PDF of this tutorialAcoustic echo apart, background noise is generated through the network when analog and digital phones are operated in hands-free mode. As additional sounds are directly and indirectly picked up by the microphone, multipath audio is created and transmitted back to the talker. The surrounding noise, whether in an automobile or in a crowded, public environment, passes through the digital cellular vocoder, causing distorted speech for the wireline caller.
Digital processing delays and speech-compression techniques further contribute to echo generation and degraded voice quality in wireless networks. Delays are encountered as signals are processed through various routes within the networks, including copper wire, fiber optic lines, microwave connections, international gateways, and satellite transmission. This is especially true with mixed technology digital networks, where calls are processed across numerous network infrastructures.
Echo-control systems are required in all networks that produce one-way time delays greater than 16 ms. In today's digital wireless networks, voice paths are processed at two points in the network within the mobile handset and at the radio frequency (RF) interface of the network. As calls are processed through vocoders in the network, speech processing delays ranging from 80 ms to 100 ms are introduced, resulting in an unacceptable total end-to-end delay of 160 ms to 200 ms. As a result, echo cancellation devices are required within the wireless network to eliminate the hybrid and acoustic echoes in a digital wireless call.
