Composite limiting sum and difference circuitry for extending the reception area of a frequency modulated stereo radio transmitter

#1. Field of the Invention

#This present invention relates to frequency modulated transmitters and more particularly to frequency modulated stereo transmitters. 2. Description of Prior Art

#Typically frequency modulated stereo transmitters have two audio stereo channels, a left and a right channel generated from a disc record or tape recording as two separate signals, one being a right signal and the other being a left signal.

#FIG. 1 is a block diagram of a typical prior art FM stereo transmitting system;

#FIG. 2A is a block diagram of an alternative embodiment of the FM stereo transmitting system according to the invention;

#FIG. 2B is a block diagram of an alternative embodiment of the FM stereo transmitting system according to the invention;

#Referring to FIGS. 2A and 2B, a transmitter system which has been modified to incorporate an embodiment of the invention is shown.

#Referring to FIG. 2A, a studio 50 is shown having an audio mixer 52 with several inputs for various stereo audio equipment such as phonograph turntables 54 and 56, microphones 58 and 60, and tape cartridge machines 62. The stereo signals from the audio mixer 52 are fed into a control room 66 having an audio mixer 64 where the stereo signals are mixed as known in the prior art. The mixed stereo signals from the mixer 64 are fed into an AGC unit 68 where their maximum amplitudes are limited. In order to prevent any large amplitude signals from exceeding the instantaneous power limitations of the transmitter system either from the design of the output states of the transmitter unit 22 or governmental or intergovernmental requirements such as the U.S. Federal Communications Commission Regulations which could subject a transmitter system user to sanctions, such as fines or even the suspension of a license to operate, a hard amplitude limiter 70 limits the signals transmitted therethrough. The signals from the hard limiter 70 are fed to a composite stereo generator and composite limiter 72 which generates sum and difference signals as described in FIG. 1 but without the 19 KHz pilot carrier. The pilot carrier may either be filtered out by a filter or not generated by the stereo generator portion of 72. Circuit details of the stereo generator portion and composite generator 72 will be described later. The limited composite stereo signals from the generator 72 are typically modulated by a modulator 74 for transmission by a link comprising 900-1000 KHz antennas, such as an antenna 76 and an antenna 78, as shown in FIG. 2B. The signal received by the antenna 78 is demodulated by a receiver 80 and fed into a high power FM transmitter 82 for transmission to FM receivers, not shown, by an FM antenna 84.

#FIG. 3 shows a typical stereo generator circuit 100 which comprises a portion of the composite stereo generator and composite limiter 72.