Line echo, electrical echo or hybrid echo are different names given to the echo generated by an impedance mismatch in the analog local loop. This occurs when there is a hybrid and multiple gauges of wire, unused taps and loading coils in use. The most common source of electrical echo is the impedance mismatch caused by the 2 wire to 4 wire conversion done by the hybrid transformer, creating an electrical reflection point that is the source of echo.
Line Echo Cancellation
A Line Echo Canceller (LEC) is made of two functional parts: the Adaptive Filter (AF) and the Non-Linear Processor (NLP). The AF is a numerical filter that models the echo path, predicts the echo of the signal present on its receive path, and subtracts the estimated echo on its send path. Since the AF can only model the linear part of the echo, it will attenuate all the linear components and let through any non-linear components of the echo signal. The remaining signal, residual echo, is handled by the NLP. The NLP basically removes any signal that is below a certain threshold and replaces them with simulated background noise.
Octasic’s patented Echo Cancellation algorithm:
- Certified carrier-grade algorithm, considered a benchmark for echo cancellation
- Field hardened: deployed in wireless, wireline and VoP networks worldwide
- Seamless, transparent echo canceller with Octasic music protection
- G.168 Compliant
Acoustic echo is caused by acoustic coupling problems between a telephone's speaker and its microphone. Acoustic echo can occur in mobile phones, wireline telephones or in a hands-free set of a speaker phone. It can be caused by handset crosstalk in poor quality handsets or by echo in the environment surrounding the caller. Handset crosstalk occurs when sound coming out of the speaker is picked-up directly by the microphone. The most common source of acoustic echo is speaker phones in hands-free mode or any type of endpoint using speakers and a microphone; like a SIP endpoint running on a PC for example.
Acoustic echo is prevalent in mobile networks and is typically installed in the Mobile Switching Center (MSC). Octasic’s solutions provide both Line Echo Cancellation (LEC) and Acoustic Echo Cancellation (AEC) simultaneously in the same device, enabling high density gateways that can be provisioned for the appropriate type of call.
Acoustic Echo Cancellation
Acoustic Echo due to its mechanical and environmental sources is mostly non-linear. It is also much more time-varying than line echo since movements of the speaker or device will cause changes in the echo path. Therefore, an AEC algorithm relies much more heavily on the NLP. The echo tail for acoustic echo can also be much longer, sometimes up to half a second.
Adaptive Noise Reduction
The ubiquity of mobile phones has meant that a large percentage of calls are now made outdoors, or in noisy settings. Airports, cars, restaurants, and busy streets, all make it difficult to hear and be heard when placing a call. Adaptive Noise Reduction (ANR) serves to remove the unnecessary noise from a conversation. Removing this noise increases intelligibility, reducing listener fatigue and improving customer satisfaction.
ANR can be run on any voice call, removing the environmental noise, without clipping, distorting or otherwise modifying the speech. The algorithm runs constantly, adapting to changes in the environment.