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Echo Cancelers: What They Are and Why They Don’t Work Quite Right

Modern cell phones are great, and for many people they are the only form of voice communication they use, having given up their old (and often expensive) landline service. However, as countless cell phone users have noticed, audio quality is not always as good as could be. On many cellular networks subscribers complain of hearing their own voices echoed back, or their calls suffer from other audio-related maladies. Without any understanding of what causes these issues, most people just write it off as a problem with their phone or a common limitation of the cellular service.

In actual fact, the problem is not caused by their phone, nor is it a given that cellular networks behave in this manner. The problem is a poorly-tuned or poorly-designed ECHO CANCELER. But what on earth is an echo canceler and why do we need one on a cellular network, when clearly this problem never existed on old-fashioned analog landline phones? To understand the need for this piece of equipment we should first have a quick look at one of the fundamental differences between analog and digital voice communication.

Analog is a simple application of an electrical current to carry audio from one location to another. As sound falls on a microphone is generates a tiny voltage that varies according to the sound waves that strike it. These tiny voltages are amplified and then applied to a wire that carries these voltage changes at the speed of light (186,000 miles per second) to the far end.

In digital communication we must first take the rapid changes in voltage coming from the microphone and turn them into a series of 1s and 0s. We can do this fairly quickly, but the raw data stream we create is horrendously wasteful and can be compressed into a much more efficient stream of data. The problem is that the compression process must gather a certain amount of information before it can generate the compressed version of it. When it gets there the data must be uncompressed to put it back into its raw form, and this too takes time. The entire process adds delays of up to 1/4 to 1/2 second before the data is turned back into changes in voltage for delivery to the earpiece.

A delay of up to 1/2 second means that if nothing was done to prevent it, your voice would come out of your earpiece as plain as day 1/2 second after you spoke it. Your brain can’t cope with this sort of delay and your own voice confuses you and it makes it difficult to talk. Long, long ago the Ontario Science Center in Toronto had a display in which the visitor put on a set of headphones and spoke into a microphone. A tape loop provided a degree of delay and the visitor was challenged to read the words to “Mary had a Little Lamb”. The visitor thought they were doing fine, but their friends would die laughing as they listened to their friend stumble through the simple verse. Clearly then, allowing such an echo would make digital communication a royal pain.

The solution is to somehow suppress the echo of the subscriber’s voice so that they aren’t annoyed by it. This could be dealt with simply by keeping the audio in one direction separate from the audio coming in the other direction. This is actually quite trivial, but much of the existing telephone infrastructure wasn’t built that...

 

 

way simply because it doubles the number of communications channels needed. Cellular phone companies can keep the audio streams separate in cellular-to-cellular calls, but if the call ever ends up on a single bi-directional communication channel the echo becomes a problem.

Hence the need for the ECHO CANCELER, but its job isn’t as simple as you might image. It must clearly identify the audio that is coming from the subscriber and literally punch it out of the combined audio channel without doing any damage whatsoever to the audio the subscribers wants to hear. This is actually a pretty tall order and the technology that goes into an echo canceler is extremely advanced. The process is quite precarious and it takes very little to upset the delicate balance it must sustain.

Needless to say, echo cancelers must be correctly installed and maintained to ensure they work correctly, but it seems that many (perhaps most) of the cellular providers don’t put high priority in setting up or maintaining these pieces of equipment. For this reason, echo canceler failures remain a common source of audio quality issues faced by subscribers.

The effects of a malfunctioning echo canceler as wide-ranging. When the canceler fails outright, the subscriber hears a clear and loud echo of their voice. However, such malfunctions are rare and they usually fall somewhere between this extreme and working correctly. For example, one of the common failures of an echo canceler is it manages to block the echo, but it does so by causing damage to the incoming audio. This damage ranges from minor (slight distortion of the incoming voice) to major (the incoming voice is blotted out completely).

This type of damage is noticed most frequently when both parties talk at the same time. The obvious solution would seem to be not to talk over one other, but we humans rarely carry on conversations this way, and without such conventions such as saying “over”, it’s very difficult to know when the person we are talking with has finished and wants us to respond.

If that weren’t bad enough, there is also the issue of background noise. Cell phone users are often NOT in a quiet room. Instead they may be in a car, walking down the street, or in a busy shopping mall. The constant background din must also be canceled, and when an echo canceler damages the incoming audio, it will do so continuously as long as background noise is present.

Some of the better-quality echo cancelers can adapt over time. This means that echo and other canceler-related issues might show up during the first 5 to 10 seconds of a phone call, but will gradually go away as the device makes the necessary adjustments to the speaker’s voice and to the overall background noise.

If you are unlucky enough to subscriber to a carrier that uses poor-quality echo cancelers, or just doesn’t put the time into maintaining them, there really isn’t much you can do. Complaining to the cellular provider about poor audio quality might get them to open a trouble ticket, but if they are already aware of the limitations of their echo cancelers, or know they don’t bother to maintain them properly, it is unlikely that your trouble ticket will be dealt with. Only an educated public that demands better audio quality, rather than a docile public that simply accepts these audio problems as inevitable, is going to push providers to make the necessary changes.

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