Applied Technology: MPX-Direct: Digital Connectivity for FM

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Should the slope of the up-sampled interpolation filter, in the SRC, appear greater than the slope of the final filter in the audio processor, then output overshoots may result in the sample rate conversion process! Unfortunately, these overshoots are generated after the processing unit. To remove them would require another limiting device. Some exciters provide an additional peak limiter, and the action of this limiter can degrade audio quality.

Enter MPX-Direct

The desired goal for the digital interconnect between audio processor and exciter, has been to emulate the older analog method, where the audio processor generates the FM stereo MPX signal, and then connect to a wide spectral input on the exciter...a digital BNC-to-BNC connection so to speak.

MPX-Direct generates the FM stereo MPX signal inside the audio processor, and scales the sampling rate to 192kHz. Through recent firmware developments, the AES3 transom can accommodate a sampling rate at 192kHz, making it possible to transport the MPX signal directly from the audio processor to the input of the exciter: MPX over AES. Now it is possible to create a digital version of the older analog method. Figure 3 illustrates the concept.

Figure 3. MPX-Direct transports the multiplexed signal via an AES3 stream. Click to enlarge.

Figure 3. MPX-Direct transports the multiplexed signal via an AES3 stream. Click to enlarge.

There are benefits to this configuration. Now it is possible to couple the output of the audio processor directly to the input of the exciter's modulator in a full linear fashion. There are no SRC functions in the path, which can degrade modulation and sonic performance due to overshoots. The absolute peak level at the output of the stereo generator, will translate to the absolute maximum deviation of the FM carrier. Accomplishing this insures the modulator will faithfully mirror the performance of the audio processor. This reduces, and basically eliminates any coloration of the audio due to the exciter's input and modulator sections.

Since the connection between the audio processor and modulator is as close to theoretically perfect as possible, there will be an improvement in sonic bass performance heard on the radio. This has always been a weakness within the analog MPX connection to an exciter, as any unwanted signal components near dc, would negatively affect the performance of the AFC (automatic frequency control) section of the exciter. This usually required the use of a high-pass filter, or dc-servo of some sort. Both of these can degrade the low frequency performance of the modulator, which reduces or smears the aural texture of on-air bass.

Additionally, any specific functions unique to the audio processor/stereo generator are maintained, as they will not be forced to rely on any limitations imposed by the stereo generator in the exciter. By example, the alternative use of SSBSC (single sideband suppressed carrier) in the stereo generator, which has shown to reduce the effect of multipath, can be employed in the audio processor, and this alternative MPX signal will easily connect and operate correctly. Also, the AES3 signal is a standardized method. This now enables a digital MPX connection to be utilized as an industry standard.

Another benefit to this method is it now enables digital distribution of the MPX signal over wide broadcast networks. There are many such infrastructures of this type internationally. Any digital STL system capable of handling 192kHz sampling should be able to carry this signal. To date, MPX-Direct is available for the Omnia.11 audio processing system, and connects to a Nautel VX transmitter that is running version 4.0 software.

Foti is the CEO of the Telos Alliance and founder of Omnia Audio, Cleveland.

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