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Many FM stations are competing directly with AM stations and have developed a need for loudness.Īn FM broadcaster must deal with modulation that is not only amplitude sensitive but frequency dependent as well. The need for loudness in FM has evolved as a result of this competitiveness and because FM is being used for broader markets such as country western, hard and soft rock, and other up-beat musical formats. The subject invention relates to a pre-processing apparatus for optimum overshoot control of an FM stereo system.įM stereo radio broadcasting is a highly competitive medium. Said clipping means producing at times asymmetry in said clipped audio signal such that when said clipped signal is capacitively coupled within the transmitter, a level shift occurs in said clipped signal producing overmodulation, means responsive to said clipped audio signal and said audio signals before clipping for detecting the absolute value of the peak energy lost due to clipping, gain control means coupled to said clipping means and said detecting means for reducing the amplitude level of the clipped audio signals in proportion to the level of the absolute level detected, andĭe-emphasis means coupled to said gain control means for de-emphasizing said gain controlled, clipped audio in a manner inverse to said second pre-emphasis means to provide essentially an overall flat frequency response.
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An audio processor for modifying an audio modulating signal waveform to be applied to an FM transmitter, said transmitter of the type including a first pre-emphasis means and a low-pass filter in the input thereof which filter normally produces ringing of that signal when the audio is limited or clipped to prevent overmodulation, comprising:Ĭlipping means coupled to said second pre-emphasis means responsive to the audio output signal level therefrom reaching a level which would cause over 100% modulation for reducing that level by an amount determined by the amplitude of ringing of said audio output signal in said low-pass filter to produce a clipped audio signal, The combination of claim 2 wherein said reducing means includes D-type flip-flops clocked by signals from said first detecting means and triggered by signals from said second detecting means.Ĥ. The combination of claim 1 wherein said limiting means includes first detecting means for detecting the 100% modulation level of said pre-emphasized audio and second detecting means for detecting a pre-emphasized audio level below the 100% modulation level corresponding to the ringing amplitude of said low-pass filter, and means coupled to said first and second detecting means for reducing the audio level of said pre-emphasized audio when such signal over said 100% level is reached by the amount of said ringing until said audio signal falls below the second detected level.ģ. Limiting means coupled to said second pre-emphasis means responsive to the audio output signal level therefrom reaching a level which would cause over 100% modulation for reducing that level by an amount determined by the amplitude of ringing of second audio output signal in said low-pass filter to produce a limited audio signal, andĭe-emphasis means coupled to said limiting means for de-emphasizing said limited audio in a manner inverse to said pre-emphasis means to provide essentially an overall flat frequency response.Ģ. Second pre-emphasis means responsive to said audio modulating signal for pre-emphasizing said audio in a manner similar to said first pre-emphasis means, An audio processor for modifying an audio modulating signal waveform to be applied to an FM transmitter, said transmitter of the type including a first pre-emphasis means and a low-pass filter in the input thereof which filter normally produces ringing of that signal when the audio is limited or clipped to prevent overmodulation, comprising: