[Asterisk-Users] Why echo occurs

[Robert Hajime Lanning]

<quote who="Rich Adamson">
> Your original posting said the sidetone was coming from the distant
> phone and did not even come close to implying that sidetone is
> something always engineered into the local phone, regardless of
> whether its analog or digital. Sidetone is always local phone
> generated by design.

I went back and read my original post.  I did not say that the
sidetone was coming from the far end, but I was completely unclear
in what I was saying.

"We have echo as A hears his own voice, but the timing makes it
perceived as "sidetone"."

Should have been more like: (using terms in the original email)
"A" hears his own voice coming from his mic's injection point,
which is close enough to his speaker to make the delay short
enough, so as it is perceived as "sidetone".

I wonder if sidetone was in the original spec when creating the
hybrid, or was it added as a "feature" when they could not get
rid of it.

Did they get a 100% working hybrid, then say "hey, I can't hear
myself!"?

> A 100% perfect hybrid would never generate any feedback or echo.
> But, to date no one has been successful at designing such a beast.
> So a better way to say that is "imperfections in the hybrid can
> cause echo" as opposed to "the hybrid is supposed to automatically
> cancel echo". There is no such thing as an echo canceller in a
> hybrid.

True.  I used the wrong wording.

> Not true at all. Sidetone _is_ designed into the hybrid in analog
> phones on purpose and has been for for at least 30 years.

My guess on that is above.

> Not true. Better take a look at the Silicon Labs chip sets that are
> used in the digium TDM card (as one example). The hybrid is 100%
> digital.

I probably shouldn't have made a blanket statement.  There really
isn't anything we can't simulate in digital, anymore.

And I doubt that sidetone is purposely put into the TDM cards.  It
just comes down to that we can't get rid of it. (hybrid imperfections)

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       -MCP