Problem: "Robotic" Speech
This is generally due to a high rate of packet loss or packet discard due to jitter. The packet loss concealment algorithms used in Voice over IP systems are able to deal with occasional packet loss but when the rate of loss is very high then they can lead to robotic sounding or choppy speech.
"Robotic" sounding speech occurs when a large number of packets are dropped, the packet loss concealment algorithm tries to compensate but introduces a synthetic or robotic sound.
"Choppy" speech can occur either when a high rate of packet loss occurs and packet loss concealment is not being used, or when the speech level is low and Voice Activity Detection is missing some parts of the voice signal.
It is important to note that packet loss can vary during a call - although the average loss rate may be low, short term congestion can lead to periods when packet loss can be as high as 30-40 percent.
Impact
Users may report "robotic sounding" or garbled speech
Example audio file- "robotic" 40% packet loss with PLC:
Example audio file- "choppy" 10% packet loss - no PLC:
Example audio file- "choppy" VAD problem:
Resolution
Investigate the source of packet loss or jitter problems and verify that the jitter buffer is set to a sufficient depth. If packet loss and jitter are low then check for VAD problems.
Tools:
Network Analyzer, Switch and Router statistics, VQmon
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