Optimizing Modulation and Minimizing Overshoots With MPX Node Pair
Scope
This document covers optimizing an MPX Node Encoder / Decoder pair for tightly-controlled modulation with minimal overshoots and distortion.
Description
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.
Setup
- 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%":
- 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).
- 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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