Simulcast transmission system

• US 4,317,220 A
• Filed: 02/05/1979
• Issued: 02/23/1982
• Est. Priority Date: 02/05/1979
• Status: Expired due to Term

• Current Assignee: Andre Martin
• Original Assignee: Andre Martin
• Inventors: Martin, Andre
• Primary Examiner(s): Ng, Jin F.
• Assistant Examiner(s): Chin, Tommy P.
• Application Number:
  US06/009,452
• Time in Patent Office: 1,114 Days
• Field of Search: 455/49, 455/51, 455/56, 455/57, 455/53, 455/54, 455/58, 455/72, 455/76, 455/112, 455/132, 455/16, 455/136, 340/291, 340/299, 340/311, 358/148, 358/149, 375/107, 375/120
• US Class Current: 455/503
• CPC Class Codes: H04H 20/67 Common-wave systems, i.e. using separate transmitters operating on substantially the same frequency

Abstract

A simulcast transmission system is disclosed. The system comprises a master station including means for generating and transmitting over a medium an audio signal containing a message signal and a pilot frequency signal inserted into the audio signal band, and at least two slave stations adapted to receive the audio signal transmitted over the medium and including a radio frequency generator of the phase locked loop type and means for isolating the pilot frequency signal from the audio signal and for synchronizing the frequency of the radio frequency generator with such pilot frequency signal.

Claims

1. A simulcast transmission system comprising:

(a) a master station including means for generating and transmitting over a medium an audio signal containing a message signal and means for generating a pilot frequency signal inserted into the audio signal band, said means for generating a pilot frequency signal comprising a precision oscillator, a frequency divider connected to the output of said precision oscillator for generating said pilot frequency signal and a band pass filter for eliminating the harmonics of said pilot frequency signal, and wherein the means for generating said audio signal further comprises a compressor amplifier for limiting the amplitude and frequency band of said message signal within predetermined limits, a notch filter connected to the output of said compressor amplifier for withdrawing from said message signal all frequencies substantially equal to said pilot frequency signal thus creating a protected audio band slot, a mixer for inserting said pilot frequency signal into said protected audio band slot, and an output amplifier for testing the level of the audio signal depending on the medium it is transmitted through; and

(b) at least two slave stations adapted to receive the audio signal transmitted over said medium and each including a radio frequency generator of the phase locked loop type and means for isolating the pilot frequency signal from the audio signal and for synchronising the frequency of the radio frequency generator with said pilot frequency signal, wherein said last named means comprises a band pass filter for filtering said pilot frequency signal from the audio signal, a square wave generator connected to the output of said band pass filter for generating a square wave signal corresponding to the frequency of said pilot frequency signal and applying it to the phase locked loop radio frequency generator, the latter comprising a voltage controlled oscillator generating a carrier frequency signal which is a predetermined multiple of said pilot frequency signal, a frequency divider connected to the output of said voltage controlled oscillator for providing an output signal corresponding to the frequency of said pilot frequency signal, a phase detector conntected to said frequency divider and to said square wave generator for comparing the phase of the output signal of said frequency divider with that of said pilot frequency signal, an integrator connected to the output of said phase detector for generating a D.C. voltage corresponding to the phase difference between the output signal of the frequency divider and that of the pilot frequency signal and applying said voltage to said voltage controlled oscillator for reducing said phase difference to zero, a notch filter connected in parallel with said band pass filter for withdrawing the pilot frequency signal from the audio signal transmitted by the master station to recover the message signal, a limiter connected to the output of the notch filter for limiting the frequency variations of the message signal within prescribed limits, a low pass filter connected to the output of the limiter to eliminate all frequencies of the message signal higher than 3000 Hz, and a modulator interconnecting the low pass filter and the voltage controlled oscillator for modulating the carrier frequency signal with the message signal; and

(c) a delay line connected to the input of each slave station for compensating for the difference in propagation times of the audio signal between the master station and each of the slave stations.

2. A simulcast transmission system as defined in claim 1, further comprising a compressor amplifier connected at the input of the slave station ahead of said band pass filter, and an amplitude detector interconnecting the output of said band pass filter and said compressor amplifier for controlling the amplitude of the pilot frequency signal.

3. A simulcast transmission system as defined in claim 1, further comprising a radio frequency amplifier interconnecting the output of the voltage controlled oscillator to a suitable antenna, and a protection circuit interconnecting the phase detector and the radio frequency amplifier for preventing operation of the radio frequency amplifier when the phase detector senses that the phase locked loop is unlocked.