MicroMPX – STL Transport Made Easy

MicroMPX: Composite MPX Transport over IP Background: Signal Degradation in the Studio-to-Transmitter Link FM stations typically perform audio processing at the studio, where the signal is optimized for loudness, spectral consistency, and modulation efficiency. In a conventional STL configuration, however, only the discrete left and right audio channels are transported to the transmitter site, that or using an analog link which adds noise. Stereo encoding, pilot insertion, RDS, and composite clipping must then be performed at the transmitter — either by additional processing hardware at each site, or not at all. In the latter case, the modulation control and loudness gains achieved at the studio are effectively lost. Approach MicroMPX addresses this by encoding and transporting the complete composite MPX signal — including the 19 kHz pilot, RDS subcarrier, and composite clipping — over IP connections at bitrates as low as 320 kbps. The full signal chain, up to and including MPX generation, remains at the studio; the transmitter site requires only decoding. MicroMPX has been in production use since 2016. Development History The concept originated from field experience with Stereo Tool, Thimeo’s software audio processor, in use since 2004. The introduction of its composite clipper yielded a 2–3 dB improvement in loudness with reduced distortion, but adoption was constrained: applying composite clipping requires processing at the point of MPX generation, and many stations were unwilling to deploy and maintain processors at each transmitter site. This motivated the development of a method for transporting the fully processed composite signal directly from the studio. Codec Design Rather than adapting an existing general-purpose audio codec, Thimeo developed a unique codec specifically for the FM composite signal. The model is tuned to the characteristics of the FM channel itself: coding artifacts are shaped so that they fall below FM’s inherent noise floor, where they are masked in reception. This allows substantially lower bitrates than would be acceptable for general audio distribution.. Operational Considerations Centralizing MPX generation at the studio removes the need for audio processing hardware at each transmitter site and permits the use of standard IP connectivity in place of dedicated high-bandwidth transmission lines. This reduces both capital and maintenance costs, particularly for stations operating multiple sites. Single Frequency Networks and MicroMPX+ Because all sites receive an identical, fully generated composite signal, MicroMPX also provides a practical basis for Single Frequency Network operation, including for smaller stations. For severely bandwidth-constrained links, MicroMPX+ extends operation down to 176 kbps, with limited quality degradation at the lowest rates. Typical applications include satellite distribution and sites with restricted connectivity.