Asterisk hardware vs software echo cancellation

Think of it as a kind of metronome to allow the system to mix multiple audio streams in a properly synchronized manner. One of the issues that can arise if you use analog interfaces on a VoIP system is echo. Echo is simply what you say being reflected back to you a short time later. The echo is caused by the far end, but you are the one that hears it.

It is a little known fact that echo would be a massive problem in the PSTN were it not for the fact that the carriers employ complex and expensive strategies to eliminate it.

We will talk about echo a bit more later on, but with respect to hardware we would suggest that you consider adding echo-cancellation hardware to any card you purchase for use as a PSTN interface.

While Asterisk can do some work with echo in software, it does not provide nearly enough power to deal with the problem. Also, echo cancellation in software imposes a load on the processor; hardware echo cancellers built into the PSTN card take this burden away from the CPU. Hardware echo cancellation can add several hundred dollars to your equipment cost, but if you are serious about having a quality system, invest the extra money now instead of suffering later.

Echo problems are not pleasant at all, and your users will hate the system if they experience it. As of this writing, several software echo cancellers have become available. We have not had a chance to evaluate any of them, but we know that they employ the same algorythems the hardware echo cancellers do.

If you have a recently purchased Digium analog card, you can call Digium sales for a keycode to allow its latest software echo canceller to work with your system.

They will place a measureable load on the CPU that needs to be taken into account when you design a system using these technologies. More Than 6 Months. What is your data connection? Need New Connection. Fill out the form below: First Name:. Last Name:. Helena St. I agree to receive quotes, newsletters and other information from voipmechanic.

I understand that I can withdraw my consent at anytime. The first, and most common cause of echo is impedance mismatches anywhere in the circuit-switched phone network. When telco cabling is spliced or terminated in connectors, or in the conversion of 4-wire phone circuits to 2 wires, a discontinuity occurs that causes an impedance mismatch on the phone circuit.

The greater the extent of impedance mismatch, the more leakage of transmit audio on the receive side of the line when it terminates on a 2-wire phone circuit or device. The main cause of this impedance mismatch is the 2 to 4 wire hybrid that converts a 4-wire audio path to a local 2-wire loop. This tip-and-ring loop is the most-used type of telephone connection made to subscribers from your telephone company central office, and directly drives common analog telephone devices.

Acoustic coupling between the microphone and speaker of a telephone device leads to Acoustic echo, the second most common cause of echo on voice communication networks.

This inherent echo led to the development of Echo Cancellers, in order to improve the quality of voice communications. Echo cancellers are very complex digital signal processing devices, and the algorithms that drive them are produced by a handful of technically proficient companies. Echo can be caused by the fact that an analog local loop circuit has to transmit and receive on the same pair of wires.

If this circuit is not electrically balanced, or if a low-quality telephone is connected to the end of the circuit, signals it receives can be reflected back, becoming part of the return transmission. When this reflected circuit gets back to you, you will hear the words you spoke just moments before.

Humans will perceive an echo beyond a certain amount of delay possibly as low as 20 milliseconds for some people. This echo will become annoying as the delay increases.

In a cheap telephone, it is possible for echo to be generated in the body of the handset.