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Applied Technology: MPX-Direct: Digital Connectivity for FM
Prior to the advent of digital exciters for FM radio, integration of an audio processor (with integrated stereo generator) was done via a baseband or multiplexed (MPX) signal connected directly to the exciter's broadband (DC - 53kHz) input. The MPX signal would be routed directly to the modulator. This tried and true method is a present day standard. Any audio processor, be it analog or digital, that contains its own stereo generator offers an analog-based MPX signal on a BNC connector. The connection of this signal, should it be directly out of the stereo generator itself or from the output of a composite STL system, is then connected to the MPX input on the exciter via another BNC connector. Anyone who has installed audio processing will understand this.
With the introduction of digital exciters, the only MPX input is analog and it must be passed through an analog-to-digital converter (ADC) before it can be digitally modulated. The sampling rate of the ADC must be relatively high due to the wide bandwidth of the MPX. Consider for a moment the audio spectrum of FM is 99kHz wide. Therefore, any analog signal connected to a digital exciter must be sampled at 200kHz or higher. Once digitized, no other processing is required before modulation.
To date, if an all-digital transmission path is desired, the only interconnect between the audio processor and exciter requires using the AES3 discrete left/right input. This path is quite a bit more complicated than the MPX path because it must condition the audio signal in preparation for stereo generation and herein lies potential for audio degradation. In this configuration, the FM stereo signal must be generated in the exciter.
Experience has proven this method less than ample in modulation efficiency. This is primarily due to the required sample rate converters (SRC) employed in the exciter's input section. The signal that is arriving at the AES3 input of the exciter might be operating at a different sampling rate than the exciter is expecting. If so, a rate converter is employed to make the proper transition. This device can pose problems as the digital filter in the rate converter can generate overshoots to the already tight peak controlled audio signal that is being adjusted.
All audio processors, both analog and digital apply some form of overshoot control to the output filtering section. In most designs, this function is a form of integrated protection clipper working around the final low pass filter to obtain control.
Knowing that the audio bandwidth used in FM stereo is 15kHz, overshoot components will begin with any nonlinear waveform above 5kHz. In this example, this would affect any signal above 5kHz that was clipped.
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