Optimizing Modulation and Minimizing Overshoots With MPX Node Pair

Updated by Mark Manolio


This document covers optimizing an MPX Node Encoder / Decoder pair for tightly-controlled modulation with minimal overshoots and distortion.


The MPX Node uses the revolutionary μMPX codec to send composite FM at a fraction of the bandwidth required by linear approaches. While this purpose-built codec offers superb fidelity in relaying the composite MPX signal, it is still a codec. Thus some overshoots can be generated, especially at minimum bitrate settings.

The MPX Node Decoder includes a safety clipper to effectively remove occasional overshoots without detrimental audible side effects when appropriately adjusted.

Follow the steps below to produce the tightest modulation control and optimum fidelity when using a pair of MPX Nodes to relay your composite MPX signal.


Ensure that your MPX Node(s) are running software v1.3.35 or higher
  1. It is easy to see when the Decoder's safety clipper will trigger by observing the "CLIP" light in the Encoder Node's μMPX Setup screen. We have deliberately increased the input level in the screenshot below to show the Encoder CLIP light in action. Here you can also see that the "Level" indicator is showing "107%":
  2. What we want to do is adjust the MPX Input Level slider so that the "Level" indicator reads up to between 98% to 102% on peaks with the CLIP indicator not lighting or only lighting infrequently: (note that you can also adjust the Level by clicking on the input level value and typing directly on the field).
If the MPX Input Level slider is set too low, loudness will be lost, and there will be more overshoot. In most cases of overshoots, the cause is an input set too low. If the MPX Input Level control is set too high, the safety clipper will be working too hard and may cause some audible distortion.
The above assumes that the FM audio processor/stereo generator feeding the MPX Encoder Node is set properly with the correct pre-emphasis and final limiter/clipper settings to provide the MPX Encoder Node with a tightly-controlled, overshoot-free signal.
  1. Once the input level to the encoder is properly adjusted, set the modulation of your exciter/transmitter using the "MPX Output Level" slider in the Decoder Node. If you cannot achieve 100% modulation of your transmitter even with the "MPX Output Level" set to 0.0dB, you'll need to check your exciter or transmitter and increase its composite input sensitivity.

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