Today’s post takes a look at what causes echo sounds on calls.
There are two primary sources of echo and one primary reason that we occasionally get a “bad connection” with that annoying echo effect.
When you yell into some canyons, after some delay, you can hear your voice echoed back to you at a lesser volume. This is caused by the energy of the soundwaves bouncing off of the canyon walls and returning to your ear, instead of the energy being dispersed or continuing away from you. If you throw a rock into the center of a huge pond, the ripples might disappear by the time they reach the shore, but in a small pool, the energy in the ripples hit the sides of the pool, and can be seen as smaller waves heading back to the source, after a delay.
The echo’s delay comes from the round-trip time it takes for the sound to play at the far end, and the microphone there to send what it hears back to your ear. The quietness (reduced loudness) of the echo is due to the less than full percentage of the original sound that was picked up by the microphones and what percentage of that signal was lost in transmission (dB, or Energy Loss).
A Telephone Hybrid was the device that converted a 4-wire telephone transmission trunk inside the phone company, to a 2-wire “subscriber” line that had a phone at the other end. Part of this conversion process reflects some of the energy each way, resulting in “Hybrid Echo,” especially from the far end of a circuit. Today, even modern fiber optic transmission lines use separate channels for transmit and receive and telephones still mimic two-wire circuits.
One of these clever engineers noticed that on most (polite) phone calls, usually only one side was talking. Early echo suppression would simply detect which side was talking (or yelling the loudest), and cut off the other side. This suppression technique was also used in half-duplex speakerphones. It also led to many a shouting-contest, as one side tried to make themselves heard enough to interrupt!
Advances in electronics, along with the invention of DSP chips has enabled the creation of echo cancellation circuits. These circuits keep a short copy of the sound waves going out (say, 1.5 seconds) and look for the same signal coming back (the echo). Once the echo is spotted, the echo canceller performs some complex high-speed math that “subtracts” the echo before the sound is played back into your annoyed ear.
So with all of these echo cancellers, why do we still sometimes hear echo and why does it occur frequently on Cellular, Sattelite, and VoIP calls? The answer lies in transmission delay. For VoIP and Cellular calls, there is an extra delay caused by sampling and converting your voice to a digital signal (and back to an analog soundwave). For Satellite calls that bounce off of a single satellite, your voice has to travel 22,223 miles (35,786 km) AND BACK. Even at the speed of light, this adds delay to your call. When the round-trip transmission delay on your phone call exceeds the length of time the echo canceler looks for an echo signal coming back, the echo is NOT cancelled out, and you get very annoyed. This is as if you shouted into the canyon and only listened for one second for an echo, completely missing the echo that came back after two seconds.
So now you know! If your Cellular or VoIP callers are complaining of hearing echoes on their lines, you know that this is caused by NETWORK (and other forms) of DELAY that can overwhelm your echo cancellation. PathSolutions tracks down these sorts of problems by measuring delays across your network. It may be that an echoing call passed through several impaired links, or individual links, are fast enough, but not when added together. Unlike other solutions that simply indicate that a call sounded bad, PathSolutions can track the problem down to the link (or links) that are causing the delays (and echo).
See also: Intro to Voice Quality Jitter