ECHO CANCELLATION AND VOIP

Echo cancellation is an important component in voice gateway products. A VoIP gateway system with inadequate echo cancellation will very likely offer less than optimal speech quality. Let’s take a closer look at the causes of “echoes” in VoIP and other voice communication systems, and explore why they need to be cancelled.

Ever heard an “Echo?”

While our major long distance carriers (on the whole) do a great job of keeping this problem to an absolute minimum, every so often I hit a call that forces me to hear whatever I have spoken back in my own earpiece, only delayed by a fraction of a second. Which can be incredibly irritating and confusing. Such calls happen when the “echo canceller” equipment at the far side of the phone connection has failed.

There are two reasons echo happens on voice communication networks: (1) impedance mismatches anywhere in the circuit-switched phone network, and (2) acoustic coupling between the microphone and speaker of a telephone device.

Impedance-induced (electrical) echo. Any time telephone cables are spliced or terminated in connectors, or when a four-wire phone circuit is converted to two wires, a discontinuity occurs that causes a less-than-perfect impedance match on the phone circuit. Why is this important? Because the greater the degree of mismatch, the more transmit audio will leak over to the receive side of the line when it terminates on a two-wire phone circuit or device. The most common cause of impedance mismatch is the two-to-four-wire hybrid that converts a four-wire audio path (two wires for transmit audio and two wires for receive audio) to a local two-wire loop. (See Figure 1.) This tip-and-ring loop directly drives common analog telephone devices and is the most-used type of subscriber connection.

Acoustic echo. The second type of echo, acoustic echo, is caused by received caller audio directly leaking via sound pressure from a speaker or earpiece device to the nearby microphone. Cheaper speakerphones and hands-free cellular phones can cause this type of echo.

What causes echoes

Echoes happen when the caller hears their own audio delayed by as little as 30 mSec. In this case, it is the round-trip delay that is important. In other words, the time it takes for a caller’s transmit (spoken) audio to travel to the receiving caller and return to their phone receiver. Referring to Figure 1; when the caller in New York speaks, their two-wire audio is converted to four-wire, hops onto the long distance public switched telephone network (PSTN) and travels to San Francisco. At this point, a portion of inbound audio is leaked over to the transmit path, then transmitted back to the original speaker. The greater the round trip delay and impedance mismatch, the worse the potential echo.

So what causes delay on the phone network? Figure 2 shows sample one-way delays for various network equipment elements. As if these delays aren’t enough, transmission lines themselves add delays to transmitted audio. Even though this is a small delay (typically 1 mSec per 100 miles of cable), it all adds up.

Echo cancellers

Just when things were looking hopeless for getting quality audio without echoes, echo cancellers came to the rescue. These clever devices listen to speech from the far end that passes through the receive path and literally subtracts that audio from the transmit path at that end. Because they do such a good job of subtracting the potential echo-inducing audio, both sides of the conversation can talk at the same time without any ill effects (assuming the echo canceller is properly designed and adjusted). For a properly designed phone network, you need an echo canceller at any end of a call that terminates into a two-wire environment.

Echo cancellers are very complex digital signal processing devices. A relatively small number of companies in the world actually produce the algorithms that make these devices possible. In other words, we’re in the realm of Very Smart People. You know, they dream of being marooned on a desert island with the Fastest Computer in the Universe. That kind of thing.

The ITU-T has produced recommendations that carefully define excruciatingly-detailed tests that ensure proper performance of echo cancellers. Recommendations G.164, G.165, and G.168 are in this recommendation set. If you want to filter out telecom vendors who really understand echo canceller performance, ask them if their product is routinely tested to G.165 and G.168, the most recent spec that was introduced in April 1997.

Some echo canceller shops claim that even G.168 does not adequately test the echo cancellation process for communication networks that have significantly greater delays (e.g., digital cellular, VoIP, etc.), all of which require a higher degree of cancellation to ensure the best speech quality. Another echo canceller caveat: when constructing voice networks be sure that tandem connections (i.e. multiple communication paths connected together, each with their own echo canceller pair) allow proper operation of each echo canceller in the talk path.

VoIP speech quality

So, are echo cancellers necessary for good speech quality in a VoIP network? Absolutely. Unfortunately, testing echo cancellers in live situations is difficult due to the wide variety of audio and test conditions that must be generated. Having said that, there are several qualities that you can listen for that might give you some indication that an echo canceller in your network may not be operating at peak efficiency: (1) if you hear an echo at the very start of a call, (2) if you hear any echo or scratching sound right after both parties have been talking at the same time (referred to as double-talk) or (3) if you notice somewhat degraded speech quality or clipped speech when both parties are talking at the same time. Your best bet: make sure your VoIP vendor is testing their embedded echo canceller to published ITU-T recommendations. So, are echo cancellers necessary for good speech quality in a VoIP network? Absolutely. Unfortunately, testing echo cancellers in live situations is difficult due to the wide variety of audio and test conditions that must be generated. Having said that, there are several qualities that you can listen for that might give you some indication that an echo canceller in your network may not be operating at peak efficiency: (1) if you hear an echo at the very start of a call, (2) if you hear any echo or scratching sound right after both parties have been talking at the same time (referred to as double-talk) or (3) if you notice somewhat degraded speech quality or clipped speech when both parties are talking at the same time. Your best bet: make sure your VoIP vendor is testing their embedded echo canceller to published ITU-T recommendations.