[Asterisk-Users] Why echo occurs

[Robert Hajime Lanning]

<quote who="Rich Adamson">
> The sidetone is 'always' generated within analog and digital phones.
> It never comes from any source outside the phone. In analog phones,
> it derived from the hybrid within the phone. On digital phones, its
> basically firmware.

I never said that sidetone was generated outside the phone.

The hybrid is the conversion from the dual channel (4 wire,
transmit/receive) to the single channel (2 wire, the POTS line).

The "audio injection point" that I was talking about in my
previous email, is the location of the hybrids.  The hybrid is
supposed to automaticaly cancel echo, but it takes precise
impedance matching to pull it off.

In an analog phone, the sidetone is a side-effect of the hybrid.
In a digital phone, the sidetone is on purpose.

> The conversion from four-wire (analog or digital) to two-wire requires
> the use of a hybrid (physical component in analog phones, mostly
> firmware in digital phones).

The hybrid is an analog device.  When I am talking digital, I am
talking about technology like ISDN.  In a single bearer channel,
I get 56Kbps out and 56Kbps in.  I do not see an echo of the
output on the input.  (This would cause massive issues when used
as a data call.)  The echo comes when and if I hit a conversion
to analog then hit a hybrid.  If the conversation is happening
purely digital end to end, then you will not get echo.  Just like
IP to IP.

> The 'inefficiencies' of that hybrid is
> the source of echo, regardless of where they happen to be in the
> end-to-end communications path. Since it is impossible to know what
> each telephone company or long distance carrier has engineered, its
> not possible to guess at where hybrids might exist in that path.
> It is fair to say the number of hybrids is very small now compared
> to twenty years ago, but they do exist at least at both ends of a
> communications path.

This is true, as long as the path has an analog 2-wire leg.  Though
where the "ends" are that the hybrid is located could be lopsided.

Say I have a PRI into the PSTN.  I call a friend who has POTS service.
Now days, the path will be digital from my PRI all the way to my
friend's central office.  At that point it gets split off the trunk,
converted to analog, passed through a hybrid, and placed on the wire
pair to my friend's house.  Then, through the hybrid in his phone.
So, the echo I hear is from the hybrid in the central office and
the echo my friend hears is from the hybrid in his phone, which is
so close to him, that it becomes sidetone.

The previous paragraph is based on where I live (Silicon Valley),
the location of the central office hybrid maybe different, depending
on your local infrastructure.


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       -MCP