Improved page party system

• US 5,179,588 A
• Filed: 07/30/1991
• Issued: 01/12/1993
• Est. Priority Date: 07/30/1991
• Status: Expired due to Term

• Current Assignee: Hubbell, Inc.
• Original Assignee: GAI-Tronics Corp. (Hubbell, Inc.)
• Inventors: Nowicki, Edmund H.
• Primary Examiner(s): SCHREYER, STAFFORD
• Application Number:
  US07/738,196
• Time in Patent Office: 532 Days
• Field of Search: 379/170, 379/171, 379/167, 379/172, 379/173, 455/114, 455/109, 455/46, 455/47, 455/48
• US Class Current: 379/170
• CPC Class Codes: H04J 1/085 Terminal station; Combined modulator and demodulator circuits
• CPC Class Codes: H04M 9/027 Frequency division multiplex systems

Abstract

A page party system for use in industrial environments, having a plurality of stations joined by a page line and by a single party line for providing multiple full-duplex conference calls over the single party line. Multiple full-duplex conferencing is achieved by combining single sideband suppressed carrier amplitude modulation and frequency division multiplexing.

Claims

1. A page party communication system having a plurality of stations joined by a page line and by a single party line for providing plural full duplex voice conferencing channels on the single party line, each station comprising:

a current source line driver selectively coupled to either the page line for transmitting a paging message or to the party line for transmitting outgoing or receiving incoming voice signals between stations;

a channel selector switch for selecting one of said conferencing channels, each of said conferencing channels having a unique frequency band corresponding thereto;

audio input means for receiving outgoing voice signals;

a voice modem, responsive to said audio input means, including

(a) means for amplitude modulating a carrier with said outgoing voice signal and for suppressing the carrier and one its sidebands to produce a single sideband suppressed carrier spectrum;

(b) means for translating that spectrum to the frequency band of the selected conferencing channel;

(c) means, responsive to said line driver, for receiving single sideband suppressed carrier spectra representing incoming voice signals from other stations on the party line and for translating incoming spectra located at the frequency of the selected channel to baseband; and

(d) means for suppressing sidetone from incoming voice signals to produce a suppressed sidetone output signal; and

means, coupled to the voice modem, for generating an audio output in response to said suppressed sidetone output signal.

2. The page party system of claim 1 in which said voice modem further comprises:

a master clock at each station for generating a master clock signal at that station, the clock at that station being non-synchronous with respect to the clock at each other station.

3. The page party system of claim 2 further comprising:

a controller responsive to the channel selector switch for generating a channel selector signal that uniquely identifies that selected channel.

4. The page party system of claim 3 in which the voice modem further comprises:

a means responsive to the channel selector signal and to the master clock signal for generating a local oscillator signal, the frequency of which varies depending upon the conferencing channel selected, the local oscillator signal being directed to both the means for translating and means for receiving.

5. The page party system of claim 4 in which the means for translating the single sideband suppressed carrier spectrum of the outgoing voice signal further comprises:

a means for modulating the single sideband suppressed carrier spectrum with the local oscillator signal to produce both a lower frequency and higher frequency image of the single sideband suppressed carrier spectrum, at least one of which is within the band of the selected conferencing channel; and

a tracking low pass filter means which rejects the image of the single sideband spectrum at one of the frequencies while passing the other, the cut off-frequency of the filter being such as to pass the sideband image within the frequency band of the selected conferencing channel.

6. The page party system of claim 5 in which the lower frequency sideband image is within the frequency band of the selected conferencing channel.

7. The page party system of claim 5 further comprising a means for shifting the single sideband suppressed carrier spectrum of the outgoing voice signal to an intermediate frequency prior to modulation with the local oscillator.

8. The page party system of claim 7 wherein the frequency of the local oscillator is equal to the frequency of the selected conferencing channel plus the intermediate frequency.

9. The page party system of claim 7 wherein the intermediate frequency is 40 KHz.

10. The page party system of claim 7 in which the means for receiving further comprises:

a means for generating a receive local oscillator signal having a frequency corresponding to the selected conferencing channel;

a modulator for modulating the incoming voice spectra with the receive local oscillator signal to produce an image of the received spectra at baseband and an image of the received spectra at a higher frequency; and

a low pass filter for rejecting the image of the received spectra at the higher frequency while passing the image of the received spectra at baseband.

11. The page party system of claim 10 wherein the frequency of the local oscillator is equal to the frequency of the selected conferencing channel plus the intermediate frequency and wherein the means for generating a receive local oscillator comprises:

a means for modulating the local oscillator signal with a carrier signal having a frequency equal to the intermediate frequency to produce a higher frequency signal and a lower frequency signal, the lower frequency signal having a frequency equal to the frequency of the selected conferencing channel; and

a tracking low pass filter responsive to the channel selector signal for rejecting the higher frequency signal and for passing the lower frequency signal to produce a receive local oscillator signal having a frequency corresponding to the selected conferencing channel.

12. The page party system of claim 5 or 11 further comprising:

a means responsive to the control means, for determining the cut-off frequency of the tracking low-pass filters depending upon the conferencing channel being selected.

13. The page party system of claim 1 in which said means for amplitude modulating a carrier with said outgoing voice signal and for suppressing the carrier and one its sidebands to produce a single sideband suppressed carrier spectrum further comprises:

a means for splitting the outgoing voice signal into identical first and second voice signals;

a means for phase-shifting the first voice signal;

a first modulator for modulating a cosine-wave carrier signal having a selected frequency with the phase-shifted voice signal, the frequency spectrum of the modulated cosine-wave signal being centered at the carrier frequency and having a lower sideband and an upper sideband, the magnitudes of said sidebands having opposite polarity;

a second modulator for modulating a sine-wave carrier signal having the selected frequency with the second voice signal, the frequency spectrum of the modulated sine-wave signal being centered at the carrier frequency and having a lower sideband and an upper sideband, the magnitudes of said sidebands having the same polarity; and

a means coupled to each of the modulators for summing the modulated carrier signals such that at least one of the sidebands of the modulated cosine-wave carrier signals cancels with a corresponding sideband of the modulated sine-wave carrier thereby producing a single sideband suppressed carrier spectrum.

14. The page party system of claim 13 wherein the means for phase-shifting comprises a Hilbert transform filter.

15. The page party system of claim 13 wherein the selected frequency is 8 kHz.

16. The page party system of claim 11 in which said means for amplitude modulating a carrier with said outgoing voice signal and for suppressing the carrier and one its sidebands to produce a single sideband suppressed carrier spectrum comprises:

a digital signal processor.

17. The page party system of claim 1 in which said means for suppressing sidetone from incoming voice signals to produce a suppressed sidetone output signal further comprises:

a digital signal processor.