GUARD CIRCUIT AND DWELL TIME LIMITING METHOD FOR THREE LEVEL ANALOG ENCODERS

• US 3,806,639 A
• Filed: 05/16/1972
• Issued: 04/23/1974
• Est. Priority Date: 05/16/1972
• Status: Expired due to Term

• Current Assignee: John Brian Evans
• Original Assignee: John Brian Evans
• Inventors: Evans, John Brian
• Primary Examiner(s): Morrison, Malcolm A.
• Assistant Examiner(s): Dildine, Jr., R. Stephen
• Application Number:
  US05/253,754
• Time in Patent Office: 707 Days
• Field of Search: 178/DIG.3,6.6R,6,66,68 179/2DP,2TV,15.55R 325/145,148,163 340/208
• US Class Current: 379/351
• CPC Class Codes: H04N 1/00095 Systems or arrangements for the transmission of the picture signal
• CPC Class Codes: H04N 1/41 Bandwidth or redundancy reduction by scanning H04N1/17 ; methods or arrangements for coding, decoding, compressing or decompressing digital video signals H04N19/00

Abstract

A guard circuit is combined with a three level encoder so that the encoder may be employed, for example, in a facsimile transmitter to drive a voltage controlled oscillator to thereby provide a facsimile signal suitable for transmission over a standard commercial telephone network, even should the network have one or more inband signaling frequencies within a predetermined one of the half bands of the nominal frequency range for the facsimile signal. The guard circuit limits the time the facsimile signal is permitted to dwell in the predetermined half band of its frequency range to a period which is short relative to the finite response time of the inband signaling equipment within the telephone network, thereby precluding the facsimile signal from interfering with the normal operation of the signaling equipment.

Claims

1. In combination with an encoder for encoding an analog input signal by selectively shifting the analog signal between a pair of ranges above and below a predetermined midpoint level, a guard circuit for limiting the time the encoded signal is permitted to dwell in a predetermined one of said ranges, said guard circuit comprising detector means coupled to said encoder for providing a first signal whenever the encoded signal is within said predetermined range, and timer means coupled between said detector means and said encoder for providing a control signal to cause said encoded signal to be shifted to the other of said ranges whenever said first signal persists for longer than a permissible dwell time, whereby the time said encoded signal is permitted to dwell in said predetermined range is limited to the permissible dwell time.

2. The combination of claim 1 wherein the permissible dwell time is adjusted downwardly from a predetermined maximum dwell period whenever the encoded signal is shifted out of and then back to said predetermined range within a time span which is short relative to a predetermined recovery time for said timer means.

3. The combination of claim 2 wherein said timer means includes a RC network having a first time constant selected to establish the maximum permissible dwell period for the encoded signal and a second time constant selected to establish the rEcovery time for the timer means.

4. In a telecommunication system including an encoder for encoding information carried by an analog input signal to provide an encoded analog signal and convertor means coupled to said encoder for providing a frequency modulated signal in response to said encoded signal, with said frequency modulated signal being modulated in accordance with said information and having a nominal frequency range divided into upper and lower half bands by a predetermined center frequency, a guard circuit for permitting said modulated signal to be carried by a telephone network without risk of being inadvertantly detected as being a control signal for inband signaling equipment included in said network even if said network has one or more inband signaling frequencies within a predetermined one of the half bands of the nominal frequency range of said modulated signal;

said guard circuit comprising detector means coupled to send encoder for providing a first signal whenever the frequency modulated signal is within said predetermined half band, and timer means coupled between said detector means and said encoder for providing a control signal to cause said encoder to shift said frequency modulated signal to the other of said half bands whenever said first signal persists for a period approaching a predetermined finite response time of said inband signaling equipment, whereby said frequency modulated signal is precluded from dwelling in said predetermined half band for a period as long as the response time of said equipment.

5. The combination of claim 4 wherein said telecommunication system is a facsimile system, and said encoder, converter means, and guard circuit are included in a facsimile transmitter.

6. The combination of claim 4 wherein said encoder includes a bistable means which is switched between first and second stable operating states to selectively shift said frequency modulated signal between said one and said other half bands, respectively;

clamp means coupled to receive said analog input signal; and

threshold detector means coupled to said clamp means and responsive to said analog input signal for operating said clamp means to hold said frequency modulated signal at said center frequency whenever said analog input signal is below a predetermined threshold level; and

said detector means comprises a gate means having one input coupled to said threshold detector means and another input coupled to said bistable means, said gate being enabled to supply said first signal only when said bistable means is in said first stable state and said analog input signal is above said threshold level.

7. The combination of claim 6 wherein said bistable means has a control input coupled to receive said control signal from said timer means and switches in response to said control signal from said first to said second stable state to thereby cause said frequency modulated signal to shift from said one to said other half band.

8. The combination of claim 7 wherein said timer means includes a RC network having a first time constant selected to establish a predetermined maximum permissible period for said frequency modulated signal to dwell in a said one band and a second time constant selected to establish a predetermined recovery time for said timer means.

9. The combination of claim 8 wherein said RC network automatically shortens the permissible dwell period for said frequency modulated signal whenever said bistable means is switched from said one stable state to said other stable state and then back to said one stable state within a time span shorter than the predetermined recovery time for said timer means.

10. The combination of claim 9 wherein said telecommunication system is a facsimile system, and said encoder, converter means, and guard circuit are included in a facsimile transmitter.

11. A method permitting a frequency modulated signal supplied by a converter means driven by an analog encoder to Be transmitted over a telephone network containing inband signaling equipment without risk of interfering with the normal operation of said equipment even should said network have an inband signaling frequency within a predetermined half band of a nominal frequency range for said signal, said method comprising the steps of initiating a timing cycle whenever said frequency modulated signal is within said predetermined half band of its nominal frequency range, and signaling said encoder to independently shift said frequency modulated signal to another half band of its nominal frequency range whenever said modulated signal tends to dwell in said predetermined half band for longer than permissible dwell period, said permissible dwell period having a predetermined maximum duration selected to be shorter than a predetermined response time of said inband signal equipment to any signal at said signaling frequency.

12. The method of claim 11 further including the step of shortening said permissible dwell period whenever said frequency modulated signal is shifted out of and back to said predetermined half band within a period short relative to a predetermined recovery time.