Echo Cancellers

Last Updated: 03-Mar-2003

If you've been hearing a lot about echo cancellers these days, you may also be wondering what they are, and why we need them. Don't underestimate the need for these obscure technical devices, as they play a major role in our perception of network quality. Note that much of what is said in this article is an oversimplification of the complex echo cancellation process. If you want to learn more, I suggest you read up on them either on the Internet or at your local library/bookstore.

What is an Echo Canceller and Why do we Need It?

Most (though not all) telephone calls in North America, whether they originate from a landline or a cell phone, eventually terminate on a two-wire system, or pass through a two-wire system at some point. This means that we have just two wires carrying our conversation in both directions. To make this work the audio from both ends of the line must travel together on those two wires. The upshot of this is that phones at both ends hear both sets of audio through their earpieces. Before the days of digital wireless phones this rarely presented a problem. In fact, hearing your own voice in the earpiece was reassuring, as it let you know the phone was working. There is even a term to describe this. We call the sample of your own voice a "side tone".

But if a side tone is such a good idea in a landline phone, why isn't it an equally good idea in a digital wireless phone? Before I can answer that I must first bring you up to speed on how a digital wireless phone transmits audio. The first step of the process is to translate your voice into a stream of 0s and 1s. This process is known as digitization. This process is not unique to wireless communications however, as just about every single landline phone system converts voice into digital anyway.

The key difference comes in the next step. RF bandwidth is a precious resource, and wireless carriers must do everything they can to stuff as many callers into as small a chunk of bandwidth as possible. The stream of bits that result from digitization is horrendously wasteful. The landline guys don't care about this, since they don't have a bandwidth restriction to worry about. We can radically reduce the stream by applying a compression algorithm to the bits. Unlike computer files, which must compress and expand without a single error, voice compression has a lot more leeway. We can selectively "throw away" parts of the audio to achieve levels of compression that far exceed that which you can get with PKZIP.

Like all compression algorithms however, the real data must be allowed to "run ahead". This gives the algorithm all the data it needs to perform the complex task of compression. Typically the audio is allowed to run ahead by approximate 1/8th of a second. You can hear this delay by using your wireless phone to call your landline. Talk into the wireless phone and listen to yourself on the landline phone.

Here's where the problem comes in. Your voice is delayed by 1/8th of a second before it is transmitted by your phone to the network. The network then delivers this audio stream to the landline network where the two-wire system echoes it back to the network. Finally, the network retransmits the audio back to you, but adds a further 1/8th of a second delay in the process. Your brain can tolerate a small amount of delay between the words leaving your mouth, and what it hears at your ears. However one quarter of a second is simply way too long, and the results of trying to speak under those conditions are comical at best.

The Ontario Science Center in Toronto had a demonstration of this for quite some time. In this demo the visitor put on a pair of headphones and then tried to read "Mary had a little lamb" into a microphone. A tape loop provided the necessary delay so that your words arrived at the headphones about a half second after you spoke them. Needless to say this was hilarious for your friends to witness. Can you honestly say you want all your callers laughing at you?

An echo canceller is a piece of technology that does away with this problem. Its function is easy to describe, but horrendously difficult to achieve. The echo canceller literally removes your voice from the returning audio stream without removing the audio coming from your caller. This is difficult because the canceller must produce a perfect inverse rendition of your audio so that only your voice is removed. Although this technology has been in development for decades, it has only recently become good enough to be called "excellent".

Echo canceller technology must be tuned to work correctly under the expected conditions. Various models are capable of on-the-fly tuning that allow them to adjust to the conditions in real time. Others seem to lack this ability completely, or simply don't implement it well. Each wireless provider uses echo cancellers that are either built into their switches, or are purchased independently. The quality of the echo canceller will determine the quality of your call.

When echo cancellers don't work right they produce a variety of unwanted side effects. The most obvious is that you hear your own voice echoed with a 1/4 second of delay. Depending upon the volume of the echoed sound, this can range from mildly annoying to downright disruptive. A more common symptom however is that the canceller does its job, but it does it too well (so to speak). In other words, it cancels your voice AND it cancels part of the voice of your caller. In this situation you'll find that talking over your caller will obliterate them, or that taking a call in a noisy environment can sometimes cause continuous damage to the audio that simply makes it sound distorted.

If you hear symptoms such as the those I've described above, you are probably hearing the side effects of a poorly tuned or poorly designed echo canceller. There isn't much you can do about this, short of complaining to your provider, or switching to a different provider. Phones whose microphones are closer to your mouth will likely suffer less problems in noisy environments than those having microphones far away from your mouth (due to the latter requiring great microphone sensitivity). Phones with louder outgoing audio will suffer more greatly in the presence of a malfunctioning echo canceller.