WO1983003939A1

WO1983003939A1 - Intercommunication system

Info

Publication number: WO1983003939A1
Application number: PCT/US1983/000502
Authority: WO
Inventors: Perry Kim White
Original assignee: Western Electric Company, Inc.
Priority date: 1982-04-22
Filing date: 1983-04-08
Publication date: 1983-11-10
Also published as: EP0106883A1; GB8310668D0; GB2120046A

Abstract

A frequency multiplexed intercom system above baseband which uses a 2-wire cable as a bus (210) interconnecting a plurality of stations (200). Simultaneous conferencing among all stations is enabled by having each station modulate identical carrier waves. A reference frequency is made available on the frequency multiplexed bus so that each station can phase-lock its carrier wave generator (240) to an identical reference. Modulators (241) and demodulators (242) utilize phase-locked loops for frequency modulation and demodulation.

Description

INTERCOMMUNICATION SYSTEM

Technical Field

This invention relates to intercommunication systems and more particularly to a frequency multiplexed bus strategy above baseband to enable simultaneous conferencing. Background of the Invention

In a residence or office location there is often a need to provide intercommunication service among a plurality of stations. Such a feature provides a user with the ability to deliver a verbal message to another person at a distant location without shouting. It is especially useful in combination with a telephone set when the need to locate another arises precisely because of an incoming telephone call.

Intercommunication systems are well known, but usually require more than two wires to implement and often require special equipment such as selector switches and separate control hardware. These items tend to be both cumbersome and costly. Further, many locations do not have more than a single pair of wires that is accessible or common to those locations seeking intercom service.

Prior art intercommunication systems typically require use of that portion of the frequency spectrum

(baseband) already in use by other equipment (a telephone perhaps), and therefore cannot operate simultaneously with that other equipment. Modulated carrier systems are known in the art of intercommunication service and provide the necessary frequency translation to allow simultaneous activity on the same pair of wires.

In U. S. Patent 3,705,412 issued to Nakamura et al on December 5, 1972, there is disclosed a Duplex Interphone wherein a master station and a substation can simultaneously communicate over a single pair of wires. The transmitters utilize amplitude modulation of carrier waves having frequency differences larger than audible frequencies from each other; this avoids interference tones in the audible band. Each transmitter in this arrangement must be configured differently which becomes increasingly complex as the number of stations increase.

It is desirable to provide a conferencing arrangement whereby several persons are able to speak at the same time. Such an arrangement allows a natural interaction to take place because it permits normal feedback such as an indication of understanding and agreement or the lack thereof. This is especially needed when conferees are not visible to each other and body language such as smiles, nods and the like are absent. In many prior art conferencing systems it is necessary to activate a switch in order to talk - the so called "press to talk" mode. In this mode of operation, it is not possible to detect comments from listeners. Conferees thus tend to deliver their messages in a somewhat stilted manner; informality is lost and spontaneity is virtually impossible.

It is therefore an object of this invention to provide an intercommunication system, having a plurality of stations, for use over an existing pair of wires without interference to signals thereon. It is another object of this invention to allow simultaneous conferencing among all stations.

It is yet another object of this invention to provide an intercommunication system capable of expanding the number of stations with identically configured units.

Summary of the Invention

In accordance with the objects of the invention, a frequency multiplexed system for providing intercommunication service among a plurality of stations is disclosed. A reference frequency is distributed to each using station over a common bus. At each station a local oscillator generates a carrier signal that is phase-locked to the reference frequency. The carrier signal is thereafter modulated by a locally supplied input signal and connected to the common bus for distribution to all stations. Each station has a receiver that detects modulated signals on the bus. The detected signal is a linear composite of the locally supplied input signals from each station. Use of identical carrier waves at each modulator minimizes non-linearities and prevents interfering tones at each demodulator. The benefits of this technique accrue to various modulation schemes that include amplitude modulation, phase modulation and frequency modulation.

In an illustrative embodiment of the invention, the distributed reference frequency is multiplied (in frequency) at each using station and used thereafter as a carrier wave. This provides a frequency separation between the reference frequency and the modulated carrier wave that facilitates the selection of the various signals. Frequency modulation is employed, and although it requires greater bandwidth than some other modulation schemes, bandwidth is not a premium in the typical intercom environment. In this embodiment a pair of telephone wires is used as the common bus. In another suggested embodiment of the invention, the 60 Hz power line frequency is used as the reference frequency and the power distribution system (115 VAC) is used as the common bus.

It is a feature of this invention that simultaneous communication among a plurality of stations is possible using identical modulators and demodulators at each station since there is no need to distinguish master and slave stations.

It is another feature of this invention that locally supplied signals are frequency multiplexed onto a single pair of wires thereby allowing the use of existing wires such as telephone or power distribution systems. It is another feature of this invention that frequency modulation provides significant immunity to noise and other signals present on the bus. Brief Description of the Drawing Other objects and features of the present invention will be apparent from the following discussion and drawings:

FIG. 1 illustrates a general interconnection arrangement for intercom stations using a telephone cable as a common bus;

FIG. 2 is a block diagram of a typical station for an intercommunication system in accordance with the invention;

FIG 3. is a block diagram of a carrier wave generator capable of phase locking to a reference frequency in accordance with the invention;

FIG. 4 is a block diagram of a frequency modulation transmitter;

FIG. 5 is a block diagram of a frequency modulation receiver;

FIG. 6 is a graph of amplitude vs. frequency illustrating the frequency spectrum of an embodiment utilizing the telephone cable as the common bus; FIG. 7 is a graph of amplitude vs frequency illustrating the frequency spectrum of an embodiment utilizing the 60 Hz power line frequency as the reference frequency and the power distribution cable as the common bus;

FIG. 8 is a schematic of a line filter for use with the invention; and

FIG. 9 is a schematic of a filter for each station when used for standard telephone communication. Detailed Description

In the embodiment of FIG. 1, a residence location having N telephone stations is illustrated. These stations are connected to an existing telephone cable - herein also called a common bus. Although telephone cables often contain 4 wires, the disclosed interconnection arrangement only requires 2. Frequency multiplexing improves utilization of existing cable, and is advantageously used in the invention to eliminate additional wiring. Stations (1, 2, N) are interconnected, for intercom use, at frequencies above those used for standard telephone communication. Line Filter 11 is used to isolate the activity on the common bus from the telephone network so that intercom signals will not be transmitted to the local central office. An acceptable line filter is shown in FIG.

8 where the inductors (designated L) each have a value of 3 millihenries, and a capacitor (designated C) has a value of 0.5 microfarads. The DC resistance of the line filter is a further consideration. Inductors having a DC resistance of 1 ohm are acceptable; thus the line filter of FIG. 8 will contribute approximately 4 ohms to the common bus.

All of the stations shown in FIG. 1 are identically configured; an advantage that accrues to the design, inventory, and installation phases of a product. Reference Frequency Generator 12 is an oscillator of any conventional design known in the art. For the purpose of illustration, it has an output frequency of 19.2 kHz and a signal level of -10 dBV.

Associated with each station of FIG. 1 is an input filter (not shown). This filter is required when the station is utilized for standard telephone service. The purpose of this filter is to isolate standard telephone signals from intercom signals. An acceptable filter is shown in FIG. 9 where each inductor (designated L) has a value of 3 millihenries. This filter is bypassed when the station is used as an intercom. The manner in which switching occurs is not shown since it is beyond the scope of the invention.

FIG. 2 illustrates, in block form, the circuitry necessary to implement any station in the intercommunication system for the specific purpose of providing intercom service. Station 200 (designated i) is shown connected to common bus 210 comprising a pair of wires. The 19.2 kHz reference frequency on bus 210 is extracted by bandpass filter 230 centered at 19.2 kHz. This filter is of conventional design and need only be wide enough to pass the reference frequency (f_r) . Frequency f_r is connected to carrier wave generator 240 for the purpose of generating a carrier wave (f_c) to be modulated by a modulating signal. Generator 240 includes an oscillator that is phase-locked to the reference frequency. This oscillator operates at twice the reference frequency (38.4 kHz) . Details of the generator are illustrative and are more fully discussed in connection with FIG. 3.

The carrier wave emanates from generator 240 on line 204. Its frequency is related to the reference frequency by a factor (M) which may assume a plurality of values, here M = 2. The spectrum of the carrier wave, after modulation, is such that it does not overlap other frequency multiplexed signals on the bus. Frequency separation among bus signals is provided in order to simplify filter design. Modulation of the 38.4 kHz carrier wave with audible sounds is arranged to avoid overlap with the reference frequency at 19.2 kHz or standard telephone service below 4 kHz.

Modulation of the carrier wave is performed in transmitter 241. In a preferred embodiment of the invention, frequency modulation (FM) is selected for several reasons. FM is a "rugged" form of modulation, being quite impervious to noise. When a telephone cable is used as the common bus, intercom signals are not affected by the presence of "ringing" on the bus. When a power distribution cable is used as the common bus, intercom signals are not affected by power line noise such as from appliances. Commercially available phase-locked loops, in chip form, are inexpensive and may readily be used for frequency modulation or demodulation as discussed hereinafter in connection with FIGS. 4 & 5 respectively. Finally, although other modulation techniques may be used in the invention and generally require less bandwidth than FM, bandwidth is not the prime consideration.

Transducer 221 converts audible sounds into electrical signals. In the present embodiment, the transducer is a microphone such as exists in the handset of a telephone. A time-varying message signal m(t) emanating from transducer 221 is connected to transmitter 241 through amplitude limiter 243 and filter 234. This signal is also known as the modulating signal. Other sources of message signals are possible and include broadcast radio or standard telephone signals.

Message signals are processed before they are used to frequency modulate the carrier wave. It is desirable to transmit voice frequencies within a 5 kHz band to provide good quality reproduction of audible sounds. Filter 234 is therefore of the low-pass type having an approximate bandwidth of 5 kHz. Amplitude limiter 243 is used to control the amplitude of the message signal and thereby effectively limit the bandwidth of the frequency modulated signal to approximately 20 kHz. The system may therefore be characterized as "narrowband FM." It provides superior performance to amplitude modulation (double sideband AM requires 10 kHz) and preserves the inherent advantages of frequency modulation. Amplitude limiter 243 is a conventional logarithmic gain control circuit with hard limiting of the highest signal amplitudes. Note that the placement of filter 234 allows it to remove any harmonics that result from the amplitude limiting process. The modulated carrier wave m(f_c) is the output signal from transmitter 241 and is connected to filter 231 over line 202. Filter 231 interconnects modulated signals to common bus 210 after removing undesirable frequencies from the modulated carrier wave. Filter 231 is of the band pass type and is centered at frequency f_c. The receiving portion of station 200 comprises filters 232, 233, receiver 242 and transducer 222. Filter 232 is similar to filter 231. It is of the band pass type and is centered at frequency f_c. It is noted that when the transmitter and receiver of a station operate at the same frequency, sidetone is created. Since there is little or no perceivable delay between speaking and hearing one's own voice, the sidetone level is not a significant problem as long as there is sufficient loss between the microphone and the loudspeaker to avoid oscillation caused by the positive feedback. Filter 232 only passes frequencies to receiver 242 that are within a predetermined range. Thus the reference frequency at 19.2 kHz and the standard telephone signals below 4 kHz are excluded.

Receiver 242 is an FM demodulator and is shown in greater detail in FIG. 5. While any conventional FM demodulator would be acceptable, a phase-locked loop type of demodulator is utilized in the instant invention. The output of receiver 242 includes a plurality of frequencies which are selectively passed to transducer 222 over lead 207. Filter 233 is of the low pass type and excludes frequencies above 5 kHz, it is similar in design to filter 234. Transducer 222 is a conventional loudspeaker.

Advantageously, transducers 221 and 222 are the microphone and loudspeaker of a telephone handset. Although not shown in any of the diagrams, when transducers 221 and 222 are contained in the telephone handset, a separate loudspeaker is utilized for alerting.

FIGS. 3, 4 and 5 use well known phase-lock techniques for: locking onto a reference frequency, frequency modulation, and frequency demodulation. The basic principles of phase-lock loop operation are taught in the text Phaselock Techniques, 2nd Edition, (Floyd M.

FIG. 3 illustrates an oscillator, harmonically locked to the reference frequency f_r (see also figure 10.3 of the above-cited text). Input reference frequency f_r is fed to phase comparator 310 over lead 201. Reference frequency f_r is a 19.2 kHz sine wave and is compared with a nominal 38.4 kHz square wave present on lead 305. Zero crossings of these signals are compared: if the zero crossings of the square wave occur when the polarity of the sine wave is positive, then for example, a negative valued correction signal is sent to the square wave oscillator source; and if the zero crossings of the square wave occur when the polarity of the sine wave is negative, then a positive valued correction signal is sent to the square wave oscillator source. The output signal from phase comparator 310 is connected to loop filter 320 over lead 302. Loop filter 320 is of the lowpass type having a cutoff frequency of approximately 100 Hz. This provides high selectivity (i.e., a stable reference).

The output signal from loop filter 320 is a smoothed correction signal whose magnitude and polarity regulate the frequency of Voltage Controlled Oscillator (VCO) 330. The output signal from the VCO is a 25 kHz square wave that is connected to the modulator of FIG. 4 on lead 204 and to divider 340 on lead 304. Divider 340 is a circuit that reduces an input frequency by a factor of M. Here, M=2 and the divider is a simple binary cell (flip- flop). Feedback techniques exist whereby divider 340 can reduce the input frequency by a plurality of rational multiples (i.e., numbers whose magnitudes may be expressed as a ratio of integers - they may be greater, equal to, or less than unity). Such techniques expand the choice of VCO frequencies that can lock to a particular reference frequency. An acceptable circuit that implements the carrier wave generator of FIG. 3 is a CD4046-type phase-locked loop, manufactured by National Semiconductor Corporation.

FIG. 4 illustrates a transmitter whereby carrier wave f_c on input lead 204 is frequency modulated by message signal m(t) present on input lead 205. The frequency modulated output signal m(f_c) is present on lead 202.

In order to readily understand the operation of the transmitter, note that it is substantially similar to the phase-locked loop of FIG. 3. Assume for the moment that message signal m(t) is identically zero. Phase comparator 410, loop filter 420, and VCO 430 are interconnected as a phase-locked loop whereby the output frequency on lead 406 is locked to input frequency f_c present on lead 204. Loop filter 420 is of the low pass type having a cutoff frequency of approximately 100 Hz. This frequency is chosen to be less than the lowest frequency component in the modulation signal. The smoothed correction signal is able to vary at a maximum rate of 100 Hz. Adder 440 combines the smoothed correction signal with the message signal m(t) to produce a net signal that regulates the frequency of VCO 430. The smoothed correction signal maintains the long term center frequency of the VCO at the value of carrier frequency f_c. The short term frequency variation of the VCO is controlled by modulation signal m(t) . An acceptable circuit that implements the modulator of FIG. 4 is the LM565-type phase-locked loop manufactured by National Semiconductor Corporation.

FIG. 5 illustrates an FM demodulator whereby a frequency modulated input signal, present on input lead 203, is processed in a manner such that variations in frequency are converted into variations in voltage. These variations in voltage are present on output lead 206. The operation of the FM demodulator is once again understood by considering the basic circuit comprising phase comparator 510, loop filter 520 and VCO 530 interconnected as a phase-locked loop. VCO 530 is arranged to track the instantaneous frequency of the incoming frequency modulated signal on lead 203. The output signal from the phase comparator is therefore a voltage whose magnitude and polarity are indicative of the frequency difference between the incoming signal and the VCO signal. Thus, this voltage is nothing more than the modulating signal itself and indeed is the ideal output from a demodulator. In the present invention, audible signals are being used for modulating signals. Therefore, loop filter 520 is a conventional low pass filter having an upper cutoff frequency sufficient for the transmission of voice band communication. Here, the cutoff frequency is selected to be approximately 5 kHz. The frequency demodulated output signal present on lead 206 is essentially the linear composite of the various message signals from all stations in the intercommunication system. An acceptable circuit that implements the demodulator of FIG. 5 is the LM565-type phase locked Ioop manufactured by National Semiconductor Corporation.

It should be noted that optimum performance is achieved when each modulated signal is of the same amplitude. When the various modulated signal amplitudes are different, distortion is created in the demodulation process. This distortion increases as the difference between modulated signal amplitudes. In order for the demodulator to reconstruct the linear combination of all message signals without distortion, certain parameters need to be the same at each station. Nevertheless, it is possible to operate a viable intercom system with some degradation. If, for example, frequency locking is utilized instead of phase locking - then interference tones and variable amplitude levels will be created at the receiver. The impairments suffered are, to a large extent, dependent upon the modulation scheme selected. Phase locking is clearly the preferable choice over frequency locking in view of wide availability and low cost of such circuits. Even in the case of a phase-locked loop some impairments are possible. Of interest is a comparison between amplitude and angle modulated systems in the presence of carrier phase differences. For simplicity only two simultaneously transmitting stations are considered.

Amplitude Modulation (AM) In an AM system we have: s_i(t) = A_im_i(t) cos(μ_it + θ_i)

where, s_i(t) is the transmitter output, A_i is the carrier amplitude, m_i(t) is the voice message, μ_i is the carrier radian frequency, and θ_i is the carrier phase. The message signal m_i(t) can be expressed as: m_i ( t) = M_i + m_iac(t) Where m_iac(t) indicates the ac portion of the i^th message signal and M_i is the dc term.

Assuming ideal envelope detection, specifically square-law detection, where the detector is ac coupled so that the dc terms are blocked, the signal and distortion terms are written:

The signal component, s_D(t), shows that the carrier phase difference, ( θ₁ - θ₂) causes an attenuation of the output message terms. The distortion component consists of three terms. The two "squared" terms represent harmonic distortion inherent in square-law detectors. The third term,

2 cos(θ₁ - θ₂) [ A₁A₂m_1ac(t)m_2ac(t) ] is an intermodulation distortion term. This term also results from the squaring operation but varies directly with cos(θ₁ - θ₂) . So, for square-law and similar detectors, carrier component phase differences also affect the output as message signal attenuation and distortion variations. Angle Modulation

For angle modulation we have: s_i(t) = A_icos[μ_it + θi + Φ_i(t)], where s_i (t) is the transmitter output, A_i is the carrier amplitude, P^ is the carrier radian frequency, θ_i is the carrier phase, and Φ_i (t) is the modulation signal. Phase Modulation (PM) would have:

Φ _i ( t) = k_im_i(t), where k_i is the modulation index and m_i (t) is the message signal. An FM system has:

Φ_i(t) = k_i ^tm_i (α) dα.

Assuming that the received signal is hard limited before detection and θ ₁ - θ ₂ is bounded and that bound is not too large, the received phase angle information is written:

If we have a PM system, our interest is in Φ(t) since a PM detector provides an output proportional to Φ(t) . Carrier phase differences, then, produce a dc term in the PM detector output. Note, however, that the sum of modulation signals is preserved in an undistorted form except for the DC term. With FM systems, we are interested in the time derivative of Φ (t). So, after making substitutions for Φ₁(t) and Φ₂(t),

Thus, the linear combination of messages is unaffected by carrier phase difference in the FM case.

In each of the above systems, transmitter frequency coherence was required. AM systems suffer from message distortion or attenuation as the result of carrier phase differences. Phase modulation displays only an additional dc term after detection, while FM exhibits no distortion of the messages. The diagram of FIG. 6 provides a frequency domain illustration of frequency multiplexing on the 2-wire telephone cable. Standard telephone service occupies the frequency band below 4 kHz. The reference frequency f_r is at 19.2 kHz and the frequency modulated intercom signal is centered at 38.4 kHz.

FIG. 7 provides a frequency domain illustration of frequency multiplexing on the power line distribution cable. The 115 VAC signal obviously occupies the 60 Hz frequency slot. However, the 60 Hz frequency itself may advantageously be used as the reference frequency. Harmonic locking is a conventional technique and has already been discussed in connection with FIG. 3. The short term stability of the 60 Hz signal coupled with a large frequency difference between the reference frequency and the harmonically locked carrier frequency may yield unacceptable results in connection with a particular modulation scheme. Clearly a separate reference frequency generator can be used in connection with the power line distribution cable as the common bus. It is to be understood that the implementations discussed are merely illustrative of the principles of my invention. A wide variety of modifications thereto may be effected by persons skilled in the telephone art without departing from the spirit and scope of my invention.

Claims

1. A method for providing simultaneous intercommunication service among a plurality of stations over a pair of wires comprising the steps of: transmitting a reference frequency to stations in the intercommunication system over the pair of wires; generating a carrier wave at each station; phase-locking said carrier wave at each station to the reference frequency, said carrier waves being substantially identical in frequency and phase at each station; modulating, at a transmitting station, the carrier wave with a message signal to be transmitted over the pair of wires; and demodulating, at a receiving station, the modulated carrier waves present on the pair of wires.

2. In an intercommunications system, apparatus for transmitting a message signal over a frequency multiplexed bus including an oscillator for generating a carrier wave,

CHARACTERIZED BY means responsive to signals on the frequency multiplexed bus for deriving a reference frequency; means responsive to the reference frequency for regulating the frequency of said oscillator; means responsive to the message signal for modulating the carrier wave; and means interconnecting the modulated carrier wave to the frequency multiplexed bus.

3. Apparatus according to claim 2 further including a receiver and CHARACTERIZED BY: means for demodulating said modulated carrier wave; means for interconnecting signals on the frequency multiplexed bus, in the frequency range of the modulated carrier wave, to the demodulator; and means for converting the demodulated signal into audible sounds.

4. Apparatus according to claim 2, CHARACTERIZED IN THAT the regulating means comprises a phase-locked loop for locking the frequency of the carrier wave to a rational multiple of the reference frequency.

5. Apparatus according to claim 3, CHARACTERIZED IN THAT the modulating means and the demodulating means comprise phase-locked loops interconnected to provide frequency modulating and frequency demodulation.