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A background hissing sound occurs during speech processing with NM-HDA

Core Issue

A low hissing sound is heard when outbound calls are placed from a Foreign Exchange Office (FXO) port through the High-Density Analog voice or fax Network Module (NM-HDA). The hissing sounds as though the noise floor has been raised incorrectly. This symptom does not occur if the two-port voice Network Module is used instead of the NM-HDA.

Note: Slips can also cause echoes and hissing.


To resolve this problem, issue the no non-linear voice port configuration command to disable non-linear processing on the echo canceler. Echo may still be observed after this workaround is implemented.

For a list of integrated releases, refer to Cisco bug ID CSCdw77268.

For more information on how to resolve hissing and static, refer to Troubleshooting Hissing and Static.

Voice Quality

Hissing (Listen to short or long hissing samples.)

Static (Listen to short or long static samples or short or long severe static samples.)