Introduction to Data Communications
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25c. Envelope Delay Distortion

If the Propagation Delay changes with frequency than we would have the condition where the lower frequencies such as 300 Hz may arrive earlier or later than the higher frequencies such as 3000 Hz. For voice communication, this would probably not be noticable but for data communication using modems, this could affect the phase of the carrier or the modulation technique used to encode the data.

When the Propagation Delay varies across the frequency range, we call this Envelope Delay Distortion. We measure propagation delay in microseconds (us) and the reference is from the worst case to the best case.

Introduction to Data Communications
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