Echo and Soft VoIP PBX Systems
Most of us have experienced telephone calls with disturbing echoes on the line. Low echo volumes together with discernible delay can make a line completely unusable, with the call being terminated after the exchange of a few halting sentences. Traditionally, problems with echo have been experienced on long-distance or international calls, particularly those involving satellite connections.
For many people new to software-based VoIP telephony systems, such as Asterisk, the phenomenon of voice echo comes as an unpleasant surprise. This is true even for those who come to the business after working with traditional PBX systems or proprietary VoIP equipment. Suddenly echo is a problem on local calls, and the traditionally troublesome long-distance and satellite calls are completely echo-free.
In this article, we discuss the origins of echo and how it manifests itself in the VoIP world with particular reference to Asterisk and other software-based telephony systems.
Echo in telephony systems is caused by two main phenomena: the first is electrical echo due to imperfect impedance matching, and the second is acoustic echo due to microphone pickup of audio output. Both these sources produce similar effects and have to be treated similarly. The major difference is electrical echo is a property of the line connection and remains mostly constant throughout the call, while acoustic echo varies in strength and delay depending on the changing acoustic environment of the echo source. For instance, on a hands-free cell-phone call, the echo characteristics change as the speaker moves around.
Electrical signals of all types always are reflected at line terminations, except when the load at the line end exactly matches the impedance rating of the line itself. In fact, the meaning of, say, “75-ohm cabling” is precisely that in order to have no signal reflections, the cable must be terminated by a 75-ohm load. Line impedance is a property of the cable that is affected only by the cable geometry. As no cables are geometrically perfect over their length and no load impedance is perfectly accurate, there always is some reflection at a line termination.
Where digital signals are concerned, as long as the reflections are a small enough fraction of the data transmission, the reflections do not cause errors in reading the bit values. Thus, digital systems can tolerate considerable echo.
The human ear has quite different characteristics, however; it is an incredibly sensitive instrument. The softest sound that can be heard has an acoustic power about a hundred thousand billion times smaller than the power at the threshold of pain. As long as sounds vary by only about a factor of 100 or so, the ear hears a similar level of sound. So even what electrically looks like a small reflection can sound about the same volume as the original signal to the human ear.
And, the traditional telephone circuits are far from perfect. Two-wire circuits from analog lines terminate at devices called hybrids that convert the two-wire analog signal to four-wire signals before digitization. The loads at the hybrids vary quite widely, as does the impedance of the low-cost subscriber loop wiring. The result is almost every call that involves an analog telephone anywhere in the circuit has electrical reflections that can be interpreted by the ear as troublesome echoes.
If this is so, why is echo not a problem on every call? The answer is, if the echo is heard at the same time as the caller is speaking, it is heard as part of the side tone and goes unnoticed. Echo becomes noticeable only when there is a delay between speaking and hearing your voice echoed. This is why echo is a problem only for traditional telephony over long distances. The round-trip delay on a coast-to-coast US call is more than 30ms, which is enough for echo to cause irritation. Satellite delays are much longer still.
VoIP intrinsically has packetization, depacketization and processing delays built into its protocols. That is why, from the point of view of echo, every VoIP call is like a very long-distance call.
Figure 1 shows a typical VoIP scenario. The echo is heard on the VoIP phone: the caller on the analog line hears only a normal side tone, because there are no signal delays. Because delay is a necessary component of perceived echo, traditional PBXes that switch analog or T1/E1 traffic have no perceived echo problems, as their intrinsic end-to-end delay is low. It is the packetization and processing delays inherent in VoIP that cause existing echo to become a problem.
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