularly, a series circuit RC 24, comprising the resistor R8 connected to the conductor 22, the capacitor
C7, conductor 27 and conductor 26, is mounted between the emitter and the base of transistor Q2. A resistor R5, mounted between the base and the collector of transistor Q2, determines the operating point of transistor Q2. A resistor in series R4 between the emitter of transistor Q2 and the microphone 17 increases the feedback current.

 Means are provided to compensate for ambient noise in the earpiece 28. More particularly, the PNP transistor Qî and associated circuits fulfill this function, as well as other important functions. The direct current of transistor Q1 and a fraction of the modulated signal are applied to the emitter of transistor Q1 by the emitter of Q2 by a conductor 29. The transmitted signal is applied to the base of transistor Q1 by a circuit which includes the conductor 16, the series circuit comprising the resistor R1, the capacitor Cl, the conductor 31, and the resistors R2 and R3. The base of the transistor Q1 is connected directly to the conductor 31. The transmitted signals applied to the base and to the emitter of the transistor Q1 are in phase. Thus, by adjusting the level of these signals individually, one can adjust the amplitude of the ambient noises or even reduce them almost completely.

 Means are provided to compensate for the effects of changing the line impedance. More particularly, a line impedance compensation circuit 32 is provided. the compensation circuit 32 includes in series the conductor 33, the capacitor C3, the capacitor C4 and the resistor R6 in parallel, and the resistor R7. One side of the transistor C3 is connected to the base of the transistor Q2 by the conductor 26. One side of the capacitor C4 is connected by the resistor R7 to the section 18 of the wire R.

 The circuit thus described is very insensitive to changes in impedance line, given the negative feedback generated by the transistor Q2. We noticed that variations of the order of five times in line impedance did not unbalance the circuit.

 Means are provided for amplifying the signal received from the telephone line. These means include the transistor Q1. The alternating signal received from section 16 passes through resistor R1, capacitor Cl and conductor 31 to the base of transistor Q1. The amplified received signal appears at the collector of transistor Q1 and is applied to the earpiece 28 by the conductor 34, the capacitor C2 and the conductor 36. The other side of the earpiece is connected to the section 18 of the wire R by the conductor 37. The earpiece is shown bridged by the on-off contacts DP-ON on the dial.

 A fraction of the received alternating signal also appears at the base of the transistor Q2, through the resistor R5, this fraction being determined by the divider network comprising the resistor R5 and the circuit 32. In this case, the transistor Q2 operates as a follower transmitter for the signal received and transmits this signal received to the transmitter of transistor Q1 by the conductor 29.

 The received signal is amplified by the transistor Q1, if the signal at the base of the transistor Q1 is greater than the signal applied to its emitter. The gain of transistor Q1 is adjusted by choosing the ratio of the incoming alternating signal directly received from the line by the transistor Q1 and the incoming signal it receives from the transistor Q2.

The coupling capacitor C2 associated with the resistor R3 prevents the DC signals from passing through the earpiece.

 Since the electronic circuit of transistor Q2, described above, functions as a constant current source, it naturally limits the current in the microphone and avoids overloads.

Thus, the varistors normally used to protect the microphone are no longer necessary in the circuit of the invention.

 The level of the transmitted signal is automatically adjusted by the feedback of the circuit described above, for different lengths and loop impedances. The longer the loop, the stronger the transmitted signal. Automatic signal level adjustment equalizes the signal at the entrance to the telephone exchange. You can also increase the gain for the received signal or for the transmitted signal or for both, if necessary, in addition to the automatic line impedance settings just described.

In Fig. 2, the same references have been used to designate the parts common with those of the subscriber station circuit in solid state of FIG. 1. Thus, the subscriber station of FIG. 2 uses the main components of that of Fige 1, except that that of
Fig. 2 is a numbering station with multifrequency signaling, instead of dialing. For example, the line input circuit 11 is exactly the same as that of FIG.

1, except that the dial contacts are deleted on wire T.

 A simplified diagram of a standard multi-frequency keyboard circuit 30 is shown in FIG. 2. The microphone 17 is shown connected by the keyboard circuit 30 to the emitter of the transistor Q2. More particularly, a path of the wire section 18 is provided by the microphone 17, the resistor R4, the conductor 20, common contacts 33, the conductor 35 and the conductor 29, towards the emitter of the transistor Q2. When a number key is pressed, the common contacts all work and contact 33 opens while contact 33a closes.

 Contact 33a then couples an oscillator to transistor Q2.

This oscillator includes a PNP transistor Q3 and its circuits.

 The collector of the transistor Q3 is connected to the line section 18 by the conductor 38, the conductor 39 and the resistor R3. The emitter of transistor Q3 is also connected to conductor 38 by a series circuit formed by the Roll resistor, the inductors L3 and L4, the varistor diode D8 and the resistor R10. The base of the transistor Q3 is connected, at the same time, to the point common to the diode D8 and to the resistor R10 by the inductors L5 and L6 in series, and to the collector of Q3 by a capacitor C11.

 The diode rectifier bridge of the telephone line is connected, in the keyboard circuit, by the conductor 41, the common contacts 51, normally closed, of the keyboard circuit 30, the inductors L1 and L2 in series, and the conductor 42, to the line section 18. A varistor D7 is mounted between the conductor 41 and the conductor 42 to shunt the inductors L1 and L2, when the contacts 51 are closed.

 The earpiece 28 is connected, through the keyboard circuit 30, to the reception amplifier transistor Q1. More particularly, a circuit goes from the section 18, through the conductor 37, the earpiece 28, the conductor 36, the common contacts 32, the conductor 40 and the capacitor C2, to the collector of the transistor Q1. The common contacts 32 are bridged by a resistor R9 which serves to leave the earpiece silent during the emission of the multifrequency dial tone tones. A headphone clamping diode D6 connects the conductor 40 to the conductor 18.

 Thus, the electronic circuit easily adapts with multi-frequency numbering systems. The transistor Q2 is used to improve the output of the oscillator as well as to perform the functions already described relating to the transmission, reception and control of ambient noise. The advantage of this light, inexpensive, affordable and compensated system is that, among other things, it can work with both dial-up and dial phones.

 There are now commercially available integrated numbering circuits, such as the Motorola M 14409. The electronic telephone set described here can also be used with such numberers.

Fig. 3 shows the interface between a numbering circuit
Motorola MC 14409 and the solid state station circuit of FIG. 1.

When possible, the components of Fig. 3 have the same reference numbers as those in FIG. 1.

 The electronic numbering circuit is represented in X1.

 I1 is shown connected to the positive voltage V + at 46. The line sections 16 and 18 are shown terminated by arrows' leading to the diode bridge already shown. The negative voltage terminal V- of X1 is connected to section 18. It should be understood that the positive and negative signals are only examples. The voltages could be reversed using PNP transistors instead of NPN transistors and vice versa, for example.

 Means are provided to leave the earpiece silent during dialing. More particularly, the point common to the resistor R2, to the capacitor C1 and to the base of the transistor Q1 is connected by a resistor R21 to the silence terminal 51 of the electronic numbering circuit.

 Means for amplifying the numbering pulses are provided. No longer, firstly, the collector of transistor Q2 - is connected directly to the line section 16. The base of transistor Q2 is connected by resistor R5 to the terminal for numbering pulses DP 52 of the electronic numbering circuit X1. The base of the transistor Q2 is also connected to the emitter of Q2 by the series feedback circuit of the capacitor C7 and the resistor R8. The microphone 17 is connected to the emitter of the transistor Q2 by the resistor T4, forming a series circuit which connects the line sections 16 and 18.

 The emitter of transistor Q2 is connected to the emitter of transistor Q1 by resistor R19. The common point of the resistor R19 and the emitter of the transistor Q1 is connected to the conductor 18 by the resistor R10 and the supply capacitor C8 in parallel. The supply capacitor is of a suitable size for supplying energy, but still allows feedback.

 Fig. 4 shows a practical time diagram when the number 3 was dialed and showing the dialing pulse lengths and the silence signal lengths.

 The difference between the operation of the circuit in FIG. 3 and that of the electronic circuits of FIGS. 1 and 2 stems from the fact that transistor Q2 is used for switching the dialing signals, as well as for controlling ambient noise. The low level pulses from the DP terminal switches the transistor Q2 to the blocked state by the resistor R5. The transistor Q2 is normally biased in the unlocked state by the high level voltage applied to the output terminal DP.

 In the circuit, the transistor Q2 operates as a voltage follower with the operating point loop current in the telephone line and in the microphone, determined by the level of the voltage at the output of the DP terminal.

 In addition to its reception amplifier function, the transistor Q1 acts to make the earpiece silent during dialing.

The PNP transistor Q1 is blocked by the high level applied to its base from the silence terminal 51 of the numbering circuit X1, through the resistor R21.

 The diagram in FIG. 5 is that of another exemplary embodiment of the present invention, which comprises a subscriber station circuit in solid state, using the numbering system with the rotary dial of FIG. 1. Thus, FIG. 5 shows a variant of part 12 of the circuit of FIG. 1.

 The wire T is identified by the reference numeral 14 and the wire R by the reference numeral 19, as in FIG. 1. In circuit 12, means are provided to protect the circuit against overcurrents. As shown, the wires T and R are shunted by the transient voltage suppressor D1. The rectifier bridge includes the diodes D2, D3, D4 and D5 and is used to maintain the section 16 of T at a positive potential with respect to the section 18 of R.

Means are provided for transmitting signals over the telephone line in a manner similar to that of the circuit of the
Fig. 1. The loop current passing through the series circuit comprising the conductor 73, the emitter-collector circuit of the transistor Q2, the conductor 74, the resistor R36, the conductor 64, the microphone 17 and the conductor 66, is modulated as a function of the audio input signal from the microphone, thereby applying an alternating signal to the emitter of transistor Q2. The amplified signal at the collector of transistor Q2 is applied to the telephone line, through conductors 73 and 16, the diode bridge and conductor 14.

The quiescent point of the NPN transistor Q2 serving as a transmission amplifier is determined by the resistors R34, R37 and the Zener diode D18. The resistor R34 is connected to the section 16 of the wire T and is in series with the circuit formed by the diode D18 in parallel with the conductors 71, 72 and the capacitor C14. The Zener diode
D18 and the capacitor C14 are connected to the section 18 of the wire R. The node between the resistor R34 and the diode D18 is connected to the base of the transmission transistor Q2, by the resistor R37 and the conductor 76. The capacitor C14 is used to reject any alternating current which could develop at the terminals of the Zener diode, when it is not fully saturated.

 The alternating gain of the transistor Q2 is determined by the resistors R34, R37 and the series circuit formed by the resistor R49 and the capacitor C16 mounted between the conductors 76 and 16. In addition, a capacitor C17 is mounted between the base of the transistor Q2 and section 16 to operate as a suppressor of high frequency signals.

 Means are provided for amplifying the signals received on the lines to the subscriber station. More particularly, a first NPN transistor Q1 is shown mounted in emitter follower mode. The base of the transistor Q1 is connected, by a capacitor C1 in series with a resistor R1, to the section 16 of the wire T. The collector of the transistor Q1 is directly connected to the section 16. The emitter of the transistor Q1 is only connected to the series transmission circuit, described above, at the node between the resistor R36 and the microphone 17, by the conductor 63 and the circuit formed by the resistor R31 in parallel with the conductor 61, a capacitor C2n, an earpiece 28 and a conductor 62. In addition, a varistor diode D17 is mounted in parallel on the earphone 28. The transmitter of Q1 is thus coupled in alternation with the earphone and receives its DC load by the resistor R31 and the transmission circuit already mentioned.

 A continuous polarization is applied to the base of the transistor Q1 by a circuit comprising a conductor 67, a resistor R32, conductors 68, 69, 71, a resistor R34 and the section 16 of the wire T.

 Means are provided for controlling ambient noise in the earpiece from the signals applied to the microphone. in particular, a resistor 38 is mounted in parallel with the series circuit formed by the resistor R36, the conductor 64 and the microphone 17.

Thus, resistors R36 and R38 form a voltage divider, R38 bypassing part of the modulated line current mentioned above.

The microphone output is directly connected to the emitter of the transistor Q1, by the resistor R31 and the conductor 63, while the divided signal, amplified by the transistor Q2 is applied directly to the collector of the transistor Qi and, by the resistor R1 and the capacitor C1, at its base. So the direct and split signals are.

phase. Thus, the listener 28 is excited by the differences, if any, of two phase signals. The ratio of the values of resistors R36 and R38 controls the relative level of the ambient noise brought back to the earpiece.

 It will be understood that the circuit of FIG. 5 can be modified in order to be able to include it in a multi-frequency numbering system or systems with integrated electronic numbering circuits, in the same way as the circuits of FIGS. 2 and 3 were obtained by modifying that of FIG. 1.

The practical values of the components of the circuit of FIG. 1 are, for example:
R1 = 22 kilohms R6 = 13 kilohms C3 = 0.22 microfarads
R2 = 22 kilohms R7 = 10 kilohms C4 = 0.22 microfarads
R3 = 390 ohms R8 = 100 ohms C7 = 10 microfarads
R4 = 100 ohms Q1 = 2 N 5401 C1 = 0.22 microfarads
R5 = 22 kilohms Q2 = 2 N 3053 C2 = 10 microfarads
Elderly, the electronic subscriber telephone station circuit fulfills the functions of stations using "iron" and Larz inductors, in a more efficient manner and may be capable of hybridization and usable in numbered systems. dial, or multi-frequency and with commercially available integrated numbering circuits.

 In addition, the electronic station described is basically a constant current device. So there is less need to compensate for current variations. For example, there is no need for varistor protection of the earphone and microphone elements, which incidentally is a factor which plays in favor of the hybridization of the circuits described.

 The constant current characteristic makes the substation less sensitive to variations in line length. and more independent of the input impedance, in general. This feature allows for more solid control of ambient noise.

 Another important characteristic of the station, considering in particular the problems of energy supply and cost, is that the electronic circuit uses significantly less energy than standard stations. For example, in the case of short loops, a standard station consumes up to 100 mA, while 20 mA is sufficient with the electronic station described. Thus, large-scale use of electronic stations would bring great savings in energy and energy costs.

 Another intrinsic and important characteristic of the electronic station is that with active devices, one can have gain and control the two signals, received and transmitted. Thus, one can easily cancel any loss due to balancing in the electronic post.

Claims (5)

 1) Telephone subscriber station circuit, comprising at least two line wires with a DC potential difference between them, the two wires being intended to connect a subscriber station to other subscriber stations, comprising means telephone receiver for converting communication signals received from said telephone line into suitable output signals, telephone transmitter means for converting input signals into suitable communication signals to be transmitted over said telephone line, characterized in that it further comprises a first solid state electronic means (Q2) coupling the transmitting means (17) to said telephone line (T, R? to transmit the communication signals on said telephone line while automatically compensating for losses thereon, said first solid state electronic means (Q2) being connected in series with said transmitter means (17) by a first coupling means (R4), said serial circuit of tr ansmission resulting being connected to said telephone line (T, R) and bridging it, a line impedance compensating means (C3, C4, R6, R7,) being connected to said first solid state electronic means (Q2) to ensure that the level of said communication signal is substantially constant, regardless of the length of the line.  2) Circuit according to claim l, characterized in that said first solid state electronic means comprises an emission transistor (Q2) for amplifying the communication signals generated by the transmitting means (17), which is connected by said first means coupling (R4) in series with the emitter-collector circuit of said transmitter transistor (Q2), and in that said line impedance compensation means comprises a voltage divider (R5, C3, C4, R6, R7) mounted between the line wires, said voltage divider comprising a first resistor in series (R5) with a line impedance compensation circuit (C3, C4, R6, R7), the base of said transmitter transistor (Q2) being connected to the point common to the first resistor (R5) and to the line impedance compensation circuit (C3, C4, R6, R7) with another means (R8, C7) for controlling the gain of said transmitter transistor (Q2).  3) Circuit according to claim 1 or 2, characterized in that it comprises a second solid state electronic means (Q1) r-cupling said receiver means to the line and which operates in association with said first solid state electronic means ( Q2) or to control the rough environment through said receiving means (28, and to amplify the signal received from the line and applied to said receiving means; 2pu.  43 Circuit according to claim 3, characterized in that said first solid state electronic means (Q2) couples, at the same time, said transmitter (17) and receiver means to said telephone line (T, R) said second electronic state means solid comprising a receiver transistor (Q2) whose base is connected alternately to said telephone line (T), whose transmitter is connected to that of said transmitter transistor (Q2), and whose collector is connected alternately to said receiving means ( 28) so that said receiving transistor (Q1) receives first signals through said transmitting transistor (Q2) and second signals at its base, the ratio and phase between the first and second signals determining the amplitude of the noise atmosphere and the received signal.  5) Circuit according to claim 3 or 4, characterized in that said second solid state electronic means (Q1) couples, at the same time, said transmitter (17) and receiver (28) means to said telephone line (T, R) , and is connected to said first solid state electronic means (Q2) by a second coupling means (28) and is connected in series with both a third coupling means (C2) and said receiving means (28) for forming a serial receiver circuit, said first solid state electronic means (Q2) being connected in series with said first coupling means (R4) to form a serial transmitter circuit, the receiver and serial transmitter circuits being connected in parallel, but in series with said transmitting means (17), the resulting serial circuit being connected to the line and bridging it, said ambient noise being controlled by the use of phase control signals obtained from said telephone line and from the first state electronic means solid (Q2).  6) Circuit according to one of claims 1 to 5, characterized in that it comprises only non-inductive components for the compensation and control of ambient noise.