LOUDSPEAKING TELEPHONE WITH A FREQUENCY SHIFTING CIRCUIT

Abstract: A loudspeaking telephone includes first level control means (4) positioned on a transmitting path, second level control means positioned on a receiving path (11), frequency shifting means provided on at least one of the trans­ mitting (3) and receiving (12) paths for shifting frequencies of at least one of a transmitting path signal and frequencies of a receiving path signal, and level comparator means (6) for comparing the levels of the transmitting path signal and the receiving path signal to control the first and second level control means.

Description: The present invention relates to a loudspeaking telephone for hands-free communication and, more particularly, to a voice-switched loudspeaking telephone which controls the gain of a transmitting and receiving paths by comparing the level of a transmitting path signal (a signal to be transmitted on a transmitting path) and that of a receiving path signal (a signal received on a receiving path). In a telephone of the type described, a seconding voice inputted through a microphone is amplified by a microphone amplifier and, then, applied to a variable loss circuit (variolosser) to be controlled in level. The output of the variolosser is fed out to, for example, a subscriber line via an output amplifier and a hybrid. On the other hand, a receiving speech signal from the subscriber line is routed through the hybrid and an input amplifier and, then, controlled in level by a variolosser which is independent of the previously mentioned one. The output of this variolosser is amplified by a speaker amplifier, and, then outputted through a loudspeaker as a receiving voice. The input to the variolosser on the input side and the output from the variolosser on the output side, i.e., the outgoing and incoming signals are compared by a level comparator with respect to level. Based on the result of comparison, the level comparator controls the amount of insertion loss to be effected by the variolossers which are respectively located on the input side and the output side. The amounts of insertion loss mentioned should be large enough to prevent "Singing", as described hereunder. In the above example, a balance network is installed for the impedance balancing of the hybrid. In practice, however, a certain amount of runaround from the transmitting path to the receiving path, i.e., sidetone coupling occurs. In addition, received voice which is outputted through the loudspeaker is coupled to the microphone, an occurrence generally referred to as acoustic coupling. It follows that, when the amount of insertion loss by the variolosser on the transmitting and receiving paths is reduced, the probability that the gain of the loop which extends from the microphone back to the microphone via the hybrid and the speaker becomes greater than 1 is increased, causing singing. To prevent such singing, the gain on one or both of the transmitting and receiving paths is made lower than those which are particular to the each path. The amount of decrease in gain, i.e., that of insertion loss in the variolosser depends upon the amount of acoustic coupling and that of sidetone coupling. Generally, the amount of acoustic coupling involves substantial peak dips with respect to frequency characteristic and noticeably fluctuates depending upon the environment in which the telephone is situated. Hence, the insertion loss should be great enough to guarantee stable operation while preventing the singing. This prevents two-way communication from being held smoothly. With respect to the details of the above-described telephone, reference may be made to the following literatures: (1) The 4A Speakerphone-Hands-Down Winner, Bell Laboratories Record, pp. 233-237, Sept. 1973 (2) Functional Design of a Voice-Switched Speakerphone, The Bell System Technical Journal, pp. 649-668, May 1961 (3) Fundamental Considerations in the Design of a Voice-Switched Speakerphone, The Bell System Technical Journal, pp. 265-293, March 1960 (4) Loudspeaking Telephone Circuit, U.S.P. 3,952,166. In German Patent Application No. 2,203,536, which was published on July 26 1973, there is proposed a loudspeaking telephone with a frequency shift circuit in the receiving path only. An object of the present invention is the provision of a loudspeaking telephone capable of preventing singing without resorting to a great insertion loss and, therefore, promoting smooth two-way communication. This object is achieved with a telephone according to Claims 1 and 8. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: Fig. 1 is a schematic block diagram showing a loudspeaking telephone embodying the present invention; Fig. 2 is a block diagram schematically showing a specific construction of a frequency shifting circuit which is included in the loudspeaking telephone of Fig. 1; Fig. 3 is a schematic block diagram showing another embodiment of the present invention; Figs. 4 and 5 are schematic block diagrams each showing a different specific example of a frequency shifting circuit of Fig. 3; and Fig. 6 is a graph showing a relationship between shifted frequency and reduction in equivalent insertion loss by actual measurement. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In Fig. 1, a loudspeaker telephone is of a voice-switched type. As shown, a sending voice inputted through a microphone 1 is amplified by a microphone amplifier 2 and, then, shifted its frequency by a frequency shifting circuit 3 to frequencies which are higher (or lower) by several tens to several hundreds of hertz. The output of the frequency shifting circuit 3 is applied to a variolosser (level control circuit) 4 to be thereby controlled its level to a level which is needed for a voice-switched operation. The controlled output of the variolosser 4 is routed through an output amplifier 5 to a hybrid 8 and, therefrom, to a subscriber line. A balance network 7 is provided to balance the impedance of the hybrid 8. On the other hand, a receiving speech signal from the subscriber line is amplified by an input amplifier 13 and, then, shifted its frequency to frequencies which are higher (or lower) by several tens to several hundreds of hertz by a frequency shifting circuit 12 as the sending voice. The output of the frequency shifting circuit 12 is controlled by a variolosser 11 to a level which is needed for the voice-switched operation, then amplified by a speaker amplifier 10, and then outputted through a speaker 9 as a receiving voice. The variolosser 11, like the circuit 4, may be replaced by an amplifier. Fig. 6 shows a relationship between the amount of frequency shift and the reduction in equivalent insertion loss on one of the transmitting and receiving paths by actual measurement. As shown, when the amount of frequency shift is, for example, 100 Hz, the insertion loss can be 7 to 8 dB smaller than when the amount of frequency shift is zero. The output of the microphone amplifier 2 and that of the variolossor 11 are fed to a level comparator 6 which controls the amount of insertion loss in the variolossor 4 and 11 which are associated with the transmitting and receiving paths, respectively. As discussed with reference to Fig. 6, the amount of insertion loss by the variolossor 4 and 11 can be reduced by a shift of frequency, compared to a prior art voice-switched telephone. This enhances the performance of two-way communication. As regards the amount of frequency shift, several tens of hertz is little unnatural to the auditory sense. Further, even a shift by more than 100 Hz hardly affects perception. No doubt, the effect of the present invention remains the same even if one of the frequency shifting circuits 3 and 12 shown in Fig. 1 is omitted. Referring to Fig. 2, a specific construction of any of the frequency shifting circuits 3 and 12 is shown. As shown, a transmitting or receiving path signal from the amplifier 2 or 13 is applied to a low-pass filter 14 having a cut-off frequency of 3.4 kHz. The band-limited output of the low-pass filter 14 and that of an oscillator 19 are fed to a balanced modulator 15. The balanced modulator 15, therefore, subjects the output frequency f₂ of the oscillator 19, e.g., 3.5 kHz to balanced modulation. The output of the balanced modulator 15 is routed through a low-pass filter 16 to another balanced modulator 17. This balanced modulator 17 modulates an output frequency f₂ of another oscillator 20, e.g., 3.6 kHz with the output of low-pass filter 16. The output of the balanced modulator 17 is applied to a low-pass filter 18 resulting that a signal whose frequency has been.shifted by 100 Hz, which is the difference between the frequencies f₁ and f₂, appears on an output terminal of the low-pass filter 18. Fig. 3 shows another embodiment of the present invention in a schematic block diagram. In this particular embodiment, not only the variolossor 4 and 11 but also the frequency shifting circuits 3A and 12A are controlled by an output of the level comparator 6. In detail, the sending voice from the microphone 1 is amplified by the microphone amplifier 2 whose output, i.e., a transmitting path signal is compared in level with a signal on the receiving path by the level comparator 6. Assume that the transmitting path signal is higher in level than the receiving path signal. In such a transmitting condition, the transmitting path signal is not shifted in frequency by the frequency shifting circuit 3 while, at the same time, the receiving path signal is shifted its frequency by the frequency shifting circuit 12A to frequencies which are higher (or lower) by a certain amount lying between several tens and several hundreds of hertz. Conversely, when the receiving path signal is higher in level than the transmitting path signal, i.e., in a receiving condition, the transmitting path signal is shifted its frequency by the frequency shifting circuit 3A to frequencies which are higher (or lower) by a certain amount lying between several tens and several hundreds of hertz, and the receiving path signal is not shifted. Subsequently, the transmitting path signal is controlled by the variolossor 4 to a level which is needed for the voice-switched operation and, then, sent over the subscriber line via the output amplifier 5 and hybrid 8. The impedance of the hybrid 8 is balanced by the balance network 7, as stated earlier. The receiving speech signal from the subscriber line is applied to the frequency shifting circuit 12A by way of the input amplifier 13. The frequency of the receiving path signal is shifted by the frequency shifting circuit 12A in a transmitting condition and not shifted in a receiving condition, as previously described. Thereafter, the output level of the circuit 12A is controlled by the variolossor 11 to a level which is needed for the voice-switched operation, then amplified by the speaker amplifier 10, and then outputted through the speaker 9. The frequency shifting circuit 3A and 12A and the variolossor 4 and 11 are each controlled by an output of the level comparator 6. It is to be noted that the frequency shift does not affect communications at all because it occurs only on the receiving path in a transmitting condition and only on the transmitting path in a receiving condition. Referring to Fig. 4, a specific construction of any of the frequency shifting circuits 3A and 12A is shown. As shown, the frequency shifting circuit 3A or 12A includes a crystal oscillator which oscillates at a frequency of 4.096 MHZ. The output of the crystal oscillator 41 is fed to a 16 frequency divider 42 and a 4 frequency divider 45. The output of the 16 frequency divider 42 is routed to a 5 frequency divider 44 via a 15 frequency divider, a frequency of 3.4 kHz appearing on an output of the frequency divider 44. The 4 frequency divider 45 divides the input frequency, i.e., 4.096 MHz to produce a frequency of 1024 kHz. The output of the 4 frequency divider 45 is applied to an N variable frequency divider 46 to be divided into a frequency which is shifted by several tens to several hundreds of hertz relative to 3.4 kHz. When it is desired to shift the frequency by 100 Hz, N is selected to be 292 and the output is 3.5 kHz. If N is 300, the output is 3.4 kHz, meaning that the frequency is not shifted at all. In a transmitting condition, for example, the level comparator 6 controls N of the N variable frequency divider 46 to "292" on the receiving path and to "300" on the transmitting path. That is, in a transmitting condition, the level comparator 6 does not shift the transmitting path signal and does shift the receiving path signal by 100 Hz. The signal outputted by the microphone amplifier 2 is limited to frequencies below 3 kHz by a low-pass filter 47 and, then, modulated by a balanced modulator 48 at 3.4 kHz first. The output of the balanced modulator 48 is applied to a low-pass filter 49 so that frequencies thereof above 3 kHz is cut off. The output of the low-pass filter 49 is in turn fed to a balanced modulator 50 which modulates a signal of 3.5 kHz with the input thereof. The output frequency of the balanced modulator 50 above 3 kHz is cut off by a low-pass filter 51 resulting that a signal which has been shifted to 100 Hz higher frequencies than the receiving voice is produced. Referring to Fig. 5, another specific construction of the frequency shifting circuit 3A or 12A is shown. As shown, the frequency shift circuit includes a voltage controlled oscillator 52. The voltage controlled oscillator 52 is controlled by the level comparator 6 such that in transmitting condition it oscillates 3.5 kHz on the receiving path and 3.4 kHz on the transmitting path. This frequency relationship will be inverted in a receiving condition. The circuitry shown in Fig. 3 may be modified as follows. Assume that the variolossor 11 which serves as a volume is so controlled in a receiving condition as to increase the volume by 10 dB. Then, the level comparator 6 maintains the amount of insertion loss of the variolossor 4 the same while increasing (or decreasing) the frequency shift as effected by the circuit 3A by an amount which is sufficient to reduce the loss by 10 dB. In summary, in accordance with the present invention, a frequency shifting circuit is installed on each of a transmitting and receiving paths so as to shift a signal inputted to the associated path by several tens to several hundreds of hertz. Consequently, while a signal is routed through a loop which is defined by the transmitting path, sidetone coupling, receiving path and acoustic coupling in this order, the frequency is brought out of the speech bandwidth and cut off by a low-pass filter. This successfully prevents singing and the like on the loop. The frequency shift does not affect a voice signal at all because it occurs only on the receiving path in a transmitting condition and only on the transmitting path in a receiving condition. With the above unprecedented advantages, the present invention realizes simultaneous two-way communication which is superior to those heretofore attained. It is to be noted that the present invention is applicable to a mobile radiotelephone without any change or modification except for the replacement of, for example, the subscriber line shown in Fig. 3 with a transmitting and receiving section.

Claims: A loudspeaking telephone, including first level control means (11) positioned on a transmitting path, second level control means (4) positioned on a receiving path and a level comparator (6) for comparing the levels of the transmitting path signal and the receiving path signal to control the first and the second level control means (11, 4) characterised in that the telephone further includes frequency shifting means (3A, 12A) provided on at least one of the transmitting and receiving paths for shifting the frequencies of at least one of a transmitting path signal and a receiving path signal, and in that the amount of frequency shift effected by the frequency shifting means (3A, 12A) is controlled by the output of the level comparator (6). A loudspeaking telephone as claimed in claim 1, wherein the level control means (4, 11) includes a variolosser. A loudspeaking telephone as claimed in either claim 1 or claim 2, wherein the amount of frequency shift is in a range between several tens and several hundreds of Hertz. A loudspeaking telephone as claimed in any one of the preceding claims, wherein the frequency shifting means (3A, 2A), includes a first and a second frequency shifting circuit (12A) (3A) which are provided on, respectively, the transmitting and the receiving path in order individually to produce frequencies which are shifted by a control signal, the level comparator (6) producing, by comparing the transmitting and the receiving path signals with respect to level, the control signal which causes the second frequency shifting circuit (3A) to shift the frequency if the transmitting path signal is at a level higher than the receiving path signal and causes the first frequency shifting circuit (12A) to shift the frequency if the receiving path signal is at a higher level than the transmitting path signal. A loudspeaking telephone as claimed in claim 4, wherein each of the first and second frequency shifting circuits includes an oscillator (4) oscillating at a predetermined frequency, a first filter (47) for limiting the transmitting and the receiving path signals with respect to frequency bandwidth, a first frequency divider (42,43,44) for dividing the predetermined frequency from the oscillator (4) by a predetermined number to produce an output, a first balanced modulator (48) for balance-modulating the output of the first filter by the output of the first frequency divider, a second filter (49) for limiting the output of the first balanced modulator (48) with respect to the frequency band to produce an output, a variable frequency divider (46) for dividing the predetermined frequency from said oscillator (4) based on the control signal output from the level comparator, a second balanced modulator (50) for balance-modulating the output of the second filter (49) by the output of the variable frequency divider, and a third filter (51) for limiting the output of the second balanced modulator (50) with respect of the frequency bandwidth to output the speech signals. A loudspeaking telephone as claimed in claim 4, wherein each of the first and second frequency shifting circuits (12A) (3A) include an oscillator (41) oscillating at a predetermined frequency, a first filter (47) for limiting the transmitting and the receiving path signals with respect to the frequency bandwidth, a first frequency divider (42,43,44) for dividing the predetermined frequency from the oscillator by a predetermined number to produce an output, a first balanced modulator (48) for balance-modulating the output of the first filter (47) by the output of the first frequency divider (42,43,44), a second filter (49) for limiting the output of the first balanced modulator with respect to bandwidth to produce an output, a voltage controlled oscillator (52) for oscillating at a frequency which is associated with the output control signal from the level comparator (6), a second balanced modulator (50) for balance-modulating the output of the second filter by the output of the voltage controlled oscillator (52), and a third filter (51) for limiting the output of the second balanced modulator (50) with respect to the frequency bandwidth to produce a frequency-shifted signal of the transmitting and receiving path signals. A loudspeaking telephone as claimed in any one of the preceding claims, further including first means for increasing the volume of one of the transmitting and receiving path signals, the level comparator (6) further including second means for, upon sensing that the first means has increased the volume, producing the control signal which causes the first variable frequency shifting circuit to effect an amount of shift which corresponds to an insertion loss which is necessary to prevent singing due to the increase in volume. A voice switched type loudspeaking telephone, including means (6) for comparing the levels of a sending voice signal and a receiving voice signal to produce a level comparison between the levels, first level control means (11) in a transmitting path responsive to the level comparison for controlling the level of a signal in the transmitting path, and second level control means (4) in a receiving path responsive to the level comparison for controlling the level of a signal in the receiving path, characterised in that there are provided first frequency shifting means (12) in the transmitting path for frequency-shifting a signal in the transmitting path, and second frequency shifting means (3) in the receiving path for frequency-shifting a signal in the receiving path.

Inventors: FUJISAKI, HISASHI C/O NEC CORPORATION, SHIMADA, KEIKO C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1988
Main classification: H04M9
IPC codes: H04M9, H04M1