[Asterisk-Users] Why echo occurs

[Robert Hajime Lanning]

<quote who="Eric Bishop">
> Just out of interest,
>
> When echo occurs (the type where I hear myself echoing as I talk) what
> is bouncing against. Is it the other caller's equipment, the central
> office or something in between?

When you are talking via 4 wire or VoIP phones there is a seperate
outbound audio channel and inbound audio channel, niether the twain
shall meet....  no echo

Except for POTS lines (2 wire)... where you have one audio channel
going in both directions.

So you have these: (fixed font spacing needed)

   A               Straight POTS                      B
--------                                         ----------
speaker---------------------------------------------speaker
         |                                        |
        mic                                      mic

A talks into mic and the audio is injected into the single
audio channel.  A almost immediatly hears his voice in his
own speaker, as the distance between the mic and the speaker
is short.  B hears A's speach a bit later traveling through
the long line.  We have echo as A hears his own voice, but the
timing makes it perceived as "sidetone".

   A               ISDN/VoIP to POTS                  B
--------                                         ----------
speaker--===================================O-------speaker
         |                                        |
        mic                                      mic

A talk into mic and the audio is sent as a seperate channel
down the line.  At some point this channel is injected into
the single channel of the POTS line for B.  The return
channel to A picks up everything on the single channel POTS
line (wanting to get B's audio, but also getting A's injected
"mic" channel.)  The distance between A's mic, the injection
point and A's speaker combines to make the delay.  This delay
causes the echo to be heard as an echo and not a sidetone.

* some (not all) VoIP/ISDN phones will simulate "sidetone" by
sampling the mic and sticking it directly in the speaker.  This
is done because us humans are used to the POTS technology and
think the line is dead if we do not hear it.  The same goes for
"comfort noise generation".  If the line is active we expect
analog white noise on it.

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END OF LINE
       -MCP