MPX over AES

A fully digital transmission chain which doesn’t sacrifice loudness now exists due to a collaboration between two of the industry’s leading organizations, Omnia and Nautel.

With the Nautel MPX over AES interface, a single AES-EBU cable between the Omnia.11 and Nautel transmitter carries the baseband signal in digital form. This 100% digital path eliminates the noise and distortion of A/D converters and reduces potential overshoots, while ensuring full FCC mask compliance.

New Digital Composite Interface Highlights:

  • Maximum loudness without noise or overshoots
  • Eliminates D/A and A/D conversions

While MPX over AES is a software feature of the Release 4.x or higher firmware update. The functionality requires hardware capabilities found in all GV and NV Series transmitters and also available in currently shipping and more recent versions of NVLT and VS transmitters. Nautel is investigating hardware upgrade options that could allow MPX over AES support for older NVLT and VS users. You can upgrade to Release 4.x software via our Latest Software page.

Nautel MPX over AES Innovation

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 over AES generates the FM-Stereo MPX signal inside the audio processor, and scales the sampling rate to 192kHz. Through recent firmware developments, the AES/EBU 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. Below is an illustration of this concept.

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.

Nautel MPX over AES

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 AES/EBU 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 192 kHz sampling should be able to carry this signal.

Nautel Omnia Direct MPX over AES spec sheet