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tschmidt
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Milford, NH
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tschmidt to coxta

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to coxta

Re: what makes a phone Echo?

said by coxta:

I had heard that it was related to satellite signal propagation.
When you say Satellite I assume you mean geosynchronous satellites.

The problem with geosynchronous satellites is the extreme distance.
»en.wikipedia.org/wiki/Ge ··· us_orbit

They are about 25,000 miles high so trip each way is 50,000 miles if satellite is used in both directions that doubles to 100,000 miles. That means it takes about a half second for round trip.

High latency are not a big issue for recorded transmission but adding that much makes voice calls virtually unusable. Any echo that is present is now much more noticeable since it is delayed so much.

/tom

Subaru
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Greenwich, CT

Subaru

Premium Member

said by tschmidt:
said by coxta:

I had heard that it was related to satellite signal propagation.
When you say Satellite I assume you mean geosynchronous satellites.

The problem with geosynchronous satellites is the extreme distance.
»en.wikipedia.org/wiki/Ge ··· us_orbit

They are about 25,000 miles high so trip each way is 50,000 miles if satellite is used in both directions that doubles to 100,000 miles. That means it takes about a half second for round trip.

High latency are not a big issue for recorded transmission but adding that much makes voice calls virtually unusable. Any echo that is present is now much more noticeable since it is delayed so much.

/tom
I don't think they use a satellite as much as they used to for very LD calls as much as they used to now..

I always been interested in this.. I went to the Air and Space in Wash. D.C and I remember seeing some equipment that was on display which was used to cure the echo issue back in the day in the early 90's.

tschmidt
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tschmidt

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said by Subaru:

I remember seeing some equipment that was on display which was used to cure the echo issue back in the day in the early 90's.
Echo cancellation is still very much in use today, now it is done with DSPs rather then dedicated hardware.

»en.wikipedia.org/wiki/Ec ··· ellation

Even though telephone circuit is full duplex (both sides can talk at the same time). In actual use only one person is talking at a time, other is silent. Echo cancellation takes advantage of that and attenuates line on the non-talking direction. The added attenuation reduces level of remote far echo improving call quality. When both sides are talking attenuation is removed. In that case speech helps mask echo.

/tom

Subaru
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Greenwich, CT

Subaru

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Hmm I did not know that.. I only on some phones if no one is talking it's like the microphone gain slowly goes up until someone speaks.

I've had phones like my HTC Wizard with it's dual speakers that is so loud it echo's back into the mic under the phone..

I've called places like Japan, Turkey and Thailand and had no echo at all.

shdesigns
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shdesigns to tschmidt

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to tschmidt
said by tschmidt:

Echo cancellation is still very much in use today, now it is done with DSPs rather then dedicated hardware.

»en.wikipedia.org/wiki/Ec ··· ellation

Even though telephone circuit is full duplex (both sides can talk at the same time). In actual use only one person is talking at a time, other is silent. Echo cancellation takes advantage of that and attenuates line on the non-talking direction. The added attenuation reduces level of remote far echo improving call quality. When both sides are talking attenuation is removed. In that case speech helps mask echo.

/tom
Actually echo cancellation works by creating a predicted echo and subtracting it from the received signal.

It works the same if one or both sides are talking. The only difference is the echo prediction is only updated when one side is talking.

I have written echo cancellers. Most implementations I have seen only handle about 30ms of delay to handle local echos. A longer echo takes more processing and memory and has issues of divergence.