Noise contrast is where the background noise level changes when the user speaks. This happens as a consequence of speech switching and echo control. This can be particularly disturbing for a PSTN user talking to a mobile terminal in a vehicle because the high background noise can be modulated with the speech and speech intelligibility can be impaired.
The main sources of background noise are
The speech transcoder operation is defined by ETSI and therefore cannot be changed. So the dominant source of noise is the acoustic noise picked up by the microphone. This can be a problem as the PSTN user will hear the background noise being modulated by their own speech with the delays inherent with the TETRA system.
GSM [GSM 03.50] states that preliminary tests with vehicles indicate that a handsfree terminal with a Signal to Noise ratio of 10dB can only tolerate a noise contrast of 2dB (based on an averaged medium term ambient noise spectrum).
This noise contrast level of 2dB is therefore proposed as a maximum figure for TETRA terminals operating in handset, handsfree and headset configurations. Any echo control devices within the processing chain should be enabled when making this measurement.
It is possible to reduce noise contrast by improving the Signal to Noise ratio. This can be achieved by the proper placement of the microphone to maximise directivity, reducing sensitivity and placing the microphone close to the mouth of the user.
As a general guideline the worst case Signal to Noise ratio for handset and headset terminals should be 15dB under noisy conditions. Handsfree systems which can have a much poorer Signal to Noise ratio should be designed to limit the change in the speech level to 5dB for all positions of the talker.
Copyright ©2002 Andy Fletcher andy@x31.com