With the introduction of stereo FM, the MPX signal was born, or was it the other way around? Regardless, the fact is that the multiplexed broadcast signal to this day represents all the artistic and technical efforts of radio production as an electrical signal. So, it is a valuable thing, this MPX signal.
In the pre-digital era, analog MPX demanded a lot of attention. Only proven experts were entrusted with monitoring and distributing MPX, which relentlessly relayed the effects of any error to all listeners.
This is the reason for the continuous effort to keep interferences away and to reduce the components within the MPX signal path. As a result, the time axis shows that there was and is an increasing integration density across both signal domains.
Digitization was a milestone in MPX signal creation and processing and resulted in the AES192 format. In the broadcast world, it describes the AES format sampled at 192kHz. It allows the transport of the entire MPX spectrum in the digital domain.
The advantages of such an approach to FM-MPX transmission are many. Most importantly, all MPX generation equipment can be located in the studio. Furthermore, if the STL, including the codecs, already supports AES192, only an AES192 capable exciter is needed at the transmitter site.
For good reasons, central signal processing is steadily expanding and being used in many places. As a result, the potential of saving hardware for signal processing and its support at each broadcasting site is enormous. Another advantage is that the station sound can be easily replicated as a sonic signature at all transmitter sites by central signal processing.
The conversion of signal distribution to the robust digital MPX/AES192 signal is not only a trend, but has the potential to become the standard.
Nevertheless, for many broadcasters, the high bandwidth required by the AES192 signal on the transmission path is an unwanted factor, even a real problem. Primary and backup paths need to provide 2-4MBit/s per broadcast.
STL suppliers and codec manufacturers have found ways to reduce the bandwidth of the linear digital MPX. This is achieved by limiting the MPX spectrum to, for example, 64kHz, or reducing the 24/16/14/12-bit resolution; a reduced MPX signal can be transmitted at less than 2MBit/s. This is regularly used today.
The distribution of radio programs as MPX over IP could eventually replace traditional broadcast feeds if MPX transmission is possible with similar network requirements as L/R audio transmission by maintaining high signal fidelity.
Recent industry efforts show promising approaches to new MPX compression algorithms. The demand for low bit-rate formats for compressed MPX transmission is high, so the trend towards centralized signal processing could prevail and become the standard.
Author: Hartmut Foerster, APT Product Manager at WorldCast Systems
